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A lenticular galaxy (denoted S0) is a type of galaxy intermediate between an elliptical (denoted E) and a spiral galaxy in galaxy morphological classification schemes. It contains a large-scale disc but does not have large-scale spiral arms. Lenticular galaxies are disc galaxies that have used up or lost most of their interstellar matter and therefore have very little ongoing star formation. They may, however, retain significant dust in their disks. As a result, they consist mainly of aging stars (like elliptical galaxies). Despite the morphological differences, lenticular and elliptical galaxies share common properties like spectral features and scaling relations. Both can be considered early-type galaxies that are passively evolving, at least in the local part of the Universe. Connecting the E galaxies with the S0 galaxies are the ES galaxies with intermediate-scale discs.
Morphology and structure
Classification
Lenticular galaxies are unique in that they have a visible disk component as well as a prominent bulge component. They have much higher bulge-to-disk ratios than typical spirals and do not have the canonical spiral arm structure of late-type galaxies, yet may exhibit a central bar. This bulge dominance can be seen in the axis ratio (i.e. the ratio between the observed minor and major axial of a disk galaxy) distribution of a lenticular galaxy sample. The distribution for lenticular galaxies rises steadily in the range 0.25 to 0.85 whereas the distribution for spirals is essentially flat in that same range. Larger axial ratios can be explained by observing face-on disk galaxies or by having a sample of spheroidal (bulge-dominated) galaxies. Imagine looking at two disk galaxies edge-on, one with a bulge and one without a bulge. The galaxy with a prominent bulge will have a larger edge-on axial ratio compared to the galaxy without a bulge based on the definition of axial ratio. Thus a sample of disk galaxies with prominent spheroidal components will have more galaxies at larger axial ratios. The fact that the lenticular galaxy distribution rises with increasing observed axial ratio implies that lenticulars are dominated by a central bulge component. | Lenticular galaxy | Wikipedia | 426 | 319126 | https://en.wikipedia.org/wiki/Lenticular%20galaxy | Physical sciences | Galaxy morphological classification | null |
Lenticular galaxies are often considered to be a poorly understood transition state between spiral and elliptical galaxies, which results in their intermediate placement on the Hubble sequence. This results from lenticulars having both prominent disk and bulge components. The disk component is usually featureless, which precludes a classification system similar to spiral galaxies. As the bulge component is usually spherical, elliptical galaxy classifications are also unsuitable. Lenticular galaxies are thus divided into subclasses based upon either the amount of dust present or the prominence of a central bar. The classes of lenticular galaxies with no bar are S01, S02, and S03 where the subscripted numbers indicate the amount of dust absorption in the disk component; the corresponding classes for lenticulars with a central bar are SB01, SB02, and SB03.
Sérsic decomposition
The surface brightness profiles of lenticular galaxies are well described by the sum of a Sérsic model for the spheroidal component plus an exponentially declining model (Sérsic index of n ≈ 1) for the disk, and often a third component for the bar. Sometimes there is an observed truncation in the surface brightness profiles of lenticular galaxies at ~ 4 disk scalelengths. These features are consistent with the general structure of spiral galaxies. However, the bulge component of lenticulars is more closely related to elliptical galaxies in terms of morphological classification. This spheroidal region, which dominates the inner structure of lenticular galaxies, has a steeper surface brightness profile (Sérsic index typically ranging from n = 1 to 4) than the disk component. Lenticular galaxy samples are distinguishable from the diskless (excluding small nuclear disks) elliptical galaxy population through analysis of their surface brightness profiles. | Lenticular galaxy | Wikipedia | 355 | 319126 | https://en.wikipedia.org/wiki/Lenticular%20galaxy | Physical sciences | Galaxy morphological classification | null |
Bars
Like spiral galaxies, lenticular galaxies can possess a central bar structure. While the classification system for normal lenticulars depends on dust content, barred lenticular galaxies are classified by the prominence of the central bar. SB01 galaxies have the least defined bar structure and are only classified as having slightly enhanced surface brightness along opposite sides of the central bulge. The prominence of the bar increases with index number, thus SB03 galaxies, like the NGC 1460 have very well defined bars that can extend through the transition region between the bulge and disk. NGC 1460 is actually the galaxy with one of the largest bars seen among lenticular galaxies. Unfortunately, the properties of bars in lenticular galaxies have not been researched in great detail. Understanding these properties, as well as understanding the formation mechanism for bars, would help clarify the formation or evolution history of lenticular galaxies.
Box-shaped bulges
NGC 1375 and NGC 1175 are examples of lenticular galaxies that have so-called box-shaped bulges. They are classified as SB0 pec. Box-shaped bulges are seen in edge-on galaxies, mostly spiral, but rarely lenticular.
Content
In many respects the composition of lenticular galaxies is like that of ellipticals. For example, they both consist of predominately older, hence redder, stars. All of their stars are thought to be older than about a billion years, in agreement with their offset from the Tully–Fisher relation (see below). In addition to these general stellar attributes, globular clusters are found more frequently in lenticular galaxies than in spiral galaxies of similar mass and luminosity. They also have little to no molecular gas (hence the lack of star formation) and no significant hydrogen α or 21-cm emission. Finally, unlike ellipticals, they may still possess significant dust.
Kinematics
Measurement difficulties and techniques | Lenticular galaxy | Wikipedia | 378 | 319126 | https://en.wikipedia.org/wiki/Lenticular%20galaxy | Physical sciences | Galaxy morphological classification | null |
Lenticular galaxies share kinematic properties with both spiral and elliptical galaxies. This is due to the significant bulge and disk nature of lenticulars. The bulge component is similar to elliptical galaxies in that it is pressure supported by a central velocity dispersion. This situation is analogous to a balloon, where the motions of the air particles (stars in a bulge's case) are dominated by random motions. However, the kinematics of lenticular galaxies are dominated by the rotationally supported disk. Rotation support implies the average circular motion of stars in the disk is responsible for the stability of the galaxy. Thus, kinematics are often used to distinguish lenticular galaxies from elliptical galaxies. Determining the distinction between elliptical galaxies and lenticular galaxies often relies on the measurements of velocity dispersion (σ), rotational velocity (v), and ellipticity (ε). In order to differentiate between lenticulars and ellipticals, one typically looks at the v/σ ratio for a fixed ε. For example, a rough criterion for distinguishing between lenticular and elliptical galaxies is that elliptical galaxies have v/σ < 0.5 for ε = 0.3. The motivation behind this criterion is that lenticular galaxies do have prominent bulge and disk components whereas elliptical galaxies have no disk structure. Thus, lenticulars have much larger v/σ ratios than ellipticals due to their non-negligible rotational velocities (due to the disk component) in addition to not having as prominent of a bulge component compared to elliptical galaxies. However, this approach using a single ratio for each galaxy is problematic due to the dependence of the v/σ ratio on the radius out to which it is measured in some early-type galaxies. For example, the ES galaxies that bridge the E and S0 galaxies, with their intermediate-scale disks, have a high v/σ ratio at intermediate radii that then drops to a low ratio at large radii. | Lenticular galaxy | Wikipedia | 397 | 319126 | https://en.wikipedia.org/wiki/Lenticular%20galaxy | Physical sciences | Galaxy morphological classification | null |
The kinematics of disk galaxies are usually determined by Hα or 21-cm emission lines, which are typically not present in lenticular galaxies due to their general lack of cool gas. Thus kinematic information and rough mass estimates for lenticular galaxies often comes from stellar absorption lines, which are less reliable than emission line measurements. There is also a considerable amount of difficulty in deriving accurate rotational velocities for lenticular galaxies. This is a combined effect from lenticulars having difficult inclination measurements, projection effects in the bulge-disk interface region, and the random motions of stars affecting the true rotational velocities. These effects make kinematic measurements of lenticular galaxies considerably more difficult compared to normal disk galaxies.
Offset Tully–Fisher relation
The kinematic connection between spiral and lenticular galaxies is most clear when analyzing the Tully–Fisher relation for spiral and lenticular samples. If lenticular galaxies are an evolved stage of spiral galaxies then they should have a similar Tully–Fisher relation with spirals, but with an offset in the luminosity / absolute magnitude axis. This would result from brighter, redder stars dominating the stellar populations of lenticulars. An example of this effect can be seen in the adjacent plot. One can clearly see that the best-fit lines for the spiral galaxy data and the lenticular galaxy have the same slope (and thus follow the same Tully–Fisher relation), but are offset by ΔI ≈ 1.5. This implies that lenticular galaxies were once spiral galaxies but are now dominated by old, red stars. | Lenticular galaxy | Wikipedia | 318 | 319126 | https://en.wikipedia.org/wiki/Lenticular%20galaxy | Physical sciences | Galaxy morphological classification | null |
Formation theories
The morphology and kinematics of lenticular galaxies each, to a degree, suggest a mode of galaxy formation. Their disk-like, possibly dusty, appearance suggests they come from faded spiral galaxies, whose arm features disappeared. However, some lenticular galaxies are more luminous than spiral galaxies, which suggests that they are not merely the faded remnants of spiral galaxies. Lenticular galaxies might result from a galaxy merger, which increase the total stellar mass and might give the newly merged galaxy a disk-like, arm-less appearance. Alternatively, it has been proposed that they grew their disks via (gas and minor merger) accretion events.
It had previously been suggested that the evolution of luminous lenticular galaxies may be closely linked to that of elliptical galaxies, whereas fainter lenticulars might be more closely associated with ram-pressure stripped spiral galaxies, although this latter galaxy harassment scenario has since been queried due to the existence of extremely isolated, low-luminosity lenticular galaxies such as LEDA 2108986.
Faded spirals
The absence of gas, presence of dust, lack of recent star formation, and rotational support are all attributes one might expect of a spiral galaxy which had used up all of its gas in the formation of stars. This possibility is further enhanced by the existence of gas poor, or "anemic", spiral galaxies. If the spiral pattern then dissipated the resulting galaxy would be similar to many lenticulars. Moore et al. also document that tidal harassment – the gravitational effects from other, near-by galaxies – could aid this process in dense regions. The clearest support for this theory, however, is their adherence to slightly shifted version of Tully–Fisher relation, discussed above.
A 2012 paper that suggests a new classification system, first proposed by the Canadian astronomer Sidney van den Bergh, for lenticular and dwarf spheroidal galaxies (S0a-S0b-S0c-dSph) that parallels the Hubble sequence for spirals and irregulars (Sa-Sb-Sc-Im) reinforces this idea showing how the spiral–irregular sequence is very similar to this new one for lenticulars and dwarf ellipticals.
Mergers | Lenticular galaxy | Wikipedia | 445 | 319126 | https://en.wikipedia.org/wiki/Lenticular%20galaxy | Physical sciences | Galaxy morphological classification | null |
The analyses of Burstein and Sandage showed that lenticular galaxies typically have surface brightness much greater than other spiral classes. It is also thought that lenticular galaxies exhibit a larger bulge-to-disk ratio than spiral galaxies and this may be inconsistent with simple fading from a spiral. If S0s were formed by mergers of other spirals these observations would be fitting and it would also account for the increased frequency of globular clusters. It should be mentioned, however, that advanced models of the central bulge which include both a general Sersic profile and bar indicate a smaller bulge, and thus a lessened inconsistency. Mergers are also unable to account for the offset from the Tully–Fisher relation without assuming that the merged galaxies were quite different from those we see today.
Disk growth via accretion
The creation of disks in, at least some, lenticular galaxies via the accretion of gas, and small galaxies, around a pre-existing spheroidal structure was first suggested as an explanation to match the high-redshift compact massive spheroidal-shaped galaxies with the equally compact massive bulges seen in nearby massive lenticular galaxies. In a "downsizing" scenario, bigger lenticular galaxies may have been built first – in a younger universe when more gas was available – and the lower-mass galaxies may have been slower to attract their disk-building material, as in the case of the isolated early-type galaxy LEDA 2108986. Within galaxy clusters, ram-pressure stripping removes gas and prevents the accretion of new gas that might be capable of furthering the development of the disk.
Examples
Cartwheel Galaxy, lenticular galaxy about 500 million light-years away in the constellation Sculptor
NGC 2787, a barred lenticular galaxy
NGC 3115
NGC 3632
NGC 4608, a barred lenticular galaxy about 56 million light years away in Virgo
NGC 5866
NGC 1533 is a prototypical lenticular galaxy in the constellation Dorado
Gallery | Lenticular galaxy | Wikipedia | 402 | 319126 | https://en.wikipedia.org/wiki/Lenticular%20galaxy | Physical sciences | Galaxy morphological classification | null |
The Eagle Nebula (catalogued as Messier 16 or M16, and as NGC 6611, and also known as the Star Queen Nebula) is a young open cluster of stars in the constellation Serpens, discovered by Jean-Philippe de Cheseaux in 1745–46. Both the "Eagle" and the "Star Queen" refer to visual impressions of the dark silhouette near the center of the nebula, an area made famous as the "Pillars of Creation" imaged by the Hubble Space Telescope. The nebula contains several active star-forming gas and dust regions, including the aforementioned Pillars of Creation. The Eagle Nebula lies in the Sagittarius Arm of the Milky Way.
Characteristics
The Eagle Nebula is part of a diffuse emission nebula, or H II region, which is catalogued as IC 4703. This region of active current star formation is about 5700 light-years distant. A spire of gas that can be seen coming off the nebula in the northeastern part is approximately 9.5 light-years or about 90 trillion kilometers long.
The cluster associated with the nebula has approximately 8100 stars, which are mostly concentrated in a gap in the molecular cloud to the north-west of the Pillars.
The brightest star (HD 168076) has an apparent magnitude of +8.24, easily visible with good binoculars. It is actually a binary star formed of an O3.5V star plus an O7.5V companion. This star has a mass of roughly 80 solar masses, and a luminosity up to 1 million times that of the Sun.
The cluster's age has been estimated to be 1–2 million years.
The descriptive names reflect impressions of the shape of the central pillar rising from the southeast into the central luminous area. The name "Star Queen Nebula" was introduced by Robert Burnham, Jr., reflecting his characterization of the central pillar as the Star Queen shown in silhouette.
"Pillars of Creation" region | Eagle Nebula | Wikipedia | 394 | 319141 | https://en.wikipedia.org/wiki/Eagle%20Nebula | Physical sciences | Notable nebulae | null |
Images produced by Jeff Hester and Paul Scowen using the Hubble Space Telescope in 1995 greatly improved scientific understanding of processes inside the nebula. One of these became famous as the "Pillars of Creation", depicting a large region of star formation. Its small dark pockets are believed to be protostars (Bok globules). The pillar structure resembles that of a much larger instance in the Soul Nebula of Cassiopeia, imaged with the Spitzer Space Telescope in 2005 equally characterized as "Pillars of Star Creation". or "Pillars of Star Formation". These columns – which resemble stalagmites protruding from the floor of a cavern – are composed of interstellar hydrogen gas and dust, which act as incubators for new stars. Inside the columns and on their surface astronomers have found knots or globules of denser gas, called EGGs ("Evaporating Gaseous Globules"). Stars are being formed inside some of these.
X-ray images from the Chandra observatory compared with Hubble's "Pillars" image have shown that X-ray sources (from young stars) do not coincide with the pillars, but rather randomly dot the nebula. Any protostars in the pillars' EGGs are not yet hot enough to emit X-rays.
Evidence from the Spitzer Space Telescope originally suggested that the pillars in M16 may be threatened by a "past supernova". Hot gas observed by Spitzer in 2007 suggested they were already – likely – being disturbed by a supernova that exploded 8,000 to 9,000 years ago. Due to the distance the main blast of light would have reached Earth for a brief time 1,000 to 2,000 years ago. A more slowly moving, theorized, shock wave would have taken a few thousand years to move through the nebula and would have blown away the delicate pillars. However, in 2014 the pillars were imaged a second time by Hubble, in both visible light and infrared light. The images being 20 years later provided a new, detailed account of the rate of evaporation occurring within the pillars. No supernova is evidenced within them, and it is estimated in some form they still exist – and will appear for at least 100,000 more years.
Gallery | Eagle Nebula | Wikipedia | 463 | 319141 | https://en.wikipedia.org/wiki/Eagle%20Nebula | Physical sciences | Notable nebulae | null |
Messier 4 or M4 (also known as NGC 6121 or the Spider Globular Cluster) is a globular cluster in the constellation of Scorpius. It was discovered by Philippe Loys de Chéseaux in 1745 and catalogued by Charles Messier in 1764. It was the first globular cluster in which individual stars were resolved.
Visibility
M4 is conspicuous in even the smallest of telescopes as a fuzzy ball of light. It appears about the same size as the Moon in the sky. It is one of the easiest globular clusters to find, being located only 1.3 degrees west of the bright star Antares, with both objects being visible in a wide-field telescope. Modestly sized telescopes will begin to resolve individual stars, of which the brightest in M4 are of apparent magnitude 10.8.
Characteristics
M4 is a rather loosely concentrated cluster of class IX and measures 75 light-years across. It features a characteristic "bar" structure across its core, visible to moderate sized telescopes. The structure consists of 11th-magnitude stars and is approximately 2.5' long and was first noted by William Herschel in 1783. At least 43 variable stars have been observed within M4.
M4 is approximately 6,000 light-years away, making it the closest globular cluster to the Solar System. It has an estimated age of 12.2 billion years.
In astronomy, the abundance of elements other than hydrogen and helium is called the metallicity, and it is usually denoted by the abundance ratio of iron to hydrogen as compared to the Sun. For this cluster, the measured abundance of iron is equal to:
This value is the logarithm of the ratio of iron to hydrogen relative to the same ratio in the Sun. Thus the cluster has an abundance of iron equal to 8.5% of the iron abundance in the Sun. This strongly suggests this cluster hosts two distinct stellar populations, differing by age. Thus the cluster probably saw two main cycles or phases of star formation. | Messier 4 | Wikipedia | 410 | 319150 | https://en.wikipedia.org/wiki/Messier%204 | Physical sciences | Notable star clusters | Astronomy |
The space velocity components are (U, V, W) = (, , ) km/s. This confirms an orbit around the Milky Way of a period of with eccentricity 0.80 ± 0.03: during periapsis it comes within from the galactic core, while at apoapsis it travels out to . The inclination is at (an angle of) from the galactic plane, thus it reaches as much as above the disk. When passing through the disk, this cluster does so at less than 5 kpc from the galactic nucleus. The cluster undergoes tidal shock during each passage, which can cause the repeated shedding of stars. Thus the cluster may have been much more massive.
Notable stars
Photographs by the Hubble Space Telescope in 1995 found white dwarf stars in M4 that are among the oldest known stars in our galaxy; aged 13 billion years. One has been found to be a binary star with a pulsar companion, PSR B1620−26 and a planet orbiting it with a mass of 2.5 times that of Jupiter (). One star in Messier 4 was also found to have much more of the rare light element lithium than expected.
CX-1 Is located in M4. It is known as a possible millisecond pulsar/neutron star binary. It orbits in 6.31 hours.
Spinthariscope analogy
The view of Messier 4 through a good telescope was likened by Robert Burnham Jr. to that of hyperkinetic luminous alpha particles seen in a spinthariscope.
Central black hole
In 2023, an analysis of Hubble Space Telescope and European Space Agency's Gaia spacecraft data from Messier 4 revealed an excess mass of roughly 800 solar masses in the center of this cluster, which appears to not be extended. This could thus be considered as kinematic evidence for an intermediate-mass black hole (even if an unusually compact cluster of compact objects like white dwarfs, neutron stars or stellar-mass black holes cannot be completely discounted). | Messier 4 | Wikipedia | 410 | 319150 | https://en.wikipedia.org/wiki/Messier%204 | Physical sciences | Notable star clusters | Astronomy |
Messier 83 or M83, also known as the Southern Pinwheel Galaxy and NGC 5236, is a barred spiral galaxy approximately 15 million light-years away in the constellation borders of Hydra and Centaurus. Nicolas-Louis de Lacaille discovered M83 on 17 February 1752 at the Cape of Good Hope. Charles Messier added it to his catalogue of nebulous objects (now known as the Messier Catalogue) in March 1781.
It is one of the closest and brightest barred spiral galaxies in the sky, and is visible with binoculars. It has an isophotal diameter at about . Its nickname of the Southern Pinwheel derives from its resemblance to the Pinwheel Galaxy (M101).
Characteristics
M83 is a massive, grand design spiral galaxy. Its morphological classification in the De Vaucouleurs system is SAB(s)c, where the 'SAB' denotes a weak-barred spiral, '(s)' indicates a pure spiral structure with no ring, and 'c' means the spiral arms are loosely wound. The peculiar dwarf galaxy NGC 5253 lies near M83, and the two likely interacted within the last billion years resulting in starburst activity in their central regions.
The star formation rate in M83 is higher along the leading edge of the spiral arms, as predicted by density wave theory. NASA's Galaxy Evolution Explorer project on 16 April 2008 reported finding large numbers of new stars in the outer reaches of the galaxy— from the center. It had been thought that these areas lacked the materials necessary for star formation. | Messier 83 | Wikipedia | 322 | 319153 | https://en.wikipedia.org/wiki/Messier%2083 | Physical sciences | Notable galaxies | Astronomy |
Supernovae
Six supernovae have been observed in M83:
SN 1923A (type unknown, mag. 14) was discovered by Carl Otto Lampland on 5 May 1923.
SN 1945B (type unknown, mag. 14.2) was discovered by William Liller on 13 July 1945.
SN 1950B (type unknown, mag. 14.5) was discovered by Guillermo Haro on 15 March 1950.
SN 1957D (type unknown, mag. 15) was discovered by H. S. Gates on 28 December 1957.
SN 1968L (type II-P, mag. 11.9) was discovered by J. C. Bennett on 17 July 1968.
SN 1983N (type Ia, mag. 11.9) was discovered by Robert Evans from Australia on July 3, 1983. On July 6, it was observed with the Very Large Array and became the first type I supernova to have a radio emission detected. The supernova reached peak optical brightness on July 17, achieving an apparent visual magnitude of 11.54. Although identified as type I, the spectrum was considered peculiar. A year after the explosion, about of iron was discovered in the ejecta. This was the first time that such a large amount of iron was unambiguously detected from a supernova explosion. SN 1983N became the modern prototype of a hydrogen deficient type Ib supernova, with the progenitor being inferred as a Wolf–Rayet star.
Environment
M83 is at the center of one of two subgroups within the Centaurus A/M83 Group, a nearby galaxy group. Centaurus A is at the center of the other subgroup. These are sometimes identified as one group, and sometimes as two. However, the galaxies around Centaurus A and the galaxies around M83 are physically close to each other, and both subgroups appear not to be moving relative to each other. | Messier 83 | Wikipedia | 397 | 319153 | https://en.wikipedia.org/wiki/Messier%2083 | Physical sciences | Notable galaxies | Astronomy |
A shield volcano is a type of volcano named for its low profile, resembling a shield lying on the ground. It is formed by the eruption of highly fluid (low viscosity) lava, which travels farther and forms thinner flows than the more viscous lava erupted from a stratovolcano. Repeated eruptions result in the steady accumulation of broad sheets of lava, building up the shield volcano's distinctive form.
Shield volcanoes are found wherever fluid, low-silica lava reaches the surface of a rocky planet. However, they are most characteristic of ocean island volcanism associated with hot spots or with continental rift volcanism. They include the largest active volcanoes on Earth, such as Mauna Loa. Giant shield volcanoes are found on other planets of the Solar System, including Olympus Mons on Mars and Sapas Mons on Venus.
Etymology
The term 'shield volcano' is taken from the German term Schildvulkan, coined by the Austrian geologist Eduard Suess in 1888 and which had been calqued into English by 1910.
Geology
Structure
Shield volcanoes are distinguished from the three other major volcanic types—stratovolcanoes, lava domes, and cinder cones—by their structural form, a consequence of their particular magmatic composition. Of these four forms, shield volcanoes erupt the least viscous lavas. Whereas stratovolcanoes and lava domes are the product of highly viscous flows, and cinder cones are constructed of explosively eruptive tephra, shield volcanoes are the product of gentle effusive eruptions of highly fluid lavas that produce, over time, a broad, gently sloped eponymous "shield". Although the term is generally applied to basaltic shields, it has also at times been applied to rarer scutiform volcanoes of differing magmatic composition—principally pyroclastic shields, formed by the accumulation of fragmentary material from particularly powerful explosive eruptions, and rarer felsic lava shields formed by unusually fluid felsic magmas. Examples of pyroclastic shields include Billy Mitchell volcano in Papua New Guinea and the Purico complex in Chile; an example of a felsic shield is the Ilgachuz Range in British Columbia, Canada. Shield volcanoes are similar in origin to vast lava plateaus and flood basalts present in various parts of the world. These are eruptive features which occur along linear fissure vents and are distinguished from shield volcanoes by the lack of an identifiable primary eruptive center. | Shield volcano | Wikipedia | 506 | 319168 | https://en.wikipedia.org/wiki/Shield%20volcano | Physical sciences | Volcanology | Earth science |
Active shield volcanoes experience near-continuous eruptive activity over extremely long periods of time, resulting in the gradual build-up of edifices that can reach extremely large dimensions. With the exclusion of flood basalts, mature shields are the largest volcanic features on Earth. The summit of the largest subaerial volcano in the world, Mauna Loa, lies above sea level, and the volcano, over wide at its base, is estimated to contain about of basalt. The mass of the volcano is so great that it has slumped the crust beneath it a further . Accounting for this subsidence and for the height of the volcano above the sea floor, the "true" height of Mauna Loa from the start of its eruptive history is about . Mount Everest, by comparison, is in height. In 2013, a team led by the University of Houston's William Sager announced the discovery of Tamu Massif, an enormous extinct submarine volcano, approximately in area, which dwarfs all previously known volcanoes on Earth. However, the extents of the volcano have not been confirmed. Although Tamu Massif was initially believed to be a shield volcano, Sanger and his colleagues acknowledged in 2019 that Tamu Massif is not a shield volcano.
Shield volcanoes feature a gentle (usually 2° to 3°) slope that gradually steepens with elevation (reaching approximately 10°) before flattening near the summit, forming an overall upwardly convex shape. These slope characteristics have a correlation with age of the forming lava, with in the case of the Hawaiian chain, steepness increasing with age, as later lavas tend to be more alkali so are more viscous, with thicker flows, that travel less distance from the summit vents. In height they are typically about one twentieth their width. Although the general form of a "typical" shield volcano varies little worldwide, there are regional differences in their size and morphological characteristics. Typical shield volcanoes found in California and Oregon measure in diameter and in height, while shield volcanoes in the central Mexican Michoacán–Guanajuato volcanic field average in height and in width, with an average slope angle of 9.4° and an average volume of . | Shield volcano | Wikipedia | 443 | 319168 | https://en.wikipedia.org/wiki/Shield%20volcano | Physical sciences | Volcanology | Earth science |
Rift zones are a prevalent feature on shield volcanoes that is rare on other volcanic types. The large, decentralized shape of Hawaiian volcanoes as compared to their smaller, symmetrical Icelandic cousins can be attributed to rift eruptions. Fissure venting is common in Hawaii; most Hawaiian eruptions begin with a so-called "wall of fire" along a major fissure line before centralizing to a small number of points. This accounts for their asymmetrical shape, whereas Icelandic volcanoes follow a pattern of central eruptions dominated by summit calderas, causing the lava to be more evenly distributed or symmetrical.
Eruptive characteristics
Most of what is currently known about shield volcanic eruptive character has been gleaned from studies done on the volcanoes of Hawaii Island, by far the most intensively studied of all shields because of their scientific accessibility; the island lends its name to the slow-moving, effusive eruptions typical of shield volcanism, known as Hawaiian eruptions. These eruptions, the least explosive of volcanic events, are characterized by the effusive emission of highly fluid basaltic lavas with low gaseous content. These lavas travel a far greater distance than those of other eruptive types before solidifying, forming extremely wide but relatively thin magmatic sheets often less than thick. Low volumes of such lavas layered over long periods of time are what slowly constructs the characteristically low, broad profile of a mature shield volcano.
Also unlike other eruptive types, Hawaiian eruptions often occur at decentralized fissure vents, beginning with large "curtains of fire" that quickly die down and concentrate at specific locations on the volcano's rift zones. Central-vent eruptions, meanwhile, often take the form of large lava fountains (both continuous and sporadic), which can reach heights of hundreds of meters or more. The particles from lava fountains usually cool in the air before hitting the ground, resulting in the accumulation of cindery scoria fragments; however, when the air is especially thick with pyroclasts, they cannot cool off fast enough because of the surrounding heat, and hit the ground still hot, accumulating into spatter cones. If eruptive rates are high enough, they may even form splatter-fed lava flows. Hawaiian eruptions are often extremely long-lived; Puʻu ʻŌʻō, a cinder cone of Kīlauea, erupted continuously from January 3, 1983, until April 2018. | Shield volcano | Wikipedia | 495 | 319168 | https://en.wikipedia.org/wiki/Shield%20volcano | Physical sciences | Volcanology | Earth science |
Flows from Hawaiian eruptions can be divided into two types by their structural characteristics: pāhoehoe lava which is relatively smooth and flows with a ropey texture, and ʻaʻā flows which are denser, more viscous (and thus slower moving) and blockier. These lava flows can be anywhere between thick. Aā lava flows move through pressure— the partially solidified front of the flow steepens because of the mass of flowing lava behind it until it breaks off, after which the general mass behind it moves forward. Though the top of the flow quickly cools down, the molten underbelly of the flow is buffered by the solidifying rock above it, and by this mechanism, aā flows can sustain movement for long periods of time. Pāhoehoe flows, in contrast, move in more conventional sheets, or by the advancement of lava "toes" in snaking lava columns. Increasing viscosity on the part of the lava or shear stress on the part of local topography can morph a pāhoehoe flow into an ʻaʻā one, but the reverse never occurs.
Although most shield volcanoes are by volume almost entirely Hawaiian and basaltic in origin, they are rarely exclusively so. Some volcanoes, such as Mount Wrangell in Alaska and Cofre de Perote in Mexico, exhibit large enough swings in their historical magmatic eruptive characteristics to cast strict categorical assignment in doubt; one geological study of de Perote went so far as to suggest the term "compound shield-like volcano" instead. Most mature shield volcanoes have multiple cinder cones on their flanks, the results of tephra ejections common during incessant activity and markers of currently and formerly active sites on the volcano. An example of these parasitic cones is at Puʻu ʻŌʻō on Kīlauea—continuous activity ongoing since 1983 has built up a tall cone at the site of one of the longest-lasting rift eruptions in known history.
The Hawaiian shield volcanoes are not located near any plate boundaries; the volcanic activity of this island chain is distributed by the movement of the oceanic plate over an upwelling of magma known as a hotspot. Over millions of years, the tectonic movement that moves continents also creates long volcanic trails across the seafloor. The Hawaiian and Galápagos shields, and other hotspot shields like them, are constructed of oceanic island basalt. Their lavas are characterized by high levels of sodium, potassium, and aluminium. | Shield volcano | Wikipedia | 503 | 319168 | https://en.wikipedia.org/wiki/Shield%20volcano | Physical sciences | Volcanology | Earth science |
Features common in shield volcanism include lava tubes. Lava tubes are cave-like volcanic straights formed by the hardening of overlaying lava. These structures help further the propagation of lava, as the walls of the tube insulate the lava within. Lava tubes can account for a large portion of shield volcano activity; for example, an estimated 58% of the lava forming Kīlauea comes from lava tubes.
In some shield volcano eruptions, basaltic lava pours out of a long fissure instead of a central vent, and shrouds the countryside with a long band of volcanic material in the form of a broad plateau. Plateaus of this type exist in Iceland, Washington, Oregon, and Idaho; the most prominent ones are situated along the Snake River in Idaho and the Columbia River in Washington and Oregon, where they have been measured to be over in thickness.
Calderas are a common feature on shield volcanoes. They are formed and reformed over the volcano's lifespan. Long eruptive periods form cinder cones, which then collapse over time to form calderas. The calderas are often filled up by progressive eruptions, or formed elsewhere, and this cycle of collapse and regeneration takes place throughout the volcano's lifespan.
Interactions between water and lava at shield volcanoes can cause some eruptions to become hydrovolcanic. These explosive eruptions are drastically different from the usual shield volcanic activity and are especially prevalent at the waterbound volcanoes of the Hawaiian Isles.
Distribution
Shield volcanoes are found worldwide. They can form over hotspots (points where magma from below the surface wells up), such as the Hawaiian–Emperor seamount chain and the Galápagos Islands, or over more conventional rift zones, such as the Icelandic shields and the shield volcanoes of East Africa. Although shield volcanoes are not usually associated with subduction, they can occur over subduction zones. Many examples are found in California and Oregon, including Prospect Peak in Lassen Volcanic National Park, as well as Pelican Butte and Belknap Crater in Oregon. Many shield volcanoes are found in ocean basins, such as Kīlauea in Hawaii, although they can be found inland as well—East Africa being one example of this. | Shield volcano | Wikipedia | 444 | 319168 | https://en.wikipedia.org/wiki/Shield%20volcano | Physical sciences | Volcanology | Earth science |
Hawaiian–Emperor seamount chain
The largest and most prominent shield volcano chain in the world is the Hawaiian–Emperor seamount chain, a chain of hotspot volcanoes in the Pacific Ocean. The volcanoes follow a distinct evolutionary pattern of growth and death. The chain contains at least 43 major volcanoes, and Meiji Seamount at its terminus near the Kuril–Kamchatka Trench is 85 million years old.
The youngest part of the chain is Hawaii, where the volcanoes are characterized by frequent rift eruptions, their large size (thousands of km3 in volume), and their rough, decentralized shape. Rift zones are a prominent feature on these volcanoes and account for their seemingly random volcanic structure. They are fueled by the movement of the Pacific Plate over the Hawaii hotspot and form a long chain of volcanoes, atolls, and seamounts long with a total volume of over .
The chain includes Mauna Loa, a shield volcano which stands above sea level and reaches a further below the waterline and into the crust, approximately of rock. Kīlauea, another Hawaiian shield volcano, is one of the most active volcanoes on Earth, with its most recent eruption occurring in 2021.
Galápagos Islands
The Galápagos Islands are an isolated set of volcanoes, consisting of shield volcanoes and lava plateaus, about west of Ecuador. They are driven by the Galápagos hotspot, and are between approximately 4.2 million and 700,000 years of age. The largest island, Isabela, consists of six coalesced shield volcanoes, each delineated by a large summit caldera. Española, the oldest island, and Fernandina, the youngest, are also shield volcanoes, as are most of the other islands in the chain. The Galápagos Islands are perched on a large lava plateau known as the Galápagos Platform. This platform creates a shallow water depth of at the base of the islands, which stretch over a diameter. Since Charles Darwin's visit to the islands in 1835 during the second voyage of HMS Beagle, there have been over 60 recorded eruptions in the islands, from six different shield volcanoes. Of the 21 emergent volcanoes, 13 are considered active. | Shield volcano | Wikipedia | 447 | 319168 | https://en.wikipedia.org/wiki/Shield%20volcano | Physical sciences | Volcanology | Earth science |
Cerro Azul is a shield volcano on the southwestern part of Isabela Island and is one of the most active in the Galapagos, with the last eruption between May and June 2008. The Geophysics Institute at the National Polytechnic School in Quito houses an international team of seismologists and volcanologists whose responsibility is to monitor Ecuador's numerous active volcanoes in the Andean Volcanic Belt and the Galapagos Islands. La Cumbre is an active shield volcano on Fernandina Island that has been erupting since April 11, 2009.
The Galápagos islands are geologically young for such a big chain, and the pattern of their rift zones follows one of two trends, one north-northwest, and one east–west. The composition of the lavas of the Galápagos shields are strikingly similar to those of the Hawaiian volcanoes. Curiously, they do not form the same volcanic "line" associated with most hotspots. They are not alone in this regard; the Cobb–Eickelberg Seamount chain in the North Pacific is another example of such a delineated chain. In addition, there is no clear pattern of age between the volcanoes, suggesting a complicated, irregular pattern of creation. How the islands were formed remains a geological mystery, although several theories have been proposed.
Iceland
Located over the Mid-Atlantic Ridge, a divergent tectonic plate boundary in the middle of the Atlantic Ocean, Iceland is the site of about 130 volcanoes of various types. Icelandic shield volcanoes are generally of Holocene age, between 5,000 and 10,000 years old. The volcanoes are also very narrow in distribution, occurring in two bands in the West and North Volcanic Zones. Like Hawaiian volcanoes, their formation initially begins with several eruptive centers before centralizing and concentrating at a single point. The main shield then forms, burying the smaller ones formed by the early eruptions with its lava.
Icelandic shields are mostly small (~), symmetrical (although this can be affected by surface topography), and characterized by eruptions from summit calderas. They are composed of either tholeiitic olivine or picritic basalt. The tholeiitic shields tend to be wider and shallower than the picritic shields. They do not follow the pattern of caldera growth and destruction that other shield volcanoes do; caldera may form, but they generally do not disappear.
Turkey
Bingöl Mountains are one of the shield volcanoes in Turkey.
East Africa | Shield volcano | Wikipedia | 498 | 319168 | https://en.wikipedia.org/wiki/Shield%20volcano | Physical sciences | Volcanology | Earth science |
In East Africa, volcanic activity is generated by the development of the East African Rift and from nearby hotspots. Some volcanoes interact with both. Shield volcanoes are found near the rift and off the coast of Africa, although stratovolcanoes are more common. Although sparsely studied, the fact that all of its volcanoes are of Holocene age reflects how young the volcanic center is. One interesting characteristic of East African volcanism is a penchant for the formation of lava lakes; these semi-permanent lava bodies, extremely rare elsewhere, form in about 9% of African eruptions.
The most active shield volcano in Africa is Nyamuragira. Eruptions at the shield volcano are generally centered within the large summit caldera or on the numerous fissures and cinder cones on the volcano's flanks. Lava flows from the most recent century extend down the flanks more than from the summit, reaching as far as Lake Kivu. Erta Ale in Ethiopia is another active shield volcano and one of the few places in the world with a permanent lava lake, which has been active since at least 1967, and possibly since 1906. Other volcanic centers include Menengai, a massive shield caldera, and Mount Marsabit in Kenya.
Extraterrestrial shield volcanoes
Shield volcanoes are not limited to Earth; they have been found on Mars, Venus, and Jupiter's moon, Io.
The shield volcanoes of Mars are very similar to the shield volcanoes on Earth. On both planets, they have gently sloping flanks, collapse craters along their central structure, and are built of highly fluid lavas. Volcanic features on Mars were observed long before they were first studied in detail during the 1976–1979 Viking mission. The principal difference between the volcanoes of Mars and those on Earth is in terms of size; Martian volcanoes range in size up to high and in diameter, far larger than the high, wide Hawaiian shields. The highest of these, Olympus Mons, is the tallest known mountain on any planet in the solar system.
Venus has over 150 shield volcanoes which are much flatter, with a larger surface area than those found on Earth, some having a diameter of more than . Although the majority of these are long extinct it has been suggested, from observations by the Venus Express spacecraft, that many may still be active. | Shield volcano | Wikipedia | 465 | 319168 | https://en.wikipedia.org/wiki/Shield%20volcano | Physical sciences | Volcanology | Earth science |
Microfilaments, also called actin filaments, are protein filaments in the cytoplasm of eukaryotic cells that form part of the cytoskeleton. They are primarily composed of polymers of actin, but are modified by and interact with numerous other proteins in the cell. Microfilaments are usually about 7 nm in diameter and made up of two strands of actin. Microfilament functions include cytokinesis, amoeboid movement, cell motility, changes in cell shape, endocytosis and exocytosis, cell contractility, and mechanical stability. Microfilaments are flexible and relatively strong, resisting buckling by multi-piconewton compressive forces and filament fracture by nanonewton tensile forces. In inducing cell motility, one end of the actin filament elongates while the other end contracts, presumably by myosin II molecular motors. Additionally, they function as part of actomyosin-driven contractile molecular motors, wherein the thin filaments serve as tensile platforms for myosin's ATP-dependent pulling action in muscle contraction and pseudopod advancement. Microfilaments have a tough, flexible framework which helps the cell in movement.
Actin was first discovered in rabbit skeletal muscle in the mid 1940s by F.B. Straub. Almost 20 years later, H.E. Huxley demonstrated that actin is essential for muscle constriction. The mechanism in which actin creates long filaments was first described in the mid 1980s. Later studies showed that actin has an important role in cell shape, motility, and cytokinesis.
Organization
Actin filaments are assembled in two general types of structures: bundles and networks. Bundles can be composed of polar filament arrays, in which all barbed ends point to the same end of the bundle, or non-polar arrays, where the barbed ends point towards both ends. A class of actin-binding proteins, called cross-linking proteins, dictate the formation of these structures. Cross-linking proteins determine filament orientation and spacing in the bundles and networks. These structures are regulated by many other classes of actin-binding proteins, including motor proteins, branching proteins, severing proteins, polymerization promoters, and capping proteins. | Microfilament | Wikipedia | 490 | 319342 | https://en.wikipedia.org/wiki/Microfilament | Biology and health sciences | Cell parts | Biology |
In vitro self-assembly
Measuring approximately 6 nm in diameter, microfilaments are the thinnest fibers of the cytoskeleton. They are polymers of actin subunits (globular actin, or G-actin), which as part of the fiber are referred to as filamentous actin, or F-actin. Each microfilament is made up of two helical, interlaced strands of subunits. Much like microtubules, actin filaments are polarized. Electron micrographs have provided evidence of their fast-growing barbed-ends and their slow-growing pointed-end. This polarity has been determined by the pattern created by the binding of myosin S1 fragments: they themselves are subunits of the larger myosin II protein complex. The pointed end is commonly referred to as the minus (−) end and the barbed end is referred to as the plus (+) end.
In vitro actin polymerization, or nucleation, starts with the self-association of three G-actin monomers to form a trimer. ATP-bound actin then itself binds the barbed end, and the ATP is subsequently hydrolyzed. ATP hydrolysis occurs with a half time of about 2 seconds, while the half time for the dissociation of the inorganic phosphate is about 6 minutes. This autocatalyzed event reduces the binding strength between neighboring subunits, and thus generally destabilizes the filament. In vivo actin polymerization is catalyzed by a class of filament end-tracking molecular motors known as actoclampins. Recent evidence suggests that the rate of ATP hydrolysis and the rate of monomer incorporation are strongly coupled. | Microfilament | Wikipedia | 361 | 319342 | https://en.wikipedia.org/wiki/Microfilament | Biology and health sciences | Cell parts | Biology |
Subsequently, ADP-actin dissociates slowly from the pointed end, a process significantly accelerated by the actin-binding protein, cofilin. ADP bound cofilin severs ADP-rich regions nearest the (−)-ends. Upon release, the free actin monomer slowly dissociates from ADP, which in turn rapidly binds to the free ATP diffusing in the cytosol, thereby forming the ATP-actin monomeric units needed for further barbed-end filament elongation. This rapid turnover is important for the cell's movement. End-capping proteins such as CapZ prevent the addition or loss of monomers at the filament end where actin turnover is unfavorable, such as in the muscle apparatus.
Actin polymerization together with capping proteins were recently used to control the 3-dimensional growth of protein filament so as to perform 3D topologies useful in technology and the making of electrical interconnect. Electrical conductivity is obtained by metallisation of the protein 3D structure.
Mechanism of force generation
As a result of ATP hydrolysis, filaments elongate approximately 10 times faster at their barbed ends than their pointed ends. At steady-state, the polymerization rate at the barbed end matches the depolymerization rate at the pointed end, and microfilaments are said to be treadmilling. Treadmilling results in elongation in the barbed end and shortening in the pointed-end, so that the filament in total moves. Since both processes are energetically favorable, this means force is generated, the energy ultimately coming from ATP.
Actin in cells
Intracellular actin cytoskeletal assembly and disassembly are tightly regulated by cell signaling mechanisms. Many signal transduction systems use the actin cytoskeleton as a scaffold, holding them at or near the inner face of the peripheral membrane. This subcellular location allows immediate responsiveness to transmembrane receptor action and the resulting cascade of signal-processing enzymes. | Microfilament | Wikipedia | 431 | 319342 | https://en.wikipedia.org/wiki/Microfilament | Biology and health sciences | Cell parts | Biology |
Because actin monomers must be recycled to sustain high rates of actin-based motility during chemotaxis, cell signalling is believed to activate cofilin, the actin-filament depolymerizing protein which binds to ADP-rich actin subunits nearest the filament's pointed-end and promotes filament fragmentation, with concomitant depolymerization in order to liberate actin monomers. In most animal cells, monomeric actin is bound to profilin and thymosin beta-4, both of which preferentially bind with one-to-one stoichiometry to ATP-containing monomers. Although thymosin beta-4 is strictly a monomer-sequestering protein, the behavior of profilin is far more complex. Profilin enhances the ability of monomers to assemble by stimulating the exchange of actin-bound ADP for solution-phase ATP to yield actin-ATP and ADP. Profilin is transferred to the leading edge by virtue of its PIP2 binding site, and it employs its poly-L-proline binding site to dock onto end-tracking proteins. Once bound, profilin-actin-ATP is loaded into the monomer-insertion site of actoclampin motors.
Another important component in filament formation is the Arp2/3 complex, which binds to the side of an already existing filament (or "mother filament"), where it nucleates the formation of a new daughter filament at a 70-degree angle relative to the mother filament, effecting a fan-like branched filament network. | Microfilament | Wikipedia | 355 | 319342 | https://en.wikipedia.org/wiki/Microfilament | Biology and health sciences | Cell parts | Biology |
Specialized unique actin cytoskeletal structures are found adjacent to the plasma membrane. Four remarkable examples include red blood cells, human embryonic kidney cells, neurons, and sperm cells. In red blood cells, a spectrin-actin hexagonal lattice is formed by interconnected short actin filaments. In human embryonic kidney cells, the cortical actin forms a scale-free fractal structure. First found in neuronal axons, actin forms periodic rings that are stabilized by spectrin and adducin and this ring structure was then found by He et al 2016 to occur in almost every neuronal type and glial cells, across seemingly every animal taxon including Caenorhabditis elegans, Drosophila, Gallus gallus and Mus musculus. And in mammalian sperm, actin forms a helical structure in the midpiece, i.e., the first segment of the flagellum.
Associated proteins
In non-muscle cells, actin filaments are formed proximal to membrane surfaces. Their formation and turnover are regulated by many proteins, including:
Filament end-tracking protein (e.g., formins, VASP, N-WASP)
Filament-nucleator known as the Actin-Related Protein-2/3 (or Arp2/3) complex
Filament cross-linkers (e.g., α-actinin, fascin, and fimbrin)
Actin monomer-binding proteins profilin and thymosin β4
Filament barbed-end cappers such as Capping Protein and CapG, etc.
Filament-severing proteins like gelsolin.
Actin depolymerizing proteins such as ADF/cofilin. | Microfilament | Wikipedia | 382 | 319342 | https://en.wikipedia.org/wiki/Microfilament | Biology and health sciences | Cell parts | Biology |
The actin filament network in non-muscle cells is highly dynamic. The actin filament network is arranged with the barbed-end of each filament attached to the cell's peripheral membrane by means of clamped-filament elongation motors, the above-mentioned "actoclampins", formed from a filament barbed-end and a clamping protein (formins, VASP, Mena, WASP, and N-WASP). The primary substrate for these elongation motors is profilin-actin-ATP complex which is directly transferred to elongating filament ends. The pointed-end of each filament is oriented toward the cell's interior. In the case of lamellipodial growth, the Arp2/3 complex generates a branched network, and in filopodia a parallel array of filaments is formed.
Actin acts as a track for myosin motor motility
Myosin motors are intracellular ATP-dependent enzymes that bind to and move along actin filaments. Various classes of myosin motors have very different behaviors, including exerting tension in the cell and transporting cargo vesicles.
A proposed model – actoclampins track filament ends
One proposed model suggests the existence of actin filament barbed-end-tracking molecular motors termed "actoclampin". The proposed actoclampins generate the propulsive forces needed for actin-based motility of lamellipodia, filopodia, invadipodia, dendritic spines, intracellular vesicles, and motile processes in endocytosis, exocytosis, podosome formation, and phagocytosis. Actoclampin motors also propel such intracellular pathogens as Listeria monocytogenes, Shigella flexneri, Vaccinia and Rickettsia. When assembled under suitable conditions, these end-tracking molecular motors can also propel biomimetic particles.
The term actoclampin is derived from acto- to indicate the involvement of an actin filament, as in actomyosin, and clamp to indicate a clasping device used for strengthening flexible/moving objects and for securely fastening two or more components, followed by the suffix -in to indicate its protein origin. An actin filament end-tracking protein may thus be termed a clampin. | Microfilament | Wikipedia | 511 | 319342 | https://en.wikipedia.org/wiki/Microfilament | Biology and health sciences | Cell parts | Biology |
Dickinson and Purich recognized that prompt ATP hydrolysis could explain the forces achieved during actin-based motility. They proposed a simple mechanoenzymatic sequence known as the Lock, Load & Fire Model, in which an end-tracking protein remains tightly bound ("locked" or clamped) onto the end of one sub-filament of the double-stranded actin filament. After binding to Glycyl-Prolyl-Prolyl-Prolyl-Prolyl-Prolyl-registers on tracker proteins, Profilin-ATP-actin is delivered ("loaded") to the unclamped end of the other sub-filament, whereupon ATP within the already clamped terminal subunit of the other subfragment is hydrolyzed ("fired"), providing the energy needed to release that arm of the end-tracker, which then can bind another Profilin-ATP-actin to begin a new monomer-addition round.
Steps involved
The following steps describe one force-generating cycle of an actoclampin molecular motor:
The polymerization cofactor profilin and the ATP·actin combine to form a profilin-ATP-actin complex that then binds to the end-tracking unit
The cofactor and monomer are transferred to the barbed-end of an actin already clamped filament
The tracking unit and cofactor dissociate from the adjacent protofilament, in a step that can be facilitated by ATP hydrolysis energy to modulate the affinity of the cofactor and/or the tracking unit for the filament; and this mechanoenzymatic cycle is then repeated, starting this time on the other sub-filament growth site.
When operating with the benefit of ATP hydrolysis, AC motors generate per-filament forces of 8–9 pN, which is far greater than the per-filament limit of 1–2 pN for motors operating without ATP hydrolysis. The term actoclampin is generic and applies to all actin filament end-tracking molecular motors, irrespective of whether they are driven actively by an ATP-activated mechanism or passively. | Microfilament | Wikipedia | 463 | 319342 | https://en.wikipedia.org/wiki/Microfilament | Biology and health sciences | Cell parts | Biology |
Some actoclampins (e.g., those involving Ena/VASP proteins, WASP, and N-WASP) apparently require Arp2/3-mediated filament initiation to form the actin polymerization nucleus that is then "loaded" onto the end-tracker before processive motility can commence. To generate a new filament, Arp2/3 requires a "mother" filament, monomeric ATP-actin, and an activating domain from Listeria ActA or the VCA region of N-WASP. The Arp2/3 complex binds to the side of the mother filament, forming a Y-shaped branch having a 70-degree angle with respect to the longitudinal axis of the mother filament. Then upon activation by ActA or VCA, the Arp complex is believed to undergo a major conformational change, bringing its two actin-related protein subunits near enough to each other to generate a new filament gate. Whether ATP hydrolysis may be required for nucleation and/or Y-branch release is a matter under active investigation. | Microfilament | Wikipedia | 233 | 319342 | https://en.wikipedia.org/wiki/Microfilament | Biology and health sciences | Cell parts | Biology |
The cattle egret (formerly genus Bubulcus) is a cosmopolitan clade of heron (family Ardeidae) in the genus Ardea found in the tropics, subtropics, and warm-temperate zones. According to the IOC bird list, it contains two species, the western cattle egret and the eastern cattle egret, although some authorities regard them as a single species. Despite the similarities in plumage to the egrets of the genus Egretta, it actually belongs to the genus Ardea. Originally native to parts of Asia, Africa, and Europe, it has undergone a rapid expansion in its distribution and successfully colonised much of the rest of the world in the last century.
They are white birds adorned with buff plumes in the breeding season. They nest in colonies, usually near bodies of water and often with other wading birds. The nest is a platform of sticks in trees or shrubs. Cattle egrets exploit drier and open habitats more than other heron species. Their feeding habitats include seasonally inundated grasslands, pastures, farmlands, wetlands, and rice paddies. They often accompany cattle or other large mammals, catching insect and small vertebrate prey disturbed by these animals. Some populations are migratory and others show postbreeding dispersal.
The adult cattle egret has few predators, but birds or mammals may raid its nests, and chicks may be lost to starvation, calcium deficiency, or disturbance from other large birds. Cattle egrets maintain a special relationship with cattle, which extends to other large grazing mammals; wider human farming is believed to be a major cause of their suddenly expanded range. The cattle egret removes ticks and flies from cattle and consumes them. This benefits both organisms, but it has been implicated in the spread of tick-borne animal diseases. | Cattle egret | Wikipedia | 375 | 319522 | https://en.wikipedia.org/wiki/Cattle%20egret | Biology and health sciences | Pelecanimorphae | Animals |
Taxonomy
Before the description of the Bubulcus by Charles Lucien Bonaparte in 1855, the western cattle egret had already been described in 1758 by Carl Linnaeus in his Systema Naturae as Ardea ibis, and the eastern cattle egret had been described in 1783 by Pieter Boddaert as Cancroma coromanda. Their generic name Bubulcus is Latin for herdsman, referring, like the English name, to their association with cattle. Ibis is a Latin and Greek word which originally referred to another white wading bird, the sacred ibis, but was applied to the western cattle egret in error. The epithet coromanda refers to the Coromandel Coast of India.
The eastern and western cattle egrets were split by McAllan and Bruce, but were regarded as conspecific by almost all other recent authors until the publication of the influential Birds of South Asia. The eastern cattle egret breeds in South Asia, Eastern Asia, and Australasia, and the western species occupies the rest of the cattle egret's range, including Western Asia, Europe, Africa, and the Americas. According to the IOC birdlist, they are both monotypic species. While some authorities recognise a third Seychelles subspecies, the Seychelles cattle egret (A. i. seychellarum), which was first described by Finn Salomonsen in 1934.
Despite superficial similarities in appearance, the cattle egret is more closely related to the other members of the genus Ardea, which comprises the great or typical herons and the great egret (A. alba), than to the majority of species termed egrets in the genus Egretta. Rare cases of hybridization with little blue herons (Egretta caerulea), little egrets (E. garzetta), and snowy egrets (E. thula) have been recorded.
An older English name for the cattle egret is buff-backed heron.
Description | Cattle egret | Wikipedia | 417 | 319522 | https://en.wikipedia.org/wiki/Cattle%20egret | Biology and health sciences | Pelecanimorphae | Animals |
The cattle egret is a stocky heron with an wingspan; it is long and weighs . It has a relatively short, thick neck, a sturdy bill, and a hunched posture. The nonbreeding adult has mainly white plumage, a yellow bill, and greyish-yellow legs. During the breeding season, adults of the western cattle egret develop orange-buff plumes on the back, breast, and crown, and the bill, legs, and irises become bright red for a brief period prior to pairing. The sexes are similar, but the male is marginally larger and has slightly longer breeding plumes than the female; juvenile birds lack coloured plumes and have a black bill.
The eastern differs from the western in breeding plumage, when the buff colour on its head extends to the cheeks and throat, and the plumes are more golden in colour. This species' bill and tarsi are longer on average than in A. ibis. A. i. seychellarum, which may or may not be a valid subspecies, is smaller and shorter-winged than the other forms. It has white cheeks and throat, like A. ibis, but the nuptial plumes are golden, as with A. coromanda. Individuals with abnormally grey, melanistic plumages have been recorded.
The positioning of the egret's eyes allows for binocular vision during feeding, and physiological studies suggest that they may be capable of crepuscular or nocturnal activity. Adapted to foraging on land, they have lost the ability possessed by their wetland relatives to accurately correct for light refraction by water.
Distribution and habitat
The western cattle egret has undergone one of the most rapid and wide-reaching natural expansions of any bird species. It was originally native to parts of southern Spain and Portugal, tropical and subtropical Africa, and humid tropical and subtropical Asia. At the end of the 19th century, it began expanding its range into southern Africa, first breeding in the Cape Province in 1908. Cattle egrets were first sighted in the Americas on the boundary of Guiana and Suriname in 1877, having apparently flown across the Atlantic Ocean. In the 1930s, the species is thought to have become established in that area. It is now widely distributed across Brazil and was first discovered in the northern region of the country in 1964, feeding along with buffalos. | Cattle egret | Wikipedia | 481 | 319522 | https://en.wikipedia.org/wiki/Cattle%20egret | Biology and health sciences | Pelecanimorphae | Animals |
The species first arrived in North America in 1941 (these early sightings were originally dismissed as escapees), bred in Florida in 1953, and spread rapidly, breeding for the first time in Canada in 1962. It is now commonly seen as far west as California. It was first recorded breeding in Cuba in 1957, in Costa Rica in 1958, and in Mexico in 1963, although it was probably established before then. In Europe, the species had historically declined in Spain and Portugal, but in the latter part of the 20th century, it expanded back through the Iberian Peninsula, and then began to colonise other parts of Europe, southern France in 1958, northern France in 1981, and Italy in 1985. Breeding in the United Kingdom was recorded for the first time in 2008, only a year after an influx seen in the previous year. In 2008, cattle egrets were also reported as having moved into Ireland for the first time. This trend has continued and cattle egrets have become more numerous in southern Britain with influxes in some numbers during the nonbreeding seasons of 2007/08 and 2016/17. They bred in Britain again in 2017, following an influx in the previous winter, and may become established there.
In Australia, the colonisation began in the 1940s, with the eastern cattle egret establishing itself in the north and east of the continent. It began to regularly visit New Zealand in the 1960s. Since 1948, the cattle egret has been permanently resident in Israel. Prior to 1948, it was only a winter visitor.
The massive and rapid expansion of the cattle egret's range is due to its relationship with humans and their domesticated animals. Originally adapted to a commensal relationship with large grazing and browsing animals, it was easily able to switch to domesticated cattle and horses. As the keeping of livestock spread throughout the world, the cattle egret was able to occupy otherwise empty niches. Many populations of cattle egrets are highly migratory and dispersive, and this has helped the genus' range expansion. The cattle egret has been seen as a vagrant in various sub-Antarctic islands, including South Georgia, Marion Island, the South Sandwich Islands, and the South Orkney Islands. A small flock of eight birds was also seen in Fiji in 2008. | Cattle egret | Wikipedia | 468 | 319522 | https://en.wikipedia.org/wiki/Cattle%20egret | Biology and health sciences | Pelecanimorphae | Animals |
In addition to the natural expansion of its range, cattle egrets have been deliberately introduced into a few areas. The western cattle egret was introduced to Hawaii in 1959, and to the Chagos Archipelago in 1955. Successful releases were also made in the Seychelles and Rodrigues, but attempts to introduce them to Mauritius failed. Numerous birds were also released by Whipsnade Zoo in England, but they were never established.
Although the cattle egret sometimes feeds in shallow water, unlike most herons, it is typically found in fields and dry grassy habitats, reflecting its greater dietary reliance on terrestrial insects rather than aquatic prey.
Migration and movements
Some populations of cattle egrets are migratory, others are dispersive, and distinguishing between the two can be difficult. In many areas, populations can be both sedentary and migratory. In the Northern Hemisphere, migration is from cooler climes to warmer areas, but cattle egrets nesting in Australia migrate to cooler Tasmania and New Zealand in the winter and return in the spring. Migration in western Africa is in response to rainfall, and in South America, migrating birds travel south of their breeding range in the nonbreeding season. Populations in southern India appear to show local migrations in response to the monsoons. They move north from Kerala after September. During winter, many birds have been seen flying at night with flocks of Indian pond herons (Ardeola grayii) on the south-eastern coast of India and a winter influx has also been noted in Sri Lanka.
Young birds are known to disperse up to from their breeding area. Flocks may fly vast distances and have been seen over seas and oceans including in the middle of the Atlantic.
Ecology and behavior
Voice
The cattle egret gives a quiet, throaty rick-rack call at the breeding colony, but is otherwise largely silent. | Cattle egret | Wikipedia | 375 | 319522 | https://en.wikipedia.org/wiki/Cattle%20egret | Biology and health sciences | Pelecanimorphae | Animals |
Breeding
The cattle egret nests in colonies, which are often found around bodies of water. The colonies are usually found in woodlands near lakes or rivers, in swamps, or on small inland or coastal islands, and are sometimes shared with other wetland birds, such as herons, egrets, ibises, and cormorants. The breeding season varies within South Asia. Nesting in northern India begins with the onset of monsoons in May. The breeding season in Australia is November to early January, with one brood laid per season. The North American breeding season lasts from April to October. In the Seychelles, the breeding season of B. i. seychellarum is April to October.
The male displays in a tree in the colony, using a range of ritualised behaviours, such as shaking a twig and sky-pointing (raising his bill vertically upwards), and the pair forms over 3–4 days. A new mate is chosen in each season and when renesting following nest failure. The nest is a small, untidy platform of sticks in a tree or shrub constructed by both parents. Sticks are collected by the male and arranged by the female, and stick-stealing is rife. The clutch size can be one to five eggs, although three or four is most common. The pale bluish-white eggs are oval-shaped and measure . Incubation lasts around 23 days, with both sexes sharing incubation duties. The chicks are partly covered with down at hatching, but are not capable of fending for themselves; they become capable of regulating their temperature at 9–12 days and are fully feathered in 13–21 days. They begin to leave the nest and climb around at 2 weeks, fledge at 30 days and become independent at around the 45th day.
The cattle egret engages in low levels of brood parasitism, and a few instances have been reported of cattle egret eggs being laid in the nests of snowy egrets and little blue herons, although these eggs seldom hatch. Also, evidence of low levels of intraspecific brood parasitism has been found, with females laying eggs in the nests of other cattle egrets. As much as 30% extra-pair copulations has been noted. | Cattle egret | Wikipedia | 465 | 319522 | https://en.wikipedia.org/wiki/Cattle%20egret | Biology and health sciences | Pelecanimorphae | Animals |
The dominant factor in nesting mortality is starvation. Sibling rivalry can be intense, and in South Africa, third and fourth chicks inevitably starve. In the dryer habitats with fewer amphibians, the diet may lack sufficient vertebrate content and may cause bone abnormalities in growing chicks due to calcium deficiency. In Barbados, nests were sometimes raided by vervet monkeys, and a study in Florida reported the fish crow and black rat as other possible nest raiders. The same study attributed some nestling mortality to brown pelicans nesting in the vicinity, which accidentally, but frequently, dislodged nests or caused nestlings to fall. In Australia, Torresian crows, wedge-tailed eagles, and white-bellied sea eagles take eggs or young, and tick infestation and viral infections may also be causes of mortality.
Feeding
The cattle egret feeds on a wide range of prey, particularly insects, especially grasshoppers, crickets, flies (adults and maggots), beetles, and moths, as well as spiders, frogs, fish, crayfish, small snakes, lizards and earthworms. In a rare instance, they have been observed foraging along the branches of a banyan tree for ripe figs. The cattle egret is usually found with cattle and other large grazing and browsing animals, and catches small creatures disturbed by the mammals. Studies have shown that cattle egret foraging success is much higher when foraging near a large animal than when feeding singly. When foraging with cattle, it has been shown to be 3.6 times more successful in capturing prey than when foraging alone. Its performance is similar when it follows farm machinery, but it is forced to move more. In urban situations, cattle egrets have also been observed foraging in peculiar situations such as railway lines.
A cattle egret will weakly defend the area around a grazing animal against others of the same species, but if the area is swamped by egrets, it will give up and continue foraging elsewhere. Where numerous large animals are present, cattle egrets selectively forage around species that move at around 5–15 steps per minute, avoiding faster and slower moving herds; in Africa, cattle egrets selectively forage behind plains zebras, waterbuck, blue wildebeest and Cape buffalo. Dominant birds feed nearest to the host, and thus obtain more food. | Cattle egret | Wikipedia | 485 | 319522 | https://en.wikipedia.org/wiki/Cattle%20egret | Biology and health sciences | Pelecanimorphae | Animals |
The cattle egret sometimes shows versatility in its diet. On islands with seabird colonies, it will prey on the eggs and chicks of terns and other seabirds. During migration, it has also been reported to eat exhausted migrating landbirds. Birds of the Seychelles race also indulge in some kleptoparasitism, chasing the chicks of sooty terns and forcing them to disgorge food.
Threats
Pairs of crested caracaras have been observed chasing cattle egrets in flight, forcing them to the ground, and killing them.
Status
The IUCN Red List treats them as a single species. They have a large range, with an estimated global extent of occurrence of . Their global population is estimated to be 3.8–6.7 million individuals. For these reasons, the genus is evaluated as least concern. The expansion and establishment of the genus over large ranges has led it to be classed as an invasive species, although little, if any, impact has been noted yet.
Relationship with humans
As a conspicuous genus, the cattle egret has attracted many common names. These mostly relate to its habit of following cattle and other large animals, and it is known variously as cow crane, cow bird or cow heron, or even elephant bird or rhinoceros egret. Its Arabic name, abu qerdan, means "father of ticks", a name derived from the huge number of parasites such as avian ticks found in its breeding colonies. The Maasai people consider the presence of large numbers of cattle egrets as an indicator of impending drought and use it to decide on moving their cattle herds.
Cattle egrets are an occurring traditional motif in fishing boats among fishermen of the Malay Peninsula east coast who believed them as a symbol of good luck and fortune.
The cattle egret is a popular bird with cattle ranchers for its perceived role as a biocontrol of cattle parasites such as ticks and flies. A study in Australia found that cattle egrets reduced the number of flies that bothered cattle by pecking them directly off the skin. It was the benefit to stock that prompted ranchers and the Hawaiian Board of Agriculture and Forestry to release the western cattle egret in Hawaii. | Cattle egret | Wikipedia | 457 | 319522 | https://en.wikipedia.org/wiki/Cattle%20egret | Biology and health sciences | Pelecanimorphae | Animals |
Not all interactions between humans and cattle egrets are beneficial. The cattle egret can be a safety hazard to aircraft due to its habit of feeding in large groups in the grassy verges of airports, and it has been implicated in the spread of animal infections such as heartwater, infectious bursal disease, and possibly Newcastle disease. | Cattle egret | Wikipedia | 69 | 319522 | https://en.wikipedia.org/wiki/Cattle%20egret | Biology and health sciences | Pelecanimorphae | Animals |
An ocean liner is a type of passenger ship primarily used for transportation across seas or oceans. Ocean liners may also carry cargo or mail, and may sometimes be used for other purposes (such as for pleasure cruises or as hospital ships). The Queen Mary 2 is the only ocean liner still in service to this day, serving with Cunard Line.
The category does not include ferries or other vessels engaged in short-sea trading, nor dedicated cruise ships where the voyage itself, and not transportation, is the primary purpose of the trip. Nor does it include tramp steamers, even those equipped to handle limited numbers of passengers. Some shipping companies refer to themselves as "lines" and their passenger ships, which often operate over set routes according to established schedules, as "liners".
Though ocean liners share certain similarities with cruise ships, they must be able to travel between continents from point A to point B on a fixed schedule, so must be faster and built to withstand the rough seas and adverse conditions encountered on long voyages across the open ocean. To protect against large waves they usually have a higher hull and promenade deck with higher positioning of lifeboats (the height above water called the freeboard), as well as a longer bow than a cruise ship. Additionally, for additional strength they are often designed with thicker hull plating than is found on cruise ships, as well as a deeper draft for greater stability, and have large capacities for fuel, food, and other consumables on long voyages. On an ocean liner, the captain's tower (bridge) is usually positioned on the upper deck for increased visibility.
The first ocean liners were built in the mid-19th century. Technological innovations such as the steam engine, Diesel engine and steel hull allowed larger and faster liners to be built, giving rise to a competition between world powers of the time, especially between the United Kingdom, the German Empire, and to a lesser extent France. Once the dominant form of travel between continents, ocean liners were rendered largely obsolete by the emergence of long-distance aircraft after World War II. Advances in automobile and railway technology also played a role. After was retired in 2008, the only ship still in service as an ocean liner is , introduced in 2004, as well as the largest ever built.
Overview | Ocean liner | Wikipedia | 464 | 319558 | https://en.wikipedia.org/wiki/Ocean%20liner | Technology | Naval transport | null |
Ocean liners were the primary mode of intercontinental travel for over a century, from the mid-19th century until they began to be supplanted by airliners in the 1950s. In addition to passengers, liners carried mail and cargo. Ships contracted to carry British Royal Mail used the designation RMS. Liners were also the preferred way to move gold and other high-value cargoes.
The busiest route for liners was on the North Atlantic with ships travelling between Europe and North America. It was on this route that the fastest, largest and most advanced liners travelled, though most ocean liners historically were mid-sized vessels which served as the common carriers of passengers and freight between nations and among other countries and their colonies and dependencies before the dawn of the jet age. Such routes included Europe to African and Asian colonies, Europe to South America, and migrant traffic from Europe to North America in the 19th and first two decades of the 20th centuries, and to Canada and Australia after the Second World War.
Shipping lines are companies engaged in shipping passengers and cargo, often on established routes and schedules. Regular scheduled voyages on a set route are called "line voyages" and vessels (passenger or cargo) trading on these routes to a timetable are called liners. The alternative to liner trade is "tramping" whereby vessels are notified on an ad hoc basis as to the availability of a cargo to be transported. (In older usage, "liner" also referred to ships of the line, that is, line-of-battle ships, but that usage is now rare.) The term "ocean liner" has come to be used interchangeably with "passenger liner", although it can refer to a cargo liner or cargo-passenger liner.
The advent of the Jet Age and the decline in transoceanic ship service brought about a gradual transition from passenger ships to modern cruise ships as a means of transportation. In order for ocean liners to remain profitable, cruise lines modified some of them to operate on cruise routes, such as the . Certain characteristics of older ocean liners made them unsuitable for cruising, such as high fuel consumption, deep draught preventing them from entering shallow ports, and cabins (often windowless) designed to maximize passenger numbers rather than comfort. The Italian Line's and , the last ocean liners to be built primarily for crossing the North Atlantic, could not be converted economically and had short careers.
History
19th century | Ocean liner | Wikipedia | 489 | 319558 | https://en.wikipedia.org/wiki/Ocean%20liner | Technology | Naval transport | null |
At the beginning of the 19th century, the Industrial Revolution and the inter-continental trade made the development of secure links between continents imperative. Being at the top among the colonial powers, the United Kingdom needed stable maritime routes to connect different parts of its empire: the Far East, India, Australia, etc. The birth of the concept of international water and the lack of any claim to it simplified navigation during this period. In 1818, the Black Ball Line, with a fleet of sailing ships, offered the first regular passenger service with emphasis on passenger comfort, from England to the United States.
In 1807, Robert Fulton succeeded in applying steam engines to ships. He built the first ship that was powered by this technology, the Clermont, which succeeded in travelling between New York City and Albany, New York in thirty hours before entering into regular service between the two cities. Soon after, other ships were built using this innovation. In 1816, the became the first steamship to cross the English Channel. Another important advance came in 1819, when became the first steamship to cross the Atlantic Ocean. She left the U.S. city of the same name and arrived in Liverpool, England in 27 days. Most of the distance was covered by sailing; the steam power was not used for more than 72 hours during the travel. Public enthusiasm for the new technology was not high, as none of the thirty-two people who had booked a seat boarded the ship for that historic voyage. Although Savannah had proven that a steamship was capable of crossing the ocean, the public was not yet prepared to trust such means of travel on an open sea, and, in 1820, the steam engine was removed from the ship.
Work on this technology continued and a new step was taken in 1833. Royal William managed to cross the Atlantic by using steam power on most of the voyage; sail was used only when the boilers were cleaned. However, there were still many skeptics, and in 1836, scientific writer Dionysius Lardner declared that: As the project of making the voyage directly from New York to Liverpool, it was perfectly chimerical, and they might as well talk of making the voyage from New York to the moon. | Ocean liner | Wikipedia | 440 | 319558 | https://en.wikipedia.org/wiki/Ocean%20liner | Technology | Naval transport | null |
The last step toward long-distance travel using steam power was taken in 1837 when left Liverpool on 4 April and arrived in New York eighteen days later on 22 April after a turbulent crossing. Too little coal was prepared for the crossing, and the crew had to burn cabin furniture in order to complete the voyage. The journey took place at a speed of 8.03 knots. The voyage was made possible by the use of a condenser, which fed the boilers with fresh water and avoiding having to periodically shut down the boilers in order to remove the salt. This new record was short-lived. The next day, , designed by railway engineer Isambard Kingdom Brunel, arrived in New York. She left Liverpool on 8 April and overtook Siriuss record with an average speed of 8.66 knots. A race for speed was born, and, with it, the tradition of the Blue Riband.
With Great Western, Isambard Kingdom Brunel laid the foundations for new shipbuilding techniques. He realised that the carrying capacity of a ship increases as the cube of its dimensions, whilst the water resistance only increases as the square of its dimensions. This means that large ships are more fuel-efficient, something very important for long voyages across the Atlantic. Constructing large ships was therefore more profitable. Moreover, migration to the Americas increased enormously. These movements of population were a financial windfall for the shipping companies, of which some of the largest were founded during this period. Examples are the P&O of the United Kingdom in 1822 and the Compagnie Générale Transatlantique of France in 1855.
The steam engine also allowed ships to provide regular service without the use of sail. This aspect particularly appealed to the postal companies, which leased the services of ships to serve clients separated by the ocean. In 1839, Samuel Cunard founded the Cunard Line and became the first to dedicate the activity of his shipping company to the transport of mails, thus ensuring regular services on a given schedule. The company's ships operated the routes between the United Kingdom and the United States. Over time, the paddle wheel, impractical on the high seas, was abandoned in favour of the propeller. In 1840, Cunard Line's began its first regular passenger and cargo service by a steamship, sailing from Liverpool to Boston, Massachusetts. | Ocean liner | Wikipedia | 472 | 319558 | https://en.wikipedia.org/wiki/Ocean%20liner | Technology | Naval transport | null |
As the size of ships increased, the wooden hull became unreliable. The start of the use of iron hulls in 1845, and then of steel hulls, solved this problem. The first ship to be both iron-hulled and equipped with a screw propeller was , a creation of Brunel. Her career was disastrous and short. She was run aground and stranded at Dundrum Bay in 1846. In 1884, she was retired to the Falkland Islands where she was used as a warehouse, quarantine ship, and coal hulk until she was scuttled in 1937. The American company Collins Line took a different approach. It equipped its ships with cold rooms, heating systems, and various other innovations but the operation was expensive. The sinking of two of its ships was a major blow to the company which was dissolved in 1858.
In 1858, Brunel built his third and last giant, . The ship was, for 43 years, the largest passenger ship ever built. She had the capacity to carry 4,000 passengers. Her career was marked by a series of failures and incidents, one of which was an explosion on board during her maiden voyage.
Many ships owned by German companies such as Hamburg America Line and Norddeutscher Lloyd were sailing from major German ports, such as Hamburg and Bremen, to the United States during this time. The year 1858 was marked by a major accident: the sinking of . The ship, built in Greenock and sailing between Hamburg and New York twice a month, suffered an accidental fire off the coast of Newfoundland and sank with the loss of all but 89 of the 542 passengers.
In the British market, Cunard Line and White Star Line competed strongly against each other in the late 1860s. The struggle was symbolised by the attainment of the Blue Riband, which the two companies achieved several times around the end of the century. The luxury and technology of ships were also evolving. Auxiliary sails became obsolete and disappeared completely at the end of the century. Possible military use of passenger ships was envisaged and, in 1889, became the first auxiliary cruiser in history. In the time of war, ships could easily be equipped with cannons and used in cases of conflict. Teutonic succeeded in impressing Emperor Wilhelm II of Germany, who wanted to see his country endowed with a modern fleet. | Ocean liner | Wikipedia | 469 | 319558 | https://en.wikipedia.org/wiki/Ocean%20liner | Technology | Naval transport | null |
In 1870, the White Star Line's set a new standard for ocean travel by having its first-class cabins amidships, with the added amenity of large portholes, electricity and running water. The size of ocean liners increased from 1880 onward to meet the needs of migration to the United States and Australia.
and her sister ship were the last two Cunard liners of the period to be fitted with auxiliary sails. Both ships were built by John Elder & Co. of Glasgow, Scotland, in 1884. They were record breakers by the standards of the time, and were the largest liners then in service, serving the Liverpool to New York route.
was a 6,814-ton steamship owned by the Orient Steamship Co., and was fitted with refrigeration equipment. She served the Suez Canal route from England to Australia during the 1890s, up until the years leading to World War I when she was converted to an armed merchant cruiser.
In 1897, Norddeutscher Lloyd launched . She was followed three years later by three sister ships. The ship was both luxurious and fast, managing to win the Blue Riband from the British. She was also the first of the fourteen ocean liners with four funnels that have emerged in maritime history. The ship needed only two funnels, but more funnels gave passengers a feeling of safety. In 1900, the Hamburg America Line competed with its own four-funnel liner, . She quickly obtained the Blue Riband for her company. This race for speed, however, was a detriment to passengers' comfort and generated strong vibration, which made her owner lose any interest in her after she lost the Blue Riband to another ship of Norddeutscher Lloyd. She was only used for ten years for transatlantic crossing before being converted into a cruise ship. Until 1907, the Blue Riband remained in the hands of the Germans.
Early 20th century
In 1902, J. P. Morgan embraced the idea of a maritime empire comprising a large number of companies. He founded the International Mercantile Marine Co., a trust which originally comprised only American shipping companies. The trust then absorbed Leyland Line and White Star Line. The British government then decided to intervene in order to regain its ascendancy. | Ocean liner | Wikipedia | 455 | 319558 | https://en.wikipedia.org/wiki/Ocean%20liner | Technology | Naval transport | null |
Although German liners dominated in terms of speed, British liners dominated in terms of size. and the Big Four of the White Star Line were the first liners to surpass Great Eastern as the largest passenger ships. Ultimately their owner was American (as mentioned above, White Star Line had been absorbed into J. P. Morgan's trust). Faced with this major competition, the British government contributed financially to Cunard Line's construction of two liners of unmatched size and speed, under the condition that they be available for conversion into armed cruisers when needed by the navy. The result of this partnership was the completion in 1907 of two sister ships: and , both of which won the Blue Riband during their respective maiden voyages. The latter retained it for twenty years. Their great speed was achieved by the use of turbines instead of conventional expansion machines.
In response to the competition from Cunard Line, White Star Line ordered the liners at the end of 1907. The first of these three liners, , completed in 1911, had a fine career, although punctuated by incidents. This was not the case for her sister, the , which sank on her maiden voyage on 15 April 1912, resulting in several major changes to maritime safety practices. As for the third sister, , she never served her intended purpose as a passenger ship, as she was drafted in the First World War as a hospital ship, and sank to a naval mine in 1916.
At the same time, France tried to mark its presence with the completion in 1912 of owned by the Compagnie Générale Transatlantique. Germany soon responded to the competition from the British. From 1912 to 1914, Hamburg America Line completed a trio of liners significantly larger than the White Star Line's Olympic-class ships. The first to be completed, in 1913, was . She was followed by SS Vaterland in 1914. The construction of the third liner, , was paused by the outbreak of World War I.
World War I was a difficult time for the liners. Some of them, like the Mauretania, , and Britannic were transformed into hospital ships during the conflict. Others became troop transports, while some, such as the Kaiser Wilhelm der Grosse, participated in the war as warships. Troop transportation was very popular due to the liners' large size. Liners converted into troop ships were painted in dazzle camouflage to reduce the risk of being torpedoed by enemy submarines. | Ocean liner | Wikipedia | 501 | 319558 | https://en.wikipedia.org/wiki/Ocean%20liner | Technology | Naval transport | null |
The war caused the loss of many liners. Britannic, while serving as a hospital ship, sank in the Aegean Sea in 1916 after she struck a mine. Numerous incidents of torpedoing took place and large numbers of ships sank. Kaiser Wilhelm der Grosse was defeated and scuttled after a fierce battle with off the coast of west Africa, while her sister ship served as a commerce raider. The torpedoing and sinking of Lusitania on 7 May 1915 caused the loss of 128 American lives at a time when the United States was still neutral. Although other factors came into play, the loss of American lives in the sinking strongly pushed the United States to favour the Allied Powers and facilitated the country's entry into the war.
The losses of the liners owned by the Allied Powers were compensated by the Treaty of Versailles in 1919. This led to the awarding of many German liners to the victorious Allies. The Hamburg America Line's trio (, Vaterland, and Bismarck) were divided between the Cunard Line, White Star Line, and the United States Lines, while the three surviving ships of the Kaiser class were requisitioned by the US Navy in the context of the conflict and then retained. The Tirpitz, whose construction was delayed by the outbreak of war, eventually became the . Of the German superliners, only Deutschland, because of her poor state, avoided this fate.
After World War I
After a period of reconstruction, the shipping companies recovered quickly from the damage caused by World War I. The ships whose construction was started before the war, such as of the French Line, were completed and put into service. Prominent British liners, such as the Olympic and the Mauretania, were also put back into service and had a successful career in the early 1920s. More modern liners were also built, such as (completed in 1927). The United States Lines, having received the Vaterland, renamed her Leviathan and made her the flagship of the company's fleet. Because all U.S. registered ships counted as an extension of U.S. territory, the National Prohibition Act made American liners alcohol-free, causing alcohol-seeking passengers to choose ships of other countries for travel and substantially reduce profits for the United States Lines. | Ocean liner | Wikipedia | 462 | 319558 | https://en.wikipedia.org/wiki/Ocean%20liner | Technology | Naval transport | null |
In 1929, Germany returned to the scene with the two ships of Norddeutscher Lloyd, and . Bremen won the Blue Riband from Britain's Mauretania after the latter had held it for twenty years. Soon, Italy also entered the scene. The Italian Line completed and in 1932, breaking the records of both luxury and speed (Rex won the westbound Blue Riband in 1933). France reentered the scene with of the French Compagnie Générale Transatlantique (CGT). The ship was the largest ship afloat at the time of her completion in 1935. She was also the fastest, winning the Blue Riband in 1935.
A crisis arose when the United States drastically reduced its immigrant quotas, causing shipping companies to lose a large part of their income and to have to adapt to this circumstance. The Great Depression also played an important role, causing a drastic decrease in the number of people crossing the Atlantic and at the same time reducing the number of profitable transatlantic voyages. In response, shipping companies redirected many of their liners to a more profitable cruise service. In 1934, in the United Kingdom, Cunard Line and White Star Line were in very bad shape financially. Chancellor of the Exchequer Neville Chamberlain proposed to merge the two companies in order to solve their financial problems. The merger took place in 1934 and launched the construction of the while progressively sending their older ships to the scrapyard. The Queen Mary was the fastest ship of her time and the largest for a short amount of time, she captured the Blue Riband twice, both off Normandie. The construction of a second ship, the , was interrupted by the outbreak of World War II.
World War II was a conflict rich in events involving liners. From the start of the conflict, German liners were requisitioned and many were turned into barracks ships. It was in the course of this activity that the Bremen caught fire while under conversion for Operation Sea Lion and was scrapped in 1941. During the conflict, Queen Elizabeth and Queen Mary provided distinguished service as troopships. | Ocean liner | Wikipedia | 419 | 319558 | https://en.wikipedia.org/wiki/Ocean%20liner | Technology | Naval transport | null |
Many liners were sunk with great loss of life; the three worst disasters were the loss of the Cunarder in 1940 off Saint-Nazaire to German bombing while attempting to evacuate troops of the British Expeditionary Force from France, with the loss of more than 3,000 lives; the sinking of , after the ship was torpedoed by a Soviet submarine, with more than 9,000 lives lost, making it the deadliest maritime disaster in history; and the sinking of with more than 7,000 lives lost, both in the Baltic Sea, in 1945.
SS Rex was bombarded and sunk in 1944, and Normandie caught fire, capsized, and sank in New York in 1942 while being converted into a troopship. Many of the superliners of the 1920s and 1930s were victims of U-boats, mines or enemy aircraft. was attacked by German planes, then torpedoed by a U-boat when tugs tried to tow her to safety. Out of all the innovative and glamorous inter-war superliners, only the Cunard Queens and Europa would survive the war.
Decline of long-distance line voyages
After the war, some ships were again transferred from the defeated nations to the winning nations as war reparations. This was the case of the Europa, which was ceded to France and renamed Liberté. The United States government was very impressed with the service of the Cunard's Queen Mary and Queen Elizabeth as troopships during the war. To ensure a reliable and fast troop transport in case of a war against the Soviet Union, the U.S. government sponsored the construction of and entered it into service for the United States Lines in 1952. She won the Blue Riband on her maiden voyage in that year and held it until Richard Branson won it in 1986 with Virgin Atlantic Challenger II. One year later, in 1953, Italy completed the , which later sank in 1956 after a collision with . | Ocean liner | Wikipedia | 391 | 319558 | https://en.wikipedia.org/wiki/Ocean%20liner | Technology | Naval transport | null |
Before World War II, aircraft had not posed a significant economic threat to ocean liners. Most pre-war aircraft were noisy, vulnerable to bad weather, and/or incapable of the range needed for transoceanic flights; all were expensive and had a small passenger capacity. The war accelerated development of large, long-ranged aircraft. Four-engined bombers, such as the Avro Lancaster and Boeing B-29 Superfortress, with their range and massive carrying capacity, were natural prototypes for post-war next-generation airliners. Jet engine technology also accelerated due to wartime development of jet aircraft. In 1953, the De Havilland Comet became the first commercial jet airliner; the Sud Aviation Caravelle, Boeing 707 and Douglas DC-8 followed, and much long-distance travel was done by air. The Italian Line's and , launched in 1962 and 1963, were two of the last ocean liners to be built primarily for liner service across the North Atlantic. Cunard's transatlantic liner, , although designed as an ocean liner, was also used as a cruise ship. By the early 1960s, 95% of passenger traffic across the Atlantic was by aircraft. Thus the reign of the ocean liners came to an end. By the early 1970s, many passenger ships continued their service in cruising.
In 1982, during the Falklands War, three active or former liners were requisitioned for war service by the British Government. The liners Queen Elizabeth 2 and , were requisitioned from Cunard and P&O to serve as troopships, carrying British Army personnel to Ascension Island and the Falkland Islands to recover the Falklands from the invading Argentine forces. The P&O educational cruise ship and former British India Steam Navigation Company liner was requisitioned as a hospital ship, and served after the war as a troopship until the RAF Mount Pleasant station was built at Stanley, which could handle trooping flights.
21st century
By the first decade of the 21st century, only a few former ocean liners were still in existence. Some, like , were sailing as cruise ships while others, like , were preserved as museums, or laid up at pier side like SS United States. After the retirement of Queen Elizabeth 2 in 2008, the only ocean liner in service was Queen Mary 2, built in 2003–04 and used for both point-to-point line voyages and for cruises. | Ocean liner | Wikipedia | 487 | 319558 | https://en.wikipedia.org/wiki/Ocean%20liner | Technology | Naval transport | null |
A proposed and planned ocean liner, the Titanic II, is a modern replica of the original RMS Titanic, which sank in 1912. The ship is owned by Blue Star Line and is bought by Australian businessman Clive Palmer. The ship is set to be launched by 2027.
Survivors
Four ocean liners built before the World War II survive today as they have been partially or fully preserved as museums and hotels. The Japanese ocean liner (1929), has been preserved in Naka-ku, Yokohama, Japan, as a museum ship, since 1961. (1934) was preserved in 1967 after her retirement, and became a museum/hotel in Long Beach, California. In the 1970s, (1843) was also preserved, and now resides in Bristol, England as another museum. The latest ship to undergo preservation is (1914). While originally being a cargo ship, it served as the Italian ocean liner Franca C. for Costa Lines from 1952 to 1959, and in 2010 it became a dry berthed luxury hotel on Bintan Island, Indonesia. | Ocean liner | Wikipedia | 209 | 319558 | https://en.wikipedia.org/wiki/Ocean%20liner | Technology | Naval transport | null |
Post-war ocean liners still existent include (1948), (1952), MV Brazil Maru (1954), (1958), (1961), (1962), Queen Elizabeth 2 (1967), and Queen Mary 2 (2003). Out of these eight ocean liners, only one is still active and three of them have since been preserved. The Rotterdam has been moored in Rotterdam as a museum and hotel since 2008, while the Queen Elizabeth 2 has been a floating luxury hotel and museum at Mina Rashid, Dubai since 2018. The Ancerville was refurbished as a hotel for use at the Sea World development in Shenzhen, China in 1984. The first of these, Astoria (originally the ocean liner MS Stockholm, which collided with Andrea Doria in 1956) has been rebuilt and refitted as a cruise ship over the years and was in active service for Cruise & Maritime Voyages until operations ceased in 2020 due to the COVID-19 pandemic. In August, 2021 she was purchased by Brock Pierce to be transformed into a hotel along with . These plans were ultimately abandoned and the ship was again made available for sale, never having left port in Rotterdam. Astoria was reported to have been sold for scrap in January 2023, but this has been denied by the ship's owner. United States has been docked in Philadelphia since 1996, but following a legal dispute between the organization that owns United States and the pier owners, she was purchased by Okaloosa County, Florida to be turned into the world's largest artificial reef. There are plans for a land-based museum and several pieces of United States are planned to be preserved. Brazil Maru was beached in Zhanjiang, China as a tourist attraction called Hai Shang Cheng Shi in 1998, though has been closed as of 2022. Funchal was purchased by Brock Pierce in 2021, with the intent of turning her into a hotel. Her future is uncertain as it was reported in July 2021 that no progress has been made since then.
Characteristics
Size and speed | Ocean liner | Wikipedia | 408 | 319558 | https://en.wikipedia.org/wiki/Ocean%20liner | Technology | Naval transport | null |
Since their beginning in the 19th century, ocean liners needed to meet growing demands. The first liners were small and overcrowded, leading to unsanitary conditions on board. Eliminating these phenomena required larger ships, to reduce the crowding of passengers, and faster ships, to reduce the duration of transatlantic crossings. The iron and steel hulls and steam power allowed for these to be achieved. Thus, SS Great Western (1,340 GRT) and SS Great Eastern (18,915 GRT) were constructed in 1838 and 1858 respectively. The record set by SS Great Eastern was not beaten until 43 years later in 1901 when (20,904 GT) was completed. The tonnage then grew profoundly: the first liners to have a tonnage that exceeded 20,000 were the Big Four of the White Star Line. The liners, first completed in 1911, were the first to have a tonnage that exceeded 45,000 and the liners first completed in 1913 became the 1st liners with tonnage exceeding 50,000. , completed in 1935, had a tonnage of 79,280. In 1940, raised the record of size to a tonnage of 83,673. She was the largest passenger ship ever constructed until 1997. In 2003, became the largest, at 149,215 GT. | Ocean liner | Wikipedia | 270 | 319558 | https://en.wikipedia.org/wiki/Ocean%20liner | Technology | Naval transport | null |
In the early 1840s, the average speed of liners was less than 10 knots (a crossing of the Atlantic thus took about 12 days or more). In the 1870s, the average speed of liners increased to around 15 knots the duration of a transatlantic crossing shortened to around 7 days, owing to the technological progress made in the propulsion of ships. The rudimentary steam boilers gave rise to more elaborate machineries and the paddlewheel gradually disappeared, replaced first by one screw then by two screws. At the beginning of the 20th century, Cunard Line's and reached a speed of 27 knots. Their records seemed unbeatable, and most shipping companies abandoned the race for speed in favor of size, luxury, and safety. The advent of ships with diesel engines, and of those whose engines were oil-burning, such as the Bremen, in the early 1930s, renewed the race for the Blue Riband. The won it in 1935 before being snatched by in 1938. It was not until 1952 that set a record that remains today: 34.5 knots (3 days and 12 hours of crossing the Atlantic). In addition, since 1935, the Blue Riband is accompanied by the Hales Trophy, which is awarded to the winner.
Passenger cabins and amenities
The first ocean liners were designed to carry mostly migrants. On-board sanitary conditions were often deplorable and epidemics were frequent. In 1848, maritime laws imposing hygiene rules were adopted and they improved on-board living conditions. Gradually, two distinct classes were developed: the cabin class and the steerage class. The passengers travelling on the former were wealthy passengers and they enjoyed certain comfort in that class. The passengers travelling on the latter were members of the middle class or the working class. In that class, they were packed in large dormitories. Until the beginning of the 20th century, they did not always have bedsheets and meals. An intermediate class for tourists and members of the middle class gradually appeared. The cabins were then divided into three classes.
The facilities offered to passengers developed over time. In the 1870s, the installation of bathtubs and oil lamps caused a sensation on board . In the following years, the number of amenities became numerous, for example: smoking rooms, lounges, and promenade deck. In 1907, even offered Turkish baths and a swimming pool. In the 1920s, was the first liner to offer a movie theatre.
Builders
British and German | Ocean liner | Wikipedia | 493 | 319558 | https://en.wikipedia.org/wiki/Ocean%20liner | Technology | Naval transport | null |
The British and the German shipyards were the most famed in shipbuilding during the era of ocean liners. In Ireland, Harland & Wolff shipyard of Belfast were particularly innovative and succeeded in winning the trust of many shipping companies, such as White Star Line. These gigantic shipyards employed a large portion of the population of cities and built hulls, machines, furnitures and lifeboats. Among the other well-known British shipyards were Swan, Hunter & Wigham Richardson, the builder of , and John Brown & Company, builders of , , , , and Queen Elizabeth 2.
Germany had many shipyards on the coast of the North Sea and the Baltic Sea, including Blohm & Voss and AG Vulcan Stettin. Many of these shipyards were destroyed during World War II; some managed to recover and continue building ships.
Other nations
In France, major shipyards included Chantiers de Penhoët in Saint-Nazaire, known for building . This shipyard merged with Ateliers et Chantiers de la Loire shipyard to form the Chantiers de l'Atlantique shipyard, which has built ships including . France also had major shipyards on the shores of the Mediterranean Sea.
Italy and the Netherlands also had shipyards capable of building large ships (for example, Fincantieri).
Shipping companies
British
There were many British shipping companies; two were particularly distinguished: Cunard Line and White Star Line. Both were founded during the 1840s and engaged in strong competition against one another, possessing the largest and fastest liners in the world in the early 20th century. It was not until 1934 that financial difficulty caused the two to merge, forming Cunard White Star Ltd. The P&O also occupied a large part of the business.
The Royal Mail Steam Packet Company operated as a state-owned enterprise with its close relationship with the government. Over the course of its history, it took over many shipping companies, becoming one of the largest companies in the world before legal problems led to its liquidation in 1931. The Union Castle Line operated in Africa and the Indian Ocean with a fleet of considerable size.
German, French and Dutch
Two rival companies, Hamburg America Line (often referred to as "HAPAG") and Norddeutscher Lloyd, competed in Germany. The First and Second World Wars dealt much damage to the two companies, both forced to cede their ships to the winning side in both wars. The two merged to form Hapag-Lloyd in 1970. | Ocean liner | Wikipedia | 499 | 319558 | https://en.wikipedia.org/wiki/Ocean%20liner | Technology | Naval transport | null |
The ocean liner industry in France also consisted of two rival companies: the Compagnie Générale Transatlantique (commonly known as "Transat" or "French Line") and Messageries Maritimes. The CGT operated on the North Atlantic route with well-known liners such as and , while the MM operated in French colonies in Asia and Africa. Decolonization in the second half of the 20th century led to a sharp decline in profit for the MM, and it merged with the CGT in 1975 to form the Compagnie Générale Maritime.
The Netherlands had three main companies. The Holland America Line operated mostly on the north Atlantic route and with well-known ships like the and . Unlike the French and German industry, the Holland America Line had no domestic rival in this trade and only had to compete with foreign lines. The other two Dutch lines were the Stoomvaart Maatschappij Nederland (SMN), otherwise known as the Netherland Line and the Koninklijke Rotterdamsche Lloyd (KRL); both offered regular service between the Netherlands and the Dutch East Indies, the Dutch colony in South East Asia now known as Indonesia, and had a long-lasting friendly rivalry.
Other nations
The United States Lines competed with European companies for the North Atlantic trade. In Italy, the Italian Line was founded in 1932 as a result of a merger of three companies. It was known for operating liners such as and . The Japanese established Nippon Yusen, also known as NYK Lines, which ran trans-Pacific liners such as the Hikawa Maru and the Asama Maru.
Routes
North Atlantic
The most important of all routes taken by ocean liners was the North Atlantic route. It accounted for a large part of the clientele, who traveled between ports of Liverpool, Southampton, Hamburg, Le Havre, Cherbourg, Cobh, and New York City. The profitability of this route came from migration to the United States. The need for speed influenced the construction of liners for this route, and the Blue Riband was awarded to the liner with the highest speed. The route was not without danger, as storm and icebergs are common in the North Atlantic. Many shipwrecks occurred on this route, among them that of , the details of which have been recounted in numerous books, films and documentaries. This route was the preferred route for major shipping companies and was the scene of fierce competition between them.
South Atlantic | Ocean liner | Wikipedia | 504 | 319558 | https://en.wikipedia.org/wiki/Ocean%20liner | Technology | Naval transport | null |
The South Atlantic was the route frequented by liners bound for South America, Africa, and sometimes Oceania. The White Star Line had some of its ships, such as the , on the Liverpool-Cape Town-Sydney route. There was not the same level of competition in the South Atlantic as there was in the North Atlantic. There were fewer shipwrecks. The Hamburg Süd operated on this route; among its ships was the famed .
Mediterranean
The Mediterranean Sea was frequented by many ocean liners. Many companies benefited from migration from Italy and the Balkans to the United States. Cunard's served on the Gibraltar-Genoa-Trieste route. Similarly, Italian liners crossed the Mediterranean Sea before entering the North Atlantic Ocean. The opening of the Suez Canal made the Mediterranean a possible route to Asia.
Indian Ocean and the East Asia
Colonization made Asia particularly attractive to shipping companies. Many government officials must travel there from time to time. As early as the 1840s, the P&O organized trips to Calcutta via the Suez Isthmus, as the canal had not yet been built. The time it took to travel on this route to India, Southeast Asia, and Japan was long, with many stopovers. The Messageries Maritimes operated on this route, notably in the 1930s, with its motor ships. Similarly, the La Marseillaise, put into service in 1949, was one of the flagships of its fleet. Decolonization caused the loss in the profitability of these ships.
Pacific
Ocean liners on the Pacific route brought large numbers of migrants from East Asia to the Americas, especially the United States, which continued despite successive laws restricting Asian immigration to the United States; the journey typically took three weeks, with many impoverished migrants travelling in steerage class conditions. Some of the finest ships on the route, such as of Canadian Pacific Steamships which operated out of Vancouver, and Hikawa Maru of Nippon Yusen, became known as 'Queen of the Pacific'.
Other | Ocean liner | Wikipedia | 397 | 319558 | https://en.wikipedia.org/wiki/Ocean%20liner | Technology | Naval transport | null |
National symbol
The construction of some ocean liners was a result of nationalism. The revival of power of the German navy stemmed from the clear affirmation of Kaiser Wilhelm II of Germany to see his country become a sea power. Thus, the of 1900 had the honor to bear the name of its mother country, an honor which she lost after ten years of a disappointing career. and of 1907 were built with the help of the British government with the desire that the United Kingdom would regain its prestige as a sea power. of 1952 was the result of a desire by the United States government to possess a large and fast ship that is convertible into a troop transport. and of 1932 were constructed at the demands of Benito Mussolini. Finally, the construction of 1961 was a result of Charles de Gaulle's desire to build on French national pride and was financed by the French government.
Some liners did gain great popularity. Mauretania and had many admirers during their careers, and their retirement and scrapping caused some sadness. The same was true of Île de France, whose scrapping aroused strong emotion from her admirers. Similarly, was very popular with the British people.
Maritime disasters and incidents
Some ocean liners are known today because of their sinking with great loss of lives. In 1873 struck an underwater rock and sank off the coast of Nova Scotia, Canada, killing at least 535 people. In 1912 the sinking of the RMS Titanic, which took approximately 1,500 lives, highlighted the overconfidence of the shipping companies in their ships, such as the failure to put enough lifeboats on board. Safety measures at sea were reexamined following the incident. Two years later, in 1914, sank in the Saint Lawrence River after colliding with the . 1,012 people died.
Among the other sinkings are the sinking by torpedo of the RMS Lusitania in 1915, which resulted in the loss of 1,198 lives and provoked an international outcry, the sinking by naval mine of in 1916, and that of , which caught fire and sank in the Gulf of Aden in 1932, killing 54 people. In 1956 the sinking of , with the loss of 46 lives, after a collision with made the headline.
In 1985, was hijacked off the coast of Egypt by members of the Palestinian Liberation Front, resulting in the death of one of the hostages being held by the hijackers. In 1994, she caught fire and sank off the coast of Somalia.
In popular culture | Ocean liner | Wikipedia | 503 | 319558 | https://en.wikipedia.org/wiki/Ocean%20liner | Technology | Naval transport | null |
Literature
Ocean liners have a strong impact on popular culture, whether during their era or afterwards. In 1867, Jules Verne recounted his experience aboard in his novel A Floating City. In 1898, writer Morgan Robertson wrote the short novel Futility, or the Wreck of the Titan, which features a British ocean liner Titan that hits an iceberg and sinks in the North Atlantic with great loss of lives. The similarities between the plot of the novel and the sinking of the 14 years later led to the assertion of conspiracy theories regarding Titanic.
Films
Ocean liners were often a setting of a love story in films, such as the 1939's Love Affair Liners were also used as a setting of disaster films. The 1960 film The Last Voyage was filmed on board the Île de France, which was used as a floating prop and was scuttled for the occasion. The 1972 film The Poseidon Adventure has become a classic of the genre and has spawned many remakes.
The sinking of Titanic also attracted attention of filmmakers. Nearly fifteen films were made to depict it, with James Cameron's 1997 film being the most well-known and commercially successful. | Ocean liner | Wikipedia | 229 | 319558 | https://en.wikipedia.org/wiki/Ocean%20liner | Technology | Naval transport | null |
In biology, a biological life cycle (or just life cycle when the biological context is clear) is a series of stages of the life of an organism, that begins as a zygote, often in an egg, and concludes as an adult that reproduces, producing an offspring in the form of a new zygote which then itself goes through the same series of stages, the process repeating in a cyclic fashion.
"The concept is closely related to those of the life history, development and ontogeny, but differs from them in stressing renewal." Transitions of form may involve growth, asexual reproduction, or sexual reproduction.
In some organisms, different "generations" of the species succeed each other during the life cycle. For plants and many algae, there are two multicellular stages, and the life cycle is referred to as alternation of generations. The term life history is often used, particularly for organisms such as the red algae which have three multicellular stages (or more), rather than two.
Life cycles that include sexual reproduction involve alternating haploid (n) and diploid (2n) stages, i.e., a change of ploidy is involved. To return from a diploid stage to a haploid stage, meiosis must occur. In regard to changes of ploidy, there are three types of cycles:
haplontic life cycle — the haploid stage is multicellular and the diploid stage is a single cell, meiosis is "zygotic".
diplontic life cycle — the diploid stage is multicellular and haploid gametes are formed, meiosis is "gametic".
haplodiplontic life cycle (also referred to as diplohaplontic, diplobiontic, or dibiontic life cycle) — multicellular diploid and haploid stages occur, meiosis is "sporic". | Biological life cycle | Wikipedia | 396 | 319610 | https://en.wikipedia.org/wiki/Biological%20life%20cycle | Biology and health sciences | Ecology | Biology |
The cycles differ in when mitosis (growth) occurs. Zygotic meiosis and gametic meiosis have one mitotic stage: mitosis occurs during the n phase in zygotic meiosis and during the 2n phase in gametic meiosis. Therefore, zygotic and gametic meiosis are collectively termed "haplobiontic" (single mitotic phase, not to be confused with haplontic). Sporic meiosis, on the other hand, has mitosis in two stages, both the diploid and haploid stages, termed "diplobiontic" (not to be confused with diplontic).
Discovery
The study of reproduction and development in organisms was carried out by many botanists and zoologists.
Wilhelm Hofmeister demonstrated that alternation of generations is a feature that unites plants, and published this result in 1851 (see plant sexuality).
Some terms (haplobiont and diplobiont) used for the description of life cycles were proposed initially for algae by Nils Svedelius, and then became used for other organisms. Other terms (autogamy and gamontogamy) used in protist life cycles were introduced by Karl Gottlieb Grell. The description of the complex life cycles of various organisms contributed to the disproof of the ideas of spontaneous generation in the 1840s and 1850s.
Haplontic life cycle
A zygotic meiosis is a meiosis of a zygote immediately after karyogamy, which is the fusion of two cell nuclei. This way, the organism ends its diploid phase and produces several haploid cells. These cells divide mitotically to form either larger, multicellular individuals, or more haploid cells. Two opposite types of gametes (e.g., male and female) from these individuals or cells fuse to become a zygote.
In the whole cycle, zygotes are the only diploid cell; mitosis occurs only in the haploid phase.
The individuals or cells as a result of mitosis are haplonts, hence this life cycle is also called haplontic life cycle. Haplonts are: | Biological life cycle | Wikipedia | 460 | 319610 | https://en.wikipedia.org/wiki/Biological%20life%20cycle | Biology and health sciences | Ecology | Biology |
In archaeplastidans: some green algae (e.g., Chlamydomonas, Zygnema, Chara)
In stramenopiles: some golden algae
In alveolates: many dinoflagellates, e.g., Ceratium, Gymnodinium, some apicomplexans (e.g., Plasmodium)
In rhizarians: some euglyphids, ascetosporeans
In excavates: some parabasalids
In amoebozoans: Dictyostelium
In opisthokonts: most fungi (some chytrids, zygomycetes, some ascomycetes, basidiomycetes)
Diplontic life cycle
In gametic meiosis, instead of immediately dividing meiotically to produce haploid cells, the zygote divides mitotically to produce a multicellular diploid individual or a group of more unicellular diploid cells. Cells from the diploid individuals then undergo meiosis to produce haploid cells or gametes. Haploid cells may divide again (by mitosis) to form more haploid cells, as in many yeasts, but the haploid phase is not the predominant life cycle phase. In most diplonts, mitosis occurs only in the diploid phase, i.e. gametes usually form quickly and fuse to produce diploid zygotes.
In the whole cycle, gametes are usually the only haploid cells, and mitosis usually occurs only in the diploid phase.
The diploid multicellular individual is a diplont, hence a gametic meiosis is also called a diplontic life cycle. Diplonts are: | Biological life cycle | Wikipedia | 371 | 319610 | https://en.wikipedia.org/wiki/Biological%20life%20cycle | Biology and health sciences | Ecology | Biology |
In archaeplastidans: some green algae (e.g., Cladophora glomerata, Acetabularia)
In stramenopiles: some brown algae (the Fucales, however, their life cycle can also be interpreted as strongly heteromorphic-diplohaplontic, with a highly reduced gametophyte phase, as in the flowering plants), some xanthophytes (e.g., Vaucheria), most diatoms, some oomycetes (e.g., Saprolegnia, Plasmopara viticola), opalines, some "heliozoans" (e.g., Actinophrys, Actinosphaerium)
In alveolates: ciliates
In excavates: some parabasalids
In opisthokonts: animals, some fungi (e.g., some ascomycetes)
Haplodiplontic life cycle
In sporic meiosis (also commonly known as intermediary meiosis), the zygote divides mitotically to produce a multicellular diploid sporophyte. The sporophyte creates spores via meiosis which also then divide mitotically producing haploid individuals called gametophytes. The gametophytes produce gametes via mitosis. In some plants the gametophyte is not only small-sized but also short-lived; in other plants and many algae, the gametophyte is the "dominant" stage of the life cycle.
Haplodiplonts are:
In archaeplastidans: red algae (which have two sporophyte generations), some green algae (e.g., Ulva), land plants
In stramenopiles: most brown algae
In rhizarians: many foraminiferans, plasmodiophoromycetes
In amoebozoa: myxogastrids
In opisthokonts: some fungi (some chytrids, some ascomycetes like the brewer's yeast)
Other eukaryotes: haptophytes
Some animals have a sex-determination system called haplodiploid, but this is not related to the haplodiplontic life cycle. | Biological life cycle | Wikipedia | 490 | 319610 | https://en.wikipedia.org/wiki/Biological%20life%20cycle | Biology and health sciences | Ecology | Biology |
Vegetative meiosis
Some red algae (such as Bonnemaisonia and Lemanea) and green algae (such as Prasiola) have vegetative meiosis, also called somatic meiosis, which is a rare phenomenon. Vegetative meiosis can occur in haplodiplontic and also in diplontic life cycles. The gametophytes remain attached to and part of the sporophyte. Vegetative (non-reproductive) diploid cells undergo meiosis, generating vegetative haploid cells. These undergo many mitosis, and produces gametes.
A different phenomenon, called vegetative diploidization, a type of apomixis, occurs in some brown algae (e.g., Elachista stellaris). Cells in a haploid part of the plant spontaneously duplicate their chromosomes to produce diploid tissue.
Parasitic life cycle
Parasites depend on the exploitation of one or more hosts. Those that must infect more than one host species to complete their life cycles are said to have complex or indirect life cycles. Dirofilaria immitis, or the heartworm, has an indirect life cycle, for example. The microfilariae must first be ingested by a female mosquito, where it develops into the infective larval stage. The mosquito then bites an animal and transmits the infective larvae into the animal, where they migrate to the pulmonary artery and mature into adults.
Those parasites that infect a single species have direct life cycles. An example of a parasite with a direct life cycle is Ancylostoma caninum, or the canine hookworm. They develop to the infective larval stage in the environment, then penetrate the skin of the dog directly and mature to adults in the small intestine.
If a parasite has to infect a given host in order to complete its life cycle, then it is said to be an obligate parasite of that host; sometimes, infection is facultative—the parasite can survive and complete its life cycle without infecting that particular host species. Parasites sometimes infect hosts in which they cannot complete their life cycles; these are accidental hosts. | Biological life cycle | Wikipedia | 452 | 319610 | https://en.wikipedia.org/wiki/Biological%20life%20cycle | Biology and health sciences | Ecology | Biology |
A host in which parasites reproduce sexually is known as the definitive, final or primary host. In intermediate hosts, parasites either do not reproduce or do so asexually, but the parasite always develops to a new stage in this type of host. In some cases a parasite will infect a host, but not undergo any development, these hosts are known as paratenic or transport hosts. The paratenic host can be useful in raising the chance that the parasite will be transmitted to the definitive host. For example, the cat lungworm (Aelurostrongylus abstrusus) uses a slug or snail as an intermediate host; the first stage larva enters the mollusk and develops to the third stage larva, which is infectious to the definitive host—the cat. If a mouse eats the slug, the third stage larva will enter the mouse's tissues, but will not undergo any development.
Evolution
The primitive type of life cycle probably had haploid individuals with asexual reproduction. Bacteria and archaea exhibit a life cycle like this, and some eukaryotes apparently do too (e.g., Cryptophyta, Choanoflagellata, many Euglenozoa, many Amoebozoa, some red algae, some green algae, the imperfect fungi, some rotifers and many other groups, not necessarily haploid). However, these eukaryotes probably are not primitively asexual, but have lost their sexual reproduction, or it just was not observed yet. Many eukaryotes (including animals and plants) exhibit asexual reproduction, which may be facultative or obligate in the life cycle, with sexual reproduction occurring more or less frequently. | Biological life cycle | Wikipedia | 357 | 319610 | https://en.wikipedia.org/wiki/Biological%20life%20cycle | Biology and health sciences | Ecology | Biology |
Individual organisms participating in a biological life cycle ordinarily age and die, while cells from these organisms that connect successive life cycle generations (germ line cells and their descendants) are potentially immortal. The basis for this difference is a fundamental problem in biology. The Russian biologist and historian Zhores A. Medvedev considered that the accuracy of genome replicative and other synthetic systems alone cannot explain the immortality of germlines. Rather Medvedev thought that known features of the biochemistry and genetics of sexual reproduction indicate the presence of unique information maintenance and restoration processes at the gametogenesis stage of the biological life cycle. In particular, Medvedev considered that the most important opportunities for information maintenance of germ cells are created by recombination during meiosis and DNA repair; he saw these as processes within the germ line cells that were capable of restoring the integrity of DNA and chromosomes from the types of damage that cause irreversible ageing in non-germ line cells, e.g. somatic cells.
The ancestry of each present day cell presumably traces back, in an unbroken lineage for over 3 billion years to the origin of life. It is not actually cells that are immortal but multi-generational cell lineages. The immortality of a cell lineage depends on the maintenance of cell division potential. This potential may be lost in any particular lineage because of cell damage, terminal differentiation as occurs in nerve cells, or programmed cell death (apoptosis) during development. Maintenance of cell division potential of the biological life cycle over successive generations depends on the avoidance and the accurate repair of cellular damage, particularly DNA damage. In sexual organisms, continuity of the germline over successive cell cycle generations depends on the effectiveness of processes for avoiding DNA damage and repairing those DNA damages that do occur. Sexual processes in eukaryotes provide an opportunity for effective repair of DNA damages in the germ line by homologous recombination. | Biological life cycle | Wikipedia | 393 | 319610 | https://en.wikipedia.org/wiki/Biological%20life%20cycle | Biology and health sciences | Ecology | Biology |
A bidet ( or ) is a bowl or receptacle designed to be sat upon in order to wash a person's genitalia, perineum, inner buttocks, and anus. The modern variety has a plumbed-in water supply and a drainage opening, and is thus a plumbing fixture subject to local hygiene regulations. The bidet is designed to promote personal hygiene and is used after defecation, and before and after sexual intercourse. It can also be used to wash feet, with or without filling it up with water. Some people even use bidets to bathe babies or pets. In several European countries, a bidet is now required by law to be present in every bathroom containing a toilet bowl. It was originally located in the bedroom, near the chamber-pot and the marital bed, but in modern times is located near the toilet bowl in the bathroom. Fixtures that combine a toilet seat with a washing facility include the electronic bidet.
Opinions as to the necessity of the bidet vary widely over different nationalities and cultures. In cultures that use it habitually, such as parts of Western, Central and Southeastern Europe (especially Italy and Portugal), Eastern Asia and some Latin American countries such as Argentina or Paraguay, it is considered an indispensable tool in maintaining good personal hygiene. It is commonly used in North African countries, such as Egypt. It is rarely used in sub-Saharan Africa, Australia, and North America.
"Bidet" is a French loanword meaning "pony" due to the straddling position adopted in its usage.
Applications
Bidets are primarily used to wash and clean the genitalia, perineum, inner buttocks, and anus. Some bidets have a vertical jet intended to give easy access for washing and rinsing the perineum and anal area. The traditional separate bidet is like a wash-basin which is used with running warm water with the help of specific soaps, and may then be used for many other purposes such as washing feet.
Types
Bidet shower | Bidet | Wikipedia | 418 | 319653 | https://en.wikipedia.org/wiki/Bidet | Technology | Hydraulics and pneumatics | null |
A bidet shower (also known as "bidet spray", "bidet sprayer", or "health faucet") is a hand-held triggered nozzle, similar to that on a kitchen sink sprayer, that delivers a spray of water to assist in anal cleansing and cleaning the genitals after defecation and urination. In contrast to a bidet that is integrated with the toilet, a bidet shower has to be held by the hands, and cleaning does not take place automatically. Bidet showers are common in countries where water is considered essential for anal cleansing.
Drawbacks include the possibility of wetting a user's clothing if used carelessly. In addition, a user must be reasonably mobile and flexible to use a hand-held bidet shower.
Conventional or standalone bidet
A bidet is a plumbing fixture that is installed as a separate unit in the bathroom besides toilet, shower and sink, which users have to straddle. Some bidets resemble a large hand basin, with taps and a stopper so they can be filled up; other designs have a nozzle that squirts a jet of water to aid in cleansing.
Add-on bidets
There are bidets that are attachable to toilet bowls, saving space and obviating additional plumbing. A bidet may be a movable or fixed nozzle, either attached to an existing toilet on the back or side toilet rim, or replacing the toilet seat. In these cases, its use is restricted to cleaning the anus and genitals. Some bidets of this type produce a vertical water jet and others a more-or-less oblique one. Other bidets have one nozzle on the side rim aimed at both anal and genital areas, and other designs have two nozzles on the back rim. The shorter one, called the "family nozzle", is used for washing the area around the anus, and the longer one ("bidet nozzle") is designed for washing the vulva. | Bidet | Wikipedia | 414 | 319653 | https://en.wikipedia.org/wiki/Bidet | Technology | Hydraulics and pneumatics | null |
Such attachable bidets (also called "combined toilets", "bidet attachments", or "add-on bidets") are controlled either mechanically, by turning a valve, or electronically. Electronic bidets are controlled with waterproof electrical switches rather than a manual valve. There are models that have a heating element which blows warm air to dry the user after washing, that offer heated seats, wireless remote controls, illumination through built in night lights, or built in deodorizers and activated carbon filters to remove odours. Further refinements include adjustable water pressure, temperature compensation, and directional spray control. Where bathroom appearance is of concern, under-the-seat mounting types have become more popular.
An add-on bidet typically connects to the existing water supply of a toilet via the addition of a threaded tee pipe adapter, and requires no soldering or other plumbing work. Electronic add-on bidets also require a GFCI protected grounded electrical outlet.
Usage and health
Personal hygiene is improved and maintained more accurately and easily with the use of both toilet paper and a bidet as compared to the use of toilet paper alone. In some add-on bidets with vertical jets, little water is used and toilet paper may not be necessary. Addressing hemorrhoids and genital health issues might also be facilitated by the use of bidet fixtures.
Because of the large surface of the basin, after-use and routine disinfection of stand-alone bidets require thoroughness, or microbial contamination from one user to the next could take place. Bidet attachments are sometimes included on hospital toilets because of their utility in maintaining hygiene. Hospitals must consider the use of bidet properly and consider the clinical background of patients to prevent cross-infection. Warm-water bidets may harbor dangerous microbes if not properly disinfected.
Environmental aspects
From an environmental standpoint, bidets can reduce the need for toilet paper. Considering that an average person uses only 0.5 litre (1/8 US gallon) of water for cleansing by using a bidet, much less water is used than for manufacturing toilet paper. An article in Scientific American concluded that using a bidet is "much less stressful on the environment than using paper". Scientific American has also reported that if the US switched to using bidets, 15 million trees could be saved every year. | Bidet | Wikipedia | 485 | 319653 | https://en.wikipedia.org/wiki/Bidet | Technology | Hydraulics and pneumatics | null |
In the US, UK, and some other countries, wet wipes are heavily marketed as an upgrade from dry toilet paper. However, this product has been criticized for its adverse environmental impact, due to the non-biodegradable plastic fibers composing most versions. Although the wipes are promoted as "flushable", they absorb waste fats and agglomerate into massive "fatbergs" which can clog sewer systems and must be cleared at great expense. Bidets are being marketed as cleaning better than toilet paper or wet wipes, with fewer negative environmental effects.
Society and culture
The bidet is common in Catholic countries and required by law in some. It is also found in some traditionally Eastern Orthodox and Protestant countries such as Greece and Finland respectively, where bidet showers are common.
In Islam, there are many strict rules concerning excretion; in particular, anal washing with water is required. Consequently, in Middle Eastern regions where Islam is the predominant religion, water for anal washing is provided in most toilets, usually in the form of a hand-held "bidet shower" or shattaf.
Prevalence
Bidets are becoming increasingly popular with the elderly and disabled. Combined toilet/bidet installations make self-care toileting possible for many people, affording greater independence. There are often special units with higher toilet seats allowing easier wheelchair transfer, and with some form of electronic remote control that benefits an individual with limited mobility or otherwise requiring assistance.
Bidets are common bathroom fixtures in the Arab world and in Catholic countries, such as Italy (the installation of a bidet in a bathroom has been mandatory since 1975), Spain (but in recent times new or renewed houses tend to have bathrooms without bidets, except the luxurious ones), and Portugal (installation is mandatory since 1975). They are also found in Southeastern European countries such as Albania, Bosnia and Herzegovina, Romania, Greece and Turkey. They are very popular in some South American countries, particularly Argentina, Paraguay and Uruguay. Electronic bidet-integrated toilets, often with functions such as toilet seat warming, are commonly found in Japan, and are becoming more popular in other Asian countries.
In Northern Europe, bidets are rare, although in Finland, bidet showers are common. Bidet showers are most commonly found in Southeast Asia, South Asia, and the Middle East. | Bidet | Wikipedia | 474 | 319653 | https://en.wikipedia.org/wiki/Bidet | Technology | Hydraulics and pneumatics | null |
In 1980, the first "paperless toilet" was launched in Japan by manufacturer Toto, a combination of toilet and bidet which also dries the user after washing. These combination toilet-bidets (washlet) with seat warmers, or attachable bidets are particularly popular in Japan and South Korea, and are found in approximately 76% of Japanese households . They are commonly found in hotels and some public facilities. These bidet-toilets, along with toilet seat and bidet units (to convert an existing toilet) are sold in many countries, including the United States.
Bidet seat conversions are much easier and lower cost to install than traditional bidets, and have disrupted the market for the older fixtures.
After a slow start in the 1990s, electronic bidets are starting to become more available in the United States. American distributors were directly influenced by their Japanese predecessors, as the founders of Brondell (established in 2003) have indicated. The popularity of add-on bidet units is steadily increasing in the United States, Canada and the United Kingdom, in part because of their ability to treat hemorrhoids or urogenital infections. In addition, shortages of toilet paper due to the coronavirus pandemic have led to an increased interest in bidets.
Etymology
Bidet is a French word for "pony", and in Old French, meant "to trot". This etymology comes from the notion that one "rides" or straddles a bidet much like a pony is ridden. The word "bidet" was used in 15th-century France to refer to the pet ponies that French royalty kept.
History
The bidet appears to have been an invention of French furniture makers in the late 17th century, although no exact date or inventor is known. The earliest written reference to the bidet is in 1726 in Italy. Even though there are records of Maria Carolina of Austria, Queen of Naples and Sicily, requesting a bidet for her personal bathroom in the Royal Palace of Caserta in the second half of the 18th century, the bidet did not become widespread in Italy until after the Second World War. The bidet is possibly associated with the chamber pot and the bourdaloue, the latter being a small, hand-held chamber pot. | Bidet | Wikipedia | 461 | 319653 | https://en.wikipedia.org/wiki/Bidet | Technology | Hydraulics and pneumatics | null |
Historical antecedents and early functions of the bidet are believed to include devices used for contraception. Bidets are considered ineffective by today's standards of contraception, and their use for that function was quickly abandoned and forgotten following the advent of modern contraceptives such as the pill.
By 1900, due to plumbing improvements, the bidet (and chamber pot) moved from the bedroom to the bathroom and became more convenient to fill and drain.
In 1928, in the United States, John Harvey Kellogg applied for a patent on an "anal douche". In his application, he used the term to describe a system comparable to what today might be called a bidet nozzle, which can be attached to a toilet to perform anal cleansing with water.
In 1965, the American Bidet Company featured an adjustable spray nozzle and warm water option, seeking to make the bidet a household item. The fixture was expensive, and required floor space to install; it was eventually discontinued without a replacement model.
The early 1980s saw the introduction of the electronic bidet from Japan, with names such as Clean Sense, Galaxy, Infinity, Novita, and of non-electric attachments such as Gobidet. These devices have attachments that connect to existing toilet water supplies, and can be used in bathrooms lacking the space for a separate bidet and toilet. Many models have additional features, such as instant-heating warm water, night lights, or a heated seat. | Bidet | Wikipedia | 296 | 319653 | https://en.wikipedia.org/wiki/Bidet | Technology | Hydraulics and pneumatics | null |
In mathematics, a binary relation is called well-founded (or wellfounded or foundational) on a set or, more generally, a class if every non-empty subset has a minimal element with respect to ; that is, there exists an such that, for every , one does not have . In other words, a relation is well-founded if:
Some authors include an extra condition that is set-like, i.e., that the elements less than any given element form a set.
Equivalently, assuming the axiom of dependent choice, a relation is well-founded when it contains no infinite descending chains, which can be proved when there is no infinite sequence of elements of such that for every natural number .
In order theory, a partial order is called well-founded if the corresponding strict order is a well-founded relation. If the order is a total order then it is called a well-order.
In set theory, a set is called a well-founded set if the set membership relation is well-founded on the transitive closure of . The axiom of regularity, which is one of the axioms of Zermelo–Fraenkel set theory, asserts that all sets are well-founded.
A relation is converse well-founded, upwards well-founded or Noetherian on , if the converse relation is well-founded on . In this case is also said to satisfy the ascending chain condition. In the context of rewriting systems, a Noetherian relation is also called terminating.
Induction and recursion
An important reason that well-founded relations are interesting is because a version of transfinite induction can be used on them: if () is a well-founded relation, is some property of elements of , and we want to show that
holds for all elements of ,
it suffices to show that:
If is an element of and is true for all such that , then must also be true.
That is,
Well-founded induction is sometimes called Noetherian induction, after Emmy Noether.
On par with induction, well-founded relations also support construction of objects by transfinite recursion. Let be a set-like well-founded relation and a function that assigns an object to each pair of an element and a function on the initial segment of . Then there is a unique function such that for every ,
That is, if we want to construct a function on , we may define using the values of for . | Well-founded relation | Wikipedia | 501 | 319712 | https://en.wikipedia.org/wiki/Well-founded%20relation | Mathematics | Order theory | null |
As an example, consider the well-founded relation , where is the set of all natural numbers, and is the graph of the successor function . Then induction on is the usual mathematical induction, and recursion on gives primitive recursion. If we consider the order relation , we obtain complete induction, and course-of-values recursion. The statement that is well-founded is also known as the well-ordering principle.
There are other interesting special cases of well-founded induction. When the well-founded relation is the usual ordering on the class of all ordinal numbers, the technique is called transfinite induction. When the well-founded set is a set of recursively-defined data structures, the technique is called structural induction. When the well-founded relation is set membership on the universal class, the technique is known as ∈-induction. See those articles for more details.
Examples
Well-founded relations that are not totally ordered include:
The positive integers , with the order defined by if and only if divides and .
The set of all finite strings over a fixed alphabet, with the order defined by if and only if is a proper substring of .
The set of pairs of natural numbers, ordered by if and only if and .
Every class whose elements are sets, with the relation ∈ ("is an element of"). This is the axiom of regularity.
The nodes of any finite directed acyclic graph, with the relation defined such that if and only if there is an edge from to .
Examples of relations that are not well-founded include:
The negative integers , with the usual order, since any unbounded subset has no least element.
The set of strings over a finite alphabet with more than one element, under the usual (lexicographic) order, since the sequence is an infinite descending chain. This relation fails to be well-founded even though the entire set has a minimum element, namely the empty string.
The set of non-negative rational numbers (or reals) under the standard ordering, since, for example, the subset of positive rationals (or reals) lacks a minimum.
Other properties | Well-founded relation | Wikipedia | 441 | 319712 | https://en.wikipedia.org/wiki/Well-founded%20relation | Mathematics | Order theory | null |
If is a well-founded relation and is an element of , then the descending chains starting at are all finite, but this does not mean that their lengths are necessarily bounded. Consider the following example:
Let be the union of the positive integers with a new element ω that is bigger than any integer. Then is a well-founded set, but
there are descending chains starting at ω of arbitrary great (finite) length;
the chain has length for any .
The Mostowski collapse lemma implies that set membership is a universal among the extensional well-founded relations: for any set-like well-founded relation on a class that is extensional, there exists a class such that is isomorphic to .
Reflexivity
A relation is said to be reflexive if holds for every in the domain of the relation. Every reflexive relation on a nonempty domain has infinite descending chains, because any constant sequence is a descending chain. For example, in the natural numbers with their usual order ≤, we have . To avoid these trivial descending sequences, when working with a partial order ≤, it is common to apply the definition of well foundedness (perhaps implicitly) to the alternate relation < defined such that if and only if and . More generally, when working with a preorder ≤, it is common to use the relation < defined such that if and only if and . In the context of the natural numbers, this means that the relation <, which is well-founded, is used instead of the relation ≤, which is not. In some texts, the definition of a well-founded relation is changed from the definition above to include these conventions. | Well-founded relation | Wikipedia | 332 | 319712 | https://en.wikipedia.org/wiki/Well-founded%20relation | Mathematics | Order theory | null |
The Rutherford scattering experiments were a landmark series of experiments by which scientists learned that every atom has a nucleus where all of its positive charge and most of its mass is concentrated. They deduced this after measuring how an alpha particle beam is scattered when it strikes a thin metal foil. The experiments were performed between 1906 and 1913 by Hans Geiger and Ernest Marsden under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester.
The physical phenomenon was explained by Rutherford in a classic 1911 paper that eventually lead to the widespread use of scattering in particle physics to study subatomic matter. Rutherford scattering or Coulomb scattering is the elastic scattering of charged particles by the Coulomb interaction. The paper also initiated the development of the planetary Rutherford model of the atom and eventually the Bohr model.
Rutherford scattering is now exploited by the materials science community in an analytical technique called Rutherford backscattering.
Summary
Thomson's model of the atom
The prevailing model of atomic structure before Rutherford's experiments was devised by J. J. Thomson. Thomson had discovered the electron through his work on cathode rays and proposed that they existed within atoms, and an electric current is electrons hopping from one atom to an adjacent one in a series. There logically had to be a commensurate amount of positive charge to balance the negative charge of the electrons and hold those electrons together. Having no idea what the source of this positive charge was, he tentatively proposed that the positive charge was everywhere in the atom, adopting a spherical shape for simplicity. Thomson imagined that the balance of electrostatic forces would distribute the electrons throughout this sphere in a more or less even manner. Thomson also believed the electrons could move around in this sphere, and in that regard he likened the substance of the sphere to a liquid. In fact the positive sphere was more of an abstraction than anything material. He did not propose a positively-charged subatomic particle; a counterpart to the electron.
Thomson was never able to develop a complete and stable model that could predict any of the other known properties of the atom, such as emission spectra and valencies. The Japanese scientist Hantaro Nagaoka rejected Thomson's model on the grounds that opposing charges cannot penetrate each other. He proposed instead that electrons orbit the positive charge like the rings around Saturn. However this model was also known to be unstable. | Rutherford scattering experiments | Wikipedia | 471 | 319834 | https://en.wikipedia.org/wiki/Rutherford%20scattering%20experiments | Physical sciences | Atomic physics | null |
Alpha particles and the Thomson atom
An alpha particle is a positively charged particle of matter that is spontaneously emitted from certain radioactive elements. Alpha particles are so tiny as to be invisible, but they can be detected with the use of phosphorescent screens, photographic plates, or electrodes. Rutherford discovered them in 1899. In 1906, by studying how alpha particle beams are deflected by magnetic and electric fields, he deduced that they were essentially helium atoms stripped of two electrons. Thomson and Rutherford knew nothing about the internal structure of alpha particles. Prior to 1911 they were thought to have a diameter similar to helium atoms and contain ten or so electrons.
Thomson's model was consistent with the experimental evidence available at the time. Thomson studied beta particle scattering which showed small angle deflections modelled as interactions of the particle with many atoms in succession. Each interaction of the particle with the electrons of the atom and the positive background sphere would lead to a tiny deflection, but many such collisions could add up. The scattering of alpha particles was expected to be similar. Rutherford's team would show that the multiple scattering model was not needed: single scattering from a compact charge at the centre of the atom would account for all of the scattering data.
Rutherford, Geiger, and Marsden
Ernest Rutherford was Langworthy Professor of Physics at the Victoria University of Manchester (now the University of Manchester). He had already received numerous honours for his studies of radiation. He had discovered the existence of alpha rays, beta rays, and gamma rays, and had proved that these were the consequence of the disintegration of atoms. In 1906, he received a visit from the German physicist Hans Geiger, and was so impressed that he asked Geiger to stay and help him with his research. Ernest Marsden was a physics undergraduate student studying under Geiger.
In 1908, Rutherford sought to independently determine the charge and mass of alpha particles. To do this, he wanted to count the number of alpha particles and measure their total charge; the ratio would give the charge of a single alpha particle. Alpha particles are too tiny to see, but Rutherford knew about Townsend discharge, a cascade effect from ionisation leading to a pulse of electric current. On this principle, Rutherford and Geiger designed a simple counting device which consisted of two electrodes in a glass tube. (See #1908 experiment.) Every alpha particle that passed through the tube would create a pulse of electricity that could be counted. It was an early version of the Geiger counter. | Rutherford scattering experiments | Wikipedia | 506 | 319834 | https://en.wikipedia.org/wiki/Rutherford%20scattering%20experiments | Physical sciences | Atomic physics | null |
The counter that Geiger and Rutherford built proved unreliable because the alpha particles were being too strongly deflected by their collisions with the molecules of air within the detection chamber. The highly variable trajectories of the alpha particles meant that they did not all generate the same number of ions as they passed through the gas, thus producing erratic readings. This puzzled Rutherford because he had thought that alpha particles were too heavy to be deflected so strongly. Rutherford asked Geiger to investigate how far matter could scatter alpha rays.
The experiments they designed involved bombarding a metal foil with a beam of alpha particles to observe how the foil scattered them in relation to its thickness and material. They used a phosphorescent screen to measure the trajectories of the particles. Each impact of an alpha particle on the screen produced a tiny flash of light. Geiger worked in a darkened lab for hours on end, counting these tiny scintillations using a microscope. For the metal foil, they tested a variety of metals, but favoured gold because they could make the foil very thin, as gold is the most malleable metal. As a source of alpha particles, Rutherford's substance of choice was radium, which is thousands of times more radioactive than uranium.
Scattering theory and the new atomic model
In a 1909 experiment, Geiger and Marsden discovered that the metal foils could scatter some alpha particles in all directions, sometimes more than 90°. This should have been impossible according to Thomson's model. According to Thomson's model, all the alpha particles should have gone straight through.
In Thomson's model of the atom, the sphere of positive charge that fills the atom and encapsulates the electrons is permeable; the electrons could move around in it, after all. Therefore, an alpha particle should be able to pass through this sphere if the electrostatic forces within permit it. Thomson himself did not study how an alpha particle might be scattered in such a collision with an atom, but he did study beta particle scattering. He calculated that a beta particle would only experience very small deflection when passing through an atom, and even after passing through many atoms in a row, the total deflection should still be less than 1°. Alpha particles typically have much more momentum than beta particles and therefore should likewise experience only the slightest deflection. | Rutherford scattering experiments | Wikipedia | 480 | 319834 | https://en.wikipedia.org/wiki/Rutherford%20scattering%20experiments | Physical sciences | Atomic physics | null |
The extreme scattering observed forced Rutherford to revise the model of the atom. The issue in Thomson's model was that the charges were too diffuse to produce a sufficiently strong electrostatic force to cause such repulsion. Therefore they had to be more concentrated. In Rutherford's new model, the positive charge does not fill the entire volume of the atom but instead constitutes a tiny nucleus at least 10,000 times smaller than the atom as a whole. All that positive charge concentrated in a much smaller volume produces a much stronger electric field near its surface. The nucleus also carried most of the atom's mass. This meant that it could deflect alpha particles by up to 180° depending on how close they pass. The electrons surround this nucleus, spread throughout the atom's volume. Because their negative charge is diffuse and their combined mass is low, they have a negligible effect on the alpha particle.
To verify his model, Rutherford developed a scientific model to predict the intensity of alpha particles at the different angles they scattered coming out of the gold foil, assuming all of the positive charge was concentrated at the centre of the atom. This model was validated in an experiment performed in 1913. His model explained both the beta scattering results of Thomson and the alpha scattering results of Geiger and Marsden.
Legacy
There was little reaction to Rutherford's now-famous 1911 paper in the first years. The paper was primarily about alpha particle scattering in an era before particle scattering was a primary tool for physics. The probability techniques he used and confusing collection of observations involved were not immediately compelling.
Nuclear physics
The first impacts were to encourage new focus on scattering experiments. For example the first results from a cloud chamber, by C.T.R. Wilson shows alpha particle scattering and also appeared in 1911. Over time, particle scattering became a major aspect of theoretical and experimental physics; Rutherford's concept of a "cross-section" now dominates the descriptions of experimental particle physics. The historian Silvan S. Schweber suggests that Rutherford's approach marked the shift to viewing all interactions and measurements in physics as scattering processes. After the nucleus - a term Rutherford introduced in 1912 - became the accepted model for the core of atoms, Rutherford's analysis of the scattering of alpha particles created a new branch of physics, nuclear physics.
Atomic model | Rutherford scattering experiments | Wikipedia | 468 | 319834 | https://en.wikipedia.org/wiki/Rutherford%20scattering%20experiments | Physical sciences | Atomic physics | null |
Rutherford's new atom model caused no stir. Rutherford explicitly ignores the electrons, only mentioning Hantaro Nagaoka's Saturnian model of electrons orbiting a tiny "sun", a model that had been previously rejected as mechanically unstable. By ignoring the electrons Rutherford also ignores any potential implications for atomic spectroscopy for chemistry. Rutherford himself did not press the case for his atomic model: his own 1913 book on "Radioactive substances and their radiations" only mentions the atom twice; other books by other authors around this time focus on Thomson's model.
The impact of Rutherford's nuclear model came after Niels Bohr arrived as a post-doctoral student in Manchester at Rutherford's invitation. Bohr dropped his work on the Thomson model in favour of Rutherford's nuclear model, developing the Rutherford–Bohr model over the next several years. Eventually Bohr incorporated early ideas of quantum mechanics into the model of the atom, allowing prediction of electronic spectra and concepts of chemistry.
Hantaro Nagaoka, who had proposed a Saturnian model of the atom, wrote to Rutherford from Tokyo in 1911: "I have been struck with the simpleness of the apparatus you employ and the brilliant results you obtain." The astronomer Arthur Eddington called Rutherford's discovery the most important scientific achievement since Democritus proposed the atom ages earlier. Rutherford has since been hailed as "the father of nuclear physics".
In a lecture delivered on 15 October 1936 at Cambridge University, Rutherford described his shock at the results of the 1909 experiment:
Rutherford's claim of surprise makes a good story but by the time of the Geiger-Marsden experiment the result confirmed suspicions Rutherford developed from his many previous experiments.
Experiments | Rutherford scattering experiments | Wikipedia | 339 | 319834 | https://en.wikipedia.org/wiki/Rutherford%20scattering%20experiments | Physical sciences | Atomic physics | null |
Alpha particle scattering: 1906 and 1908 experiments
Rutherford's first steps towards his discovery of the nature of the atom came from his work to understand alpha particles.
In 1906, Rutherford noticed that alpha particles passing through sheets of mica were deflected by the sheets by as much as 2 degrees. Rutherford placed a radioactive source in a sealed tube ending with a narrow slits followed by a photographic plate. Half of the slit was covered by a thin layer of mica. A magnetic field around the tube was altered every 10 minutes to reject the effect of beta rays, known to be sensitive to magnetic fields. The tube was evacuated to different amounts and a series of images recorded. At the lowest pressure the image of the open slit was clear, while images of the mica covered slit or the open slit at higher pressures were fuzzy. Rutherford explained these results as alpha-particle scattering in a paper published in 1906. He already understood the implications of the observation for models of atoms: "such a result brings out clearly the fact that the atoms of matter must be the seat of very intense electrical forces".
A 1908 paper by Geiger, On the Scattering of α-Particles by Matter, describes the following experiment. He constructed a long glass tube, nearly two metres long. At one end of the tube was a quantity of "radium emanation" (R) as a source of alpha particles. The opposite end of the tube was covered with a phosphorescent screen (Z). In the middle of the tube was a 0.9 mm-wide slit. The alpha particles from R passed through the slit and created a glowing patch of light on the screen. A microscope (M) was used to count the scintillations on the screen and measure their spread. Geiger pumped all the air out of the tube so that the alpha particles would be unobstructed, and they left a neat and tight image on the screen that corresponded to the shape of the slit. Geiger then allowed some air into the tube, and the glowing patch became more diffuse. Geiger then pumped out the air and placed one or two gold foils over the slit at AA. This too caused the patch of light on the screen to become more spread out, with the larger spread for two layers. This experiment demonstrated that both air and solid matter could markedly scatter alpha particles.
Alpha particle reflection: the 1909 experiment | Rutherford scattering experiments | Wikipedia | 486 | 319834 | https://en.wikipedia.org/wiki/Rutherford%20scattering%20experiments | Physical sciences | Atomic physics | null |
The results of the initial alpha particle scattering experiments were confusing. The angular spread of the particle on the screen varied greatly with the shape of the apparatus and its internal pressure. Rutherford suggested that Ernest Marsden, a physics undergraduate student studying under Geiger, should look for diffusely reflected or back-scattered alpha particles, even though these were not expected. Marsden's first crude reflector got results, so Geiger helped him create a more sophisticated apparatus. They were able to demonstrate that 1 in 8000 alpha particle collisions were diffuse reflections. Although this fraction was small, it was much larger than the Thomson model of the atom could explain.
These results where published in a 1909 paper, On a Diffuse Reflection of the α-Particles, where Geiger and Marsden described the experiment by which they proved that alpha particles can indeed be scattered by more than 90°. In their experiment, they prepared a small conical glass tube (AB) containing "radium emanation" (radon), "radium A" (actual radium), and "radium C" (bismuth-214); its open end was sealed with mica. This was their alpha particle emitter. They then set up a lead plate (P), behind which they placed a fluorescent screen (S). The tube was held on the opposite side of plate, such that the alpha particles it emitted could not directly strike the screen. They noticed a few scintillations on the screen because some alpha particles got around the plate by bouncing off air molecules. They then placed a metal foil (R) to the side of the lead plate. They tested with lead, gold, tin, aluminium, copper, silver, iron, and platinum. They pointed the tube at the foil to see if the alpha particles would bounce off it and strike the screen on the other side of the plate, and observed an increase in the number of scintillations on the screen. Counting the scintillations, they observed that metals with higher atomic mass, such as gold, reflected more alpha particles than lighter ones such as aluminium. | Rutherford scattering experiments | Wikipedia | 425 | 319834 | https://en.wikipedia.org/wiki/Rutherford%20scattering%20experiments | Physical sciences | Atomic physics | null |
Geiger and Marsden then wanted to estimate the total number of alpha particles that were reflected. The previous setup was unsuitable for doing this because the tube contained several radioactive substances (radium plus its decay products) and thus the alpha particles emitted had varying ranges, and because it was difficult for them to ascertain at what rate the tube was emitting alpha particles. This time, they placed a small quantity of radium C (bismuth-214) on the lead plate, which bounced off a platinum reflector (R) and onto the screen. They concluded that approximately 1 in 8,000 of the alpha particles that struck the reflector bounced onto the screen. By measuring the reflection from thin foils they showed that the effect due to a volume and not a surface effect. When contrasted with the vast number of alpha particles that pass unhindered through a metal foil, this small number of large angle reflections was a strange result that meant very large forces were involved.
Dependence on foil material and thickness: the 1910 experiment
A 1910 paper by Geiger, The Scattering of the α-Particles by Matter, describes an experiment to measure how the most probable angle through which an alpha particle is deflected varies with the material it passes through, the thickness of the material, and the velocity of the alpha particles. He constructed an airtight glass tube from which the air was pumped out. At one end was a bulb (B) containing "radium emanation" (radon-222). By means of mercury, the radon in B was pumped up the narrow glass pipe whose end at A was plugged with mica. At the other end of the tube was a fluorescent zinc sulfide screen (S). The microscope which he used to count the scintillations on the screen was affixed to a vertical millimetre scale with a vernier, which allowed Geiger to precisely measure where the flashes of light appeared on the screen and thus calculate the particles' angles of deflection. The alpha particles emitted from A was narrowed to a beam by a small circular hole at D. Geiger placed a metal foil in the path of the rays at D and E to observe how the zone of flashes changed. He tested gold, tin, silver, copper, and aluminium. He could also vary the velocity of the alpha particles by placing extra sheets of mica or aluminium at A. | Rutherford scattering experiments | Wikipedia | 487 | 319834 | https://en.wikipedia.org/wiki/Rutherford%20scattering%20experiments | Physical sciences | Atomic physics | null |
From the measurements he took, Geiger came to the following conclusions:
the most probable angle of deflection increases with the thickness of the material
the most probable angle of deflection is proportional to the atomic mass of the substance
the most probable angle of deflection decreases with the velocity of the alpha particles
Rutherford's Structure of the Atom paper (1911)
Considering the results of these experiments, Rutherford published a landmark paper in 1911 titled "The Scattering of α and β Particles by Matter and the Structure of the Atom" wherein he showed that single scattering from a very small and intense electric charge predicts primarily small-angle scattering with small but measurable amounts of backscattering. For the purpose of his mathematical calculations he assumed this central charge was positive, but he admitted he could not prove this and that he had to wait for other experiments to develop his theory.
Rutherford developed a mathematical equation that modelled how the foil should scatter the alpha particles if all the positive charge and most of the atomic mass was concentrated in a point at the centre of an atom.
From the scattering data, Rutherford estimated the central charge qn to be about +100 units.
Rutherford's paper does not discuss any electron arrangement beyond discussions on the scattering from Thomson's plum pudding model and Nagaoka's Saturnian model. He shows that the scattering results predicted by Thomson's model are also explained by single scattering, but that Thomson's model does not explain large angle scattering. He says that Nagaoka's model, having a compact charge, would agree with the scattering data. The Saturnian model had previously been rejected on other grounds. The so-called Rutherford model of the atom with orbiting electrons was not proposed by Rutherford in the 1911 paper.
Confirming the scattering theory: the 1913 experiment
In a 1913 paper, The Laws of Deflexion of α Particles through Large Angles, Geiger and Marsden describe a series of experiments by which they sought to experimentally verify Rutherford's equation. Rutherford's equation predicted that the number of scintillations per minute s that will be observed at a given angle Φ should be proportional to:
cosec4
thickness of foil t
magnitude of the square of central charge Qn
Their 1913 paper describes four experiments by which they proved each of these four relationships. | Rutherford scattering experiments | Wikipedia | 462 | 319834 | https://en.wikipedia.org/wiki/Rutherford%20scattering%20experiments | Physical sciences | Atomic physics | null |
To test how the scattering varied with the angle of deflection (i.e. if s ∝ csc4). Geiger and Marsden built an apparatus that consisted of a hollow metal cylinder mounted on a turntable. Inside the cylinder was a metal foil (F) and a radiation source containing radon (R), mounted on a detached column (T) which allowed the cylinder to rotate independently. The column was also a tube by which air was pumped out of the cylinder. A microscope (M) with its objective lens covered by a fluorescent zinc sulfide screen (S) penetrated the wall of the cylinder and pointed at the metal foil. They tested with silver and gold foils. By turning the table, the microscope could be moved a full circle around the foil, allowing Geiger to observe and count alpha particles deflected by up to 150°. Correcting for experimental error, Geiger and Marsden found that the number of alpha particles that are deflected by a given angle Φ is indeed proportional to csc4.
Geiger and Marsden then tested how the scattering varied with the thickness of the foil (i.e. if s ∝ t). They constructed a disc (S) with six holes drilled in it. The holes were covered with metal foil (F) of varying thickness, or none for control. This disc was then sealed in a brass ring (A) between two glass plates (B and C). The disc could be rotated by means of a rod (P) to bring each window in front of the alpha particle source (R). On the rear glass pane was a zinc sulfide screen (Z). Geiger and Marsden found that the number of scintillations that appeared on the screen was indeed proportional to the thickness, as long as the thickness was small. | Rutherford scattering experiments | Wikipedia | 370 | 319834 | https://en.wikipedia.org/wiki/Rutherford%20scattering%20experiments | Physical sciences | Atomic physics | null |
Geiger and Marsden reused the apparatus to measure how the scattering pattern varied with the square of the nuclear charge (i.e. if s ∝ Qn2). Geiger and Marsden did not know what the positive charge of the nucleus of their metals were (they had only just discovered the nucleus existed at all), but they assumed it was proportional to the atomic weight, so they tested whether the scattering was proportional to the atomic weight squared. Geiger and Marsden covered the holes of the disc with foils of gold, tin, silver, copper, and aluminium. They measured each foil's stopping power by equating it to an equivalent thickness of air. They counted the number of scintillations per minute that each foil produced on the screen. They divided the number of scintillations per minute by the respective foil's air equivalent, then divided again by the square root of the atomic weight (Geiger and Marsden knew that for foils of equal stopping power, the number of atoms per unit area is proportional to the square root of the atomic weight). Thus, for each metal, Geiger and Marsden obtained the number of scintillations that a fixed number of atoms produce. For each metal, they then divided this number by the square of the atomic weight, and found that the ratios were about the same. Thus they proved that s ∝ Qn2.
Finally, Geiger and Marsden tested how the scattering varied with the velocity of the alpha particles (i.e. if s ∝ ). Using the same apparatus, they slowed the alpha particles by placing extra sheets of mica in front of the alpha particle source. They found that, within the range of experimental error, the number of scintillations was indeed proportional to .
Positive charge on nucleus: 1913
In his 1911 paper (see above), Rutherford assumed that the central charge of the atom was positive, but a negative charge would have fitted his scattering model just as well. In a 1913 paper, Rutherford declared that the "nucleus" (as he now called it) was indeed positively charged, based on the result of experiments exploring the scattering of alpha particles in various gases. | Rutherford scattering experiments | Wikipedia | 439 | 319834 | https://en.wikipedia.org/wiki/Rutherford%20scattering%20experiments | Physical sciences | Atomic physics | null |
In 1917, Rutherford and his assistant William Kay began exploring the passage of alpha particles through gases such as hydrogen and nitrogen. In this experiment, they shot a beam of alpha particles through hydrogen, and they carefully placed their detector—a zinc sulfide screen—just beyond the range of the alpha particles, which were absorbed by the gas. They nonetheless picked up charged particles of some sort causing scintillations on the screen. Rutherford interpreted this as alpha particles knocking the hydrogen nuclei forwards in the direction of the beam, not backwards.
Rutherford's scattering model
Rutherford begins his 1911 paper with a discussion of Thomson's results on scattering of beta particles, a form of radioactivity that results in high velocity electrons. Thomson's model had electrons circulating inside of a sphere of positive charge. Rutherford highlights the need for compound or multiple scattering events: the deflections predicted for each collision are much less than one degree. He then proposes a model which will produce large deflections on a single encounter: place all of the positive charge at the centre of the sphere and ignore the electron scattering as insignificant. The concentrated charge will explain why most alpha particles do not scatter to any measurable degree – they fly past too far from the charge – and yet particles that do pass very close to the centre scatter through large angles.
Maximum nuclear size estimate
Rutherford begins his analysis by considering a head-on collision between the alpha particle and atom. This will establish the minimum distance between them, a value which will be used throughout his calculations.
Assuming there are no external forces and that initially the alpha particles are far from the nucleus, the inverse-square law between the charges on the alpha particle and nucleus gives the potential energy gained by the particle as it approaches the nucleus. For head-on collisions between alpha particles and the nucleus, all the kinetic energy of the alpha particle is turned into potential energy and the particle stops and turns back.
Where the particle stops at a distance from the centre, the potential energy matches the original kinetic energy:
where
Rearranging: | Rutherford scattering experiments | Wikipedia | 411 | 319834 | https://en.wikipedia.org/wiki/Rutherford%20scattering%20experiments | Physical sciences | Atomic physics | null |
For an alpha particle:
(mass) = =
(for the alpha particle) = 2 × =
(for gold) = 79 × =
(initial velocity) = (for this example)
The distance from the alpha particle to the centre of the nucleus () at this point is an upper limit for the nuclear radius.
Substituting these in gives the value of about , or 27 fm. (The true radius is about 7.3 fm.) The true radius of the nucleus is not recovered in these experiments because the alphas do not have enough energy to penetrate to more than 27 fm of the nuclear centre, as noted, when the actual radius of gold is 7.3 fm.
Rutherford's 1911 paper started with a slightly different formula suitable for head-on collision with a sphere of positive charge:
In Rutherford's notation, e is the elementary charge, N is the charge number of the nucleus (now also known as the atomic number), and E is the charge of an alpha particle. The convention in Rutherford's time was to measure charge in electrostatic units, distance in centimeters, force in dynes, and energy in ergs. The modern convention is to measure charge in coulombs, distance in meters, force in newtons, and energy in joules. Using coulombs requires using the Coulomb constant (k) in the equation. Rutherford used b as the turning point distance (called rmin above) and R is the radius of the atom. The first term is the Coulomb repulsion used above. This form assumes the alpha particle could penetrate the positive charge. At the time of Rutherford's paper, Thomson's plum pudding model proposed a positive charge with the radius of an atom, thousands of times larger than the rmin found above. Figure 1 shows how concentrated this potential is compared to the size of the atom.
Many of Rutherford's results are expressed in terms of this turning point distance rmin, simplifying the results and limiting the need for units to this calculation of turning point. | Rutherford scattering experiments | Wikipedia | 416 | 319834 | https://en.wikipedia.org/wiki/Rutherford%20scattering%20experiments | Physical sciences | Atomic physics | null |
Single scattering by a heavy nucleus
From his results for a head on collision, Rutherford knows that alpha particle scattering occurs close to the centre of an atom, at a radius 10,000 times smaller than the atom. The electrons have negligible effect. He begins by assuming no energy loss in the collision, that is he ignores the recoil of the target atom. He will revisit each of these issues later in his paper.
Under these conditions, the alpha particle and atom interact through a central force, a physical problem studied first by Isaac Newton.
A central force only acts along a line between the particles and when the force varies with the inverse square, like Coulomb force in this case, a detailed theory was developed under the name of the Kepler problem. The well-known solutions to the Kepler problem are called orbits and unbound orbits are hyperbolas.
Thus Rutherford proposed that the alpha particle will take a hyperbolic trajectory in the repulsive force near the centre of the atom as shown in Figure 2.
To apply the hyperbolic trajectory solutions to the alpha particle problem, Rutherford expresses the parameters of the hyperbola in terms of the scattering geometry and energies. He starts with conservation of angular momentum. When the particle of mass and initial velocity is far from the atom, its angular momentum around the centre of the atom will be where is the impact parameter, which is the lateral distance between the alpha particle's path and the atom. At the point of closest approach, labeled A in Figure 2, the angular momentum will be . Therefore
Rutherford also applies the law of conservation of energy between the same two points:
The left hand side and the first term on the right hand side are the kinetic energies of the particle at the two points; the last term is the potential energy due to the Coulomb force between the alpha particle and atom at the point of closest approach (A). qa is the charge of the alpha particle, qg is the charge of the nucleus, and k is the Coulomb constant.
The energy equation can then be rearranged thus:
For convenience, the non-geometric physical variables in this equation can be contained in a variable , which is the point of closest approach in a head-on collision scenario which was explored in a previous section of this article:
This allows Rutherford simplify the energy equation to:
This leaves two simultaneous equations for , the first derived from the conservation of momentum equation and the second from the conservation of energy equation. Eliminating and gives at a new formula for : | Rutherford scattering experiments | Wikipedia | 507 | 319834 | https://en.wikipedia.org/wiki/Rutherford%20scattering%20experiments | Physical sciences | Atomic physics | null |
The next step is to find a formula for . From Figure 2, is the sum of two distances related to the hyperbola, SO and OA. Using the following logic, these distances can be expressed in terms of angle and impact parameter .
The eccentricity of a hyperbola is a value that describes the hyperbola's shape. It can be calculated by dividing the focal distance by the length of the semi-major axis, which per Figure 2 is . As can be seen in Figure 3, the eccentricity is also equal to , where is the angle between the major axis and the asymptote. Therefore:
As can be deduced from Figure 2, the focal distance SO is
and therefore
With these formulas for SO and OA, the distance can be written in terms of and simplified using a trigonometric identity known as a half-angle formula:
Applying a trigonometric identity known as the cotangent double angle formula and the previous equation for gives a simpler relationship between the physical and geometric variables:
The scattering angle of the particle is and therefore . With the help of a trigonometric identity known as a reflection formula, the relationship between θ and b can be resolved to:
which can be rearranged to give
Rutherford gives some illustrative values as shown in this table:
Rutherford's approach to this scattering problem remains a standard treatment in textbooks on classical mechanics.
Intensity vs angle
To compare to experiments the relationship between impact parameter and scattering angle needs to be converted to probability versus angle. The scattering cross section gives the relative intensity by angles:
In classical mechanics, the scattering angle is uniquely determined the initial kinetic energy of the incoming particles and the impact parameter . Therefore, the number of particles scattered into an angle between and must be the same as the number of particles with associated impact parameters between and . For an incident intensity , this implies:
Thus the cross section depends on scattering angle as:
Using the impact parameter as a function of angle, , from the single scattering result above produces the Rutherford scattering cross section:
s = the number of alpha particles falling on unit area at an angle of deflection Φ
r = distance from point of incidence of α rays on scattering material
X = total number of particles falling on the scattering material
n = number of atoms in a unit volume of the material
t = thickness of the foil
qn = positive charge of the atomic nucleus
qa = positive charge of the alpha particles
m = mass of an alpha particle
v = velocity of the alpha particle | Rutherford scattering experiments | Wikipedia | 502 | 319834 | https://en.wikipedia.org/wiki/Rutherford%20scattering%20experiments | Physical sciences | Atomic physics | null |
This formula predicted the results that Geiger measured in the coming year. The scattering probability into small angles greatly exceeds the probability in to larger angles, reflecting the tiny nucleus surrounded by empty space. However, for rare close encounters, large angle scattering occurs with just a single target.
At the end of his development of the cross section formula, Rutherford emphasises that the results apply to single scattering and thus require measurements with thin foils. For thin foils the degree of scattering is proportional to the foil thickness in agreement with Geiger's measurements.
Comparison to JJ Thomson's results
At the time of Rutherford's paper, JJ Thomson was the "undisputed world master in the design of atoms". Rutherford needed to compare his new approach to Thomson's. Thomson's model, presented in 1910, modelled the electron collisions with hyperbolic orbits from his 1906 paper combined with a factor for the positive sphere. Multiple resulting small deflections compounded using a random walk.
In his paper Rutherford emphasised that single scattering alone could account for Thomson's results if the positive charge were concentrated in the centre.
Rutherford computes the probability of single scattering from a compact charge and demonstrates that it is 3 times larger than Thomson's multiple scattering probability. Rutherford completes his analysis including the effects of density and foil thickness, then concludes that thin foils are governed by single scattering, not multiple scattering.
Later analysis showed Thomson's scattering model could not account for large scattering.
The maximum angular deflection from electron scattering or from the positive sphere each come to less than 0.02°; even many such scattering events compounded would result in less than a one degree average deflection and a probability of scattering through 90° of less than one in 103500.
Target recoil
Rutherford's analysis assumed that alpha particle trajectories turned at the centre of the atom but the exit velocity was not reduced. This is equivalent to assuming that the concentrated charge at the centre had infinite mass or was anchored in place. Rutherford discusses the limitations of this assumption by comparing scattering from lighter atoms like aluminium with heavier atoms like gold. If the concentrated charge is lighter it will recoil from the interaction, gaining momentum while the alpha particle loses momentum and consequently slows down. | Rutherford scattering experiments | Wikipedia | 453 | 319834 | https://en.wikipedia.org/wiki/Rutherford%20scattering%20experiments | Physical sciences | Atomic physics | null |
Modern treatments analyze this type of Coulomb scattering in the centre of mass reference frame. The six coordinates of the two particles (also called "bodies") are converted into three relative coordinates between the two particles and three centre-of-mass coordinates moving in space (called the lab frame). The interaction only occurs in the relative coordinates, giving an equivalent one-body problem just as Rutherford solved, but with different interpretations for the mass and scattering angle.
Rather than the mass of the alpha particle, the more accurate formula including recoil uses reduced mass:
For Rutherford's alpha particle scattering from gold, with mass of 197, the reduced mass is very close to the mass of the alpha particle:
For lighter aluminium, with mass 27, the effect is greater:
a 13% difference in mass. Rutherford notes this difference and suggests experiments be performed with lighter atoms.
The second effect is a change in scattering angle. The angle in the relative coordinate system or centre of mass frame needs to be converted to an angle in the lab frame. In the lab frame, denoted by a subscript L, the scattering angle for a general central potential is
For a heavy particle like gold used by Rutherford, the factor can be neglected at almost all angles. Then the lab and relative angles are the same, .
The change in scattering angle alters the formula for differential cross-section needed for comparison to experiment. For any central potential, the differential cross-section in the lab frame is related to that in the centre-of-mass frame by
where
Limitations to Rutherford's scattering formula
Very light nuclei and higher energies
In 1919 Rutherford analyzed alpha particle scattering from hydrogen atoms, showing the limits of the 1911 formula even with corrections for reduced mass. Similar issues with smaller deviations for helium, magnesium, aluminium lead to the conclusion that the alpha particle was penetrating the nucleus in these cases. This allowed the first estimates of the size of atomic nuclei. Later experiments based on cyclotron acceleration of alpha particles striking heavier nuclei provided data for analysis of interaction between the alpha particle and the nuclear surface. However at energies that push the alpha particles deeper they are strongly absorbed by the nuclei, a more complex interaction.
Quantum mechanics
Rutherford's treatment of alpha particle scattering seems to rely on classical mechanics and yet the particles are of sub-atomic dimensions. However the critical aspects of the theory ultimately rely on conservation of momentum and energy. These concepts apply equally in classical and quantum regimes: the scattering ideas developed by Rutherford apply to subatomic elastic scattering problems like neutron-proton scattering. | Rutherford scattering experiments | Wikipedia | 510 | 319834 | https://en.wikipedia.org/wiki/Rutherford%20scattering%20experiments | Physical sciences | Atomic physics | null |
An alternative method to find the scattering angle
This section presents an alternative method to find the relation between the impact parameter and deflection angle in a single-atom encounter, using a force-centric approach as opposed to the energy-centric one that Rutherford used.
The scattering geometry is shown in this diagram
The impact parameter b is the distance between the alpha particle's initial trajectory and a parallel line that goes through the nucleus. Smaller values of b bring the particle closer to the atom so it feels more deflection force resulting in a larger deflection angle θ. The goal is to find the relationship between b and the deflection angle.
The alpha particle's path is a hyperbola and the net change in momentum runs along the axis of symmetry. From the geometry in the diagram and the magnitude of the initial and final momentum vectors, , the magnitude of can be related to the deflection angle:
A second formula for involving b will give the relationship to the deflection angle.
The net change in momentum can also be found by adding small increments to momentum all along the trajectory using the integral
where is the distance between the alpha particle and the centre of the nucleus and is its angle from the axis of symmetry. These two are the polar coordinates of the alpha particle at time .
Here the Coulomb force exerted along the line between the alpha particle and the atom is and the factor gives that part of the force causing deflection.
The polar coordinates r and φ depend on t in the integral, but they must be related to each other as they both vary as the particle moves. Changing the variable and limits of integration from t to φ makes this connection explicit:
The factor is the reciprocal of the angular velocity the particle. Since the force is only along the line between the particle and the atom, the angular momentum, which is proportional to the angular velocity, is constant:
This law of conservation of angular momentum gives a formula for :
Replacing in the integral for ΔP simultaneously eliminates the dependence on r:
Applying the trigonometric identities and to simplify this result gives the second formula for :
Solving for θ as a function of b gives the final result | Rutherford scattering experiments | Wikipedia | 442 | 319834 | https://en.wikipedia.org/wiki/Rutherford%20scattering%20experiments | Physical sciences | Atomic physics | null |
Why the plum pudding model was wrong
J. J. Thomson himself didn't study alpha particle scattering, but he did study beta particle scattering. In his 1910 paper "On the Scattering of rapidly moving Electrified Particles", Thomson presented equations that modelled how beta particles scatter in a collision with an atom. Rutherford adapted those equations to alpha particle scattering in his 1911 paper "The Scattering of α and β Particles by Matter and the Structure of the Atom".
Deflection by the positive sphere
In Thomson's 1910 paper "On the Scattering of rapidly moving Electrified Particles", Thomson presented the following equation (in this article's notation) that isolates the effect of the positive sphere in the plum pudding model on an incoming beta particle.
Thomson did not explain how he arrived at this equation, but this section provides an educated guess and at the same time adapts the equation to alpha particle scattering.
Consider an alpha particle passing by a positive sphere of pure positive charge (no electrons) with a radius R and mass equal to those of a gold atom. The alpha particle passes just close enough to graze the edge of the sphere, which is where the electric field of the sphere is strongest.
An earlier section of this article presented an equation which models how an incoming charged particle is deflected by another charged particle at a fixed position.
This equation can be used to calculate the deflection angle in the special case in Figure 4 by setting the impact parameter b to the same value as the radius of the sphere R. So long as the alpha particle does not penetrate the sphere, there is no practical difference between a sphere of charge and a point charge.
qg = positive charge of the gold atom = =
qa = charge of the alpha particle = =
R = radius of the gold atom =
v = speed of the alpha particle =
m = mass of the alpha particle =
k = Coulomb constant =
This shows that the largest possible deflection will be very small, to the point that the path of the alpha particle passing through the positive sphere of a gold atom is almost a straight line. Therefore in computing the average deflection, which will be smaller still, we will treat the particle's path through the sphere as a chord of length L.
Inside a sphere of uniformly distributed positive charge, the force exerted on the alpha particle at any point along its path through the sphere is
The lateral component of this force is
The lateral change in momentum py is therefore
The deflection angle is given by | Rutherford scattering experiments | Wikipedia | 507 | 319834 | https://en.wikipedia.org/wiki/Rutherford%20scattering%20experiments | Physical sciences | Atomic physics | null |
where px is the average horizontal momentum, which is first reduced then restored as horizontal force changes direction as the alpha particle goes across the sphere. Since the deflection is very small, can be treated as equal to . The chord length , per Pythagorean theorem.
The average deflection angle sums the angle for values of b and L across the entire sphere and divides by the cross-section of the sphere:
This matches Thomson's formula in his 1910 paper.
Deflection by the electrons
Consider an alpha particle passing through an atom of radius R along a path of length L. The effect of the positive sphere is ignored so as to isolate the effect of the atomic electrons. As with the positive sphere, deflection by the electrons is expected to be very small, to the point that the path is practically a straight line.
For the electrons within an arbitrary distance s of the alpha particle's path, their mean distance will be s. Therefore, the average deflection per electron will be
where qe is the elementary charge. The average net deflection by all the electrons within this arbitrary cylinder of effect around the alpha particle's path is
where N0 is the number of electrons per unit volume and is the volume of this cylinder.
Treating L as a straight line, where b is the distance of this line from the centre. The mean of is therefore
To obtain the mean deflection , replace in the equation for :
where N is the number of electrons in the atom, equal to .
Cumulative effect
Applying Thomson's equations described above to an alpha particle colliding with a gold atom, using the following values:
qg = positive charge of the gold atom = =
qa = charge of the alpha particle = =
qe = elementary charge =
R = radius of the gold atom =
v = speed of the alpha particle =
m = mass of the alpha particle =
k = Coulomb constant =
N = number of electrons in the gold atom = 79
gives the average angle by which the alpha particle should be deflected by the atomic electrons as:
The average angle by which an alpha particle should be deflected by the positive sphere is:
The net deflection for a single atomic collision is: | Rutherford scattering experiments | Wikipedia | 455 | 319834 | https://en.wikipedia.org/wiki/Rutherford%20scattering%20experiments | Physical sciences | Atomic physics | null |
On average the positive sphere and the electrons alike provide very little deflection in a single collision. Thomson's model combined many single-scattering events from the atom's electrons and a positive sphere. Each collision may increase or decrease the total scattering angle. Only very rarely would a series of collisions all line up in the same direction. The result is similar to the standard statistical problem called a random walk. If the average deflection angle of the alpha particle in a single collision with an atom is , then the average deflection after n collisions is
The probability that an alpha particle will be deflected by a total of more than 90° after n deflections is given by:
where e is Euler's number (≈2.71828...). A gold foil with a thickness of 1.5 micrometers would be about 10,000 atoms thick. If the average deflection per atom is 0.008°, the average deflection after 10,000 collisions would be 0.8°. The probability of an alpha particle being deflected by more than 90° will be
While in Thomson's plum pudding model it is mathematically possible that an alpha particle could be deflected by more than 90° after 10,000 collisions, the probability of such an event is so low as to be undetectable. This extremely small number shows that Thomson's model cannot explain the results of the Geiger-Mardsen experiment of 1909. | Rutherford scattering experiments | Wikipedia | 306 | 319834 | https://en.wikipedia.org/wiki/Rutherford%20scattering%20experiments | Physical sciences | Atomic physics | null |
A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions. Between the generating station and consumer, electric power may flow through several substations at different voltage levels. A substation may include transformers to change voltage levels between high transmission voltages and lower distribution voltages, or at the interconnection of two different transmission voltages. They are a common component of the infrastructure. There are 55,000 substations in the United States.
Substations may be owned and operated by an electrical utility, or may be owned by a large industrial or commercial customer. Generally substations are unattended, relying on SCADA for remote supervision and control.
The word substation comes from the days before the distribution system became a grid. As central generation stations became larger, smaller generating plants were converted to distribution stations, receiving their energy supply from a larger plant instead of using their own generators. The first substations were connected to only one power station, where the generators were housed, and were subsidiaries of that power station.
Construction
Substations may be designed and built by a contractor or alternately all phases of its development may be handled by the electrical utility. Most commonly, the utility does the engineering and procurement while hiring a contractor for actual construction. Major design constraints for construction of substations include land availability and cost, limitations on the construction period, transportation restrictions, and the need to get the substation running quickly. Prefabrication is a common way to reduce the construction cost. For connecting the new substation, a partial outage at another substation may be required, but the utility often tries to minimize downtime.
Types
Substations typically serve at least one of the following purposes:
Increasing the voltage produced by electric power generation for efficient transmission over long distances, using step-up transformers
Interconnection of different power grids
Reducing the voltage from transmission to lower-voltage distribution lines that supply individual homes or businesses
Converting from alternating current (AC) to direct current (DC) | Substation | Wikipedia | 432 | 320217 | https://en.wikipedia.org/wiki/Substation | Technology | Electricity transmission and distribution | null |
Transmission substation
A transmission substation connects two or more transmission lines. The simplest case is where all transmission lines have the same voltage. In such cases, substation contains high-voltage switches that allow lines to be connected or isolated for fault clearance or maintenance. A transmission station may have transformers to convert between two transmission voltages, voltage control/power factor correction devices such as capacitors, reactors or static VAR compensators and equipment such as phase shifting transformers to control power flow between two adjacent power systems.
Transmission substations can range from simple to complex. A small "switching station" may be little more than a bus plus some circuit breakers. The largest transmission substations can cover a large area (several acres/hectares) with multiple voltage levels, many circuit breakers, and a large amount of protection and control equipment (voltage and current transformers, relays and SCADA systems). Modern substations may be implemented using international standards such as IEC Standard 61850.
Distribution substation
A distribution substation transfers power from the transmission system to the distribution system of an area. It is uneconomical to directly connect electricity consumers to the main transmission network, unless they use large amounts of power, so the distribution station reduces voltage to a level suitable for local distribution.
The input for a distribution substation is typically at least two transmission or sub-transmission lines. Input voltage may be, for example, 115 kV, or whatever is common in the area. The output is a number of feeders. Distribution voltages are typically medium voltage, between 2.4 kV and 33 kV, depending on the size of the area served and the practices of the local utility. The feeders run along streets overhead (or underground, in some cases) and power the distribution transformers at or near the customer premises.
In addition to transforming voltage, distribution substations also isolate faults in either the transmission or distribution systems. Distribution substations are typically the points of voltage regulation, although on long distribution circuits (of several miles/kilometers), voltage regulation equipment may also be installed along the line.
The downtown areas of large cities feature complicated distribution substations, with high-voltage switching, and switching and backup systems on the low-voltage side. More typical distribution substations have a switch, one transformer, and minimal facilities on the low-voltage side. | Substation | Wikipedia | 479 | 320217 | https://en.wikipedia.org/wiki/Substation | Technology | Electricity transmission and distribution | null |
Collector substation
In distributed generation projects such as a wind farm or photovoltaic power station, a collector substation may be required. It resembles a distribution substation although power flow is in the opposite direction, from many wind turbines or inverters up into the transmission grid. Usually for economy of construction the collector system operates around 35 kV, although some collector systems are 12 kV, and the collector substation steps up voltage to a transmission voltage for the grid. The collector substation can also provide power factor correction if it is needed, metering, and control of the wind farm. In some special cases a collector substation can also contain an HVDC converter station.
Collector substations also exist where multiple thermal or hydroelectric power plants of comparable output power are in proximity. Examples for such substations are Brauweiler in Germany and Hradec in the Czech Republic, where power is collected from nearby lignite-fired power plants. If no transformers are required for increasing the voltage to transmission level, the substation is a switching station.
Converter substations
Converter substations may be associated with HVDC converter plants, traction current, or interconnected non-synchronous networks. These stations contain power electronic devices to change the frequency of current, or else convert from alternating to direct current or the reverse. Formerly rotary converters changed frequency to interconnect two systems; nowadays such substations are rare.
Switching station
A switching station is a substation without transformers and operating only at a single voltage level. Switching stations are sometimes used as collector and distribution stations. Sometimes they are used for switching the current to back-up lines or for parallelizing circuits in case of failure. An example is the switching stations for the HVDC Inga–Shaba transmission line.
A switching station may also be known as a switchyard, and these are commonly located directly adjacent to or nearby a power station. In this case the generators from the power station supply their power into the yard onto the generator bus on one side of the yard, and the transmission lines take their power from a Feeder Bus on the other side of the yard. | Substation | Wikipedia | 442 | 320217 | https://en.wikipedia.org/wiki/Substation | Technology | Electricity transmission and distribution | null |
An important function performed by a substation is switching, which is the connecting and disconnecting of transmission lines or other components to and from the system. Switching events may be planned or unplanned. A transmission line or other component may need to be de-energized for maintenance or for new construction, for example, adding or removing a transmission line or a transformer. To maintain reliability of supply, companies aim at keeping the system up and running while performing maintenance. All work to be performed, from routine testing to adding entirely new substations, should be done while keeping the whole system running.
Unplanned switching events are caused by a fault in a transmission line or any other component, for example:
a line is hit by lightning and develops an arc,
a tower is blown down by high wind.
The function of the switching station is to isolate the faulty portion of the system in the shortest possible time. De-energizing faulty equipment protects it from further damage, and isolating a fault helps keep the rest of the electrical grid operating with stability.
Railways
Electrified railways also use substations, often distribution substations. In some cases a conversion of the current type takes place, commonly with rectifiers for direct current (DC) trains, or rotary converters for trains using alternating current (AC) at frequencies other than that of the public grid. Sometimes they are also transmission substations or collector substations if the railway network also operates its own grid and generators to supply the other stations.
Mobile substation
A mobile substation is a substation on wheels, containing a transformer, breakers and buswork mounted on a self-contained semi-trailer, meant to be pulled by a truck. They are designed to be compact for travel on public roads, and are used for temporary backup in times of natural disaster or war. Mobile substations are usually rated much lower than permanent installations, and may be built in several units to meet road travel limitations.
Design
Substation design is aimed at minimizing cost while ensuring power availability and reliability, and enabling changes to the substation in the future.
Substations may be built outdoors, indoors, or underground or in a combination of these locations. | Substation | Wikipedia | 454 | 320217 | https://en.wikipedia.org/wiki/Substation | Technology | Electricity transmission and distribution | null |
Location selection
Selection of the location of a substation must consider many factors. Sufficient land area is required for installation of equipment with necessary clearances for electrical safety, and for access to maintain large apparatus such as transformers. The site must have room for expansion due to load growth or planned transmission additions. Environmental effects of the substation must be considered, such as drainage, noise and road traffic effects.
The substation site must be reasonably central to the distribution area to be served. The site must be secure from intrusion by passers-by, both to protect people from injury by electric shock or arcs, and to protect the electrical system from misoperation due to vandalism.
If not owned and operated by a utility company, substations are typically occupied on a long lease such as a renewable 99-year lease, giving the utility company security of tenure.
Design diagrams
The first step in planning a substation layout is the preparation of a one-line diagram, which shows in simplified form the switching and protection arrangement required, as well as the incoming supply lines and outgoing feeders or transmission lines. It is a usual practice by many electrical utilities to prepare one-line diagrams with principal elements (lines, switches, circuit breakers, transformers) arranged on the page similarly to the way the apparatus would be laid out in the actual station.
In a common design, incoming lines have a disconnector and a circuit breaker. In some cases, the lines will not have both, with either a switch or a circuit breaker being all that is considered necessary. A disconnect switch is used to provide isolation, since it cannot interrupt load current. A circuit breaker is used as a protection device to interrupt fault currents automatically, and may be used to switch loads on and off, or to cut off a line when power is flowing in the 'wrong' direction. When a large fault current flows through the circuit breaker, this is detected through the use of current transformers. The magnitude of the current transformer outputs may be used to trip the circuit breaker resulting in a disconnection of the load supplied by the circuit break from the feeding point. This seeks to isolate the fault point from the rest of the system, and allow the rest of the system to continue operating with minimal impact. Both switches and circuit breakers may be operated locally (within the substation) or remotely from a supervisory control center. | Substation | Wikipedia | 479 | 320217 | https://en.wikipedia.org/wiki/Substation | Technology | Electricity transmission and distribution | null |
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