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= = = Magnetic field = = =
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The magnetic field of a star is generated within regions of the interior where convective circulation occurs . This movement of conductive plasma functions like a dynamo , wherein the movement of elecrical charges induce magnetic fields , as does a mechanical dynamo . Those magnetic fields have a great range that exte... |
Young , rapidly rotating stars tend to have high levels of surface activity because of their magnetic field . The magnetic field can act upon a star 's stellar wind , functioning as a brake to gradually slow the rate of rotation with time . Thus , older stars such as the Sun have a much slower rate of rotation and a l... |
= = = Mass = = =
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One of the most massive stars known is Eta Carinae , which , with 100 β 150 times as much mass as the Sun , will have a lifespan of only several million years . Studies of the most massive open clusters suggests 150 M β as an upper limit for stars in the current era of the universe . This represents an empirical value... |
The first stars to form after the Big Bang may have been larger , up to 300 M β , due to the complete absence of elements heavier than lithium in their composition . This generation of supermassive population III stars is likely to have existed in the very early universe ( i.e. , they are observed to have a high redsh... |
With a mass only 80 times that of Jupiter ( MJ ) , 2MASS J0523 @-@ 1403 is the smallest known star undergoing nuclear fusion in its core . For stars with metallicity similar to the Sun , the theoretical minimum mass the star can have and still undergo fusion at the core , is estimated to be about 75 MJ . When the meta... |
The combination of the radius and the mass of a star determines its surface gravity . Giant stars have a much lower surface gravity than do main sequence stars , while the opposite is the case for degenerate , compact stars such as white dwarfs . The surface gravity can influence the appearance of a star 's spectrum ,... |
= = = Rotation = = =
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The rotation rate of stars can be determined through spectroscopic measurement , or more exactly determined by tracking their starspots . Young stars can have a rotation greater than 100 km / s at the equator . The B @-@ class star Achernar , for example , has an equatorial velocity of about 225 km / s or greater , ca... |
Degenerate stars have contracted into a compact mass , resulting in a rapid rate of rotation . However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum β the tendency of a rotating body to compensate for a contraction in size by increasing its rate of sp... |
= = = Temperature = = =
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The surface temperature of a main sequence star is determined by the rate of energy production of its core and by its radius , and is often estimated from the star 's color index . The temperature is normally given in terms of an effective temperature , which is the temperature of an idealized black body that radiates... |
The stellar temperature will determine the rate of ionization of various elements , resulting in characteristic absorption lines in the spectrum . The surface temperature of a star , along with its visual absolute magnitude and absorption features , is used to classify a star ( see classification below ) .
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Massive main sequence stars can have surface temperatures of 50 @,@ 000 K. Smaller stars such as the Sun have surface temperatures of a few thousand K. Red giants have relatively low surface temperatures of about 3 @,@ 600 K ; but they also have a high luminosity due to their large exterior surface area .
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= = Radiation = =
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The energy produced by stars , a product of nuclear fusion , radiates to space as both electromagnetic radiation and particle radiation . The particle radiation emitted by a star is manifested as the stellar wind , which streams from the outer layers as electrically charged protons and alpha and beta particles . Altho... |
The production of energy at the core is the reason stars shine so brightly : every time two or more atomic nuclei fuse together to form a single atomic nucleus of a new heavier element , gamma ray photons are released from the nuclear fusion product . This energy is converted to other forms of electromagnetic energy o... |
The color of a star , as determined by the most intense frequency of the visible light , depends on the temperature of the star 's outer layers , including its photosphere . Besides visible light , stars also emit forms of electromagnetic radiation that are invisible to the human eye . In fact , stellar electromagneti... |
Using the stellar spectrum , astronomers can also determine the surface temperature , surface gravity , metallicity and rotational velocity of a star . If the distance of the star is found , such as by measuring the parallax , then the luminosity of the star can be derived . The mass , radius , surface gravity , and r... |
= = = Luminosity = = =
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The luminosity of a star is the amount of light and other forms of radiant energy it radiates per unit of time . It has units of power . The luminosity of a star is determined by its radius and surface temperature . Many stars do not radiate uniformly across their entire surface . The rapidly rotating star Vega , for ... |
Patches of the star 's surface with a lower temperature and luminosity than average are known as starspots . Small , dwarf stars such as our Sun generally have essentially featureless disks with only small starspots . Giant stars have much larger , more obvious starspots , and they also exhibit strong stellar limb dar... |
= = = Magnitude = = =
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The apparent brightness of a star is expressed in terms of its apparent magnitude . It is a function of the star 's luminosity , its distance from Earth , and the altering of the star 's light as it passes through Earth 's atmosphere . Intrinsic or absolute magnitude is directly related to a star 's luminosity , and i... |
Both the apparent and absolute magnitude scales are logarithmic units : one whole number difference in magnitude is equal to a brightness variation of about 2 @.@ 5 times ( the 5th root of 100 or approximately 2 @.@ 512 ) . This means that a first magnitude star ( + 1 @.@ 00 ) is about 2 @.@ 5 times brighter than a se... |
On both apparent and absolute magnitude scales , the smaller the magnitude number , the brighter the star ; the larger the magnitude number , the fainter the star . The brightest stars , on either scale , have negative magnitude numbers . The variation in brightness ( ΞL ) between two stars is calculated by subtractin... |
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Relative to both luminosity and distance from Earth , a star 's absolute magnitude ( M ) and apparent magnitude ( m ) are not equivalent ; for example , the bright star Sirius has an apparent magnitude of β 1 @.@ 44 , but it has an absolute magnitude of + 1 @.@ 41 .
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The Sun has an apparent magnitude of β 26 @.@ 7 , but its absolute magnitude is only + 4 @.@ 83 . Sirius , the brightest star in the night sky as seen from Earth , is approximately 23 times more luminous than the Sun , while Canopus , the second brightest star in the night sky with an absolute magnitude of β 5 @.@ 53 ... |
As of 2006 , the star with the highest known absolute magnitude is LBV 1806 @-@ 20 , with a magnitude of β 14 @.@ 2 . This star is at least 5 @,@ 000 @,@ 000 times more luminous than the Sun . The least luminous stars that are currently known are located in the NGC 6397 cluster . The faintest red dwarfs in the cluster... |
= = Classification = =
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The current stellar classification system originated in the early 20th century , when stars were classified from A to Q based on the strength of the hydrogen line . It thought that the hydrogen line strength was a simple linear function of temperature . Rather , it was more complicated ; it strengthened with increasin... |
Stars are given a single @-@ letter classification according to their spectra , ranging from type O , which are very hot , to M , which are so cool that molecules may form in their atmospheres . The main classifications in order of decreasing surface temperature are : O , B , A , F , G , K , and M. A variety of rare s... |
In addition , stars may be classified by the luminosity effects found in their spectral lines , which correspond to their spatial size and is determined by their surface gravity . These range from 0 ( hypergiants ) through III ( giants ) to V ( main sequence dwarfs ) ; some authors add VII ( white dwarfs ) . Most star... |
Additional nomenclature , in the form of lower @-@ case letters added to the end of the spectral type to indicate peculiar features of the spectrum . For example , an " e " can indicate the presence of emission lines ; " m " represents unusually strong levels of metals , and " var " can mean variations in the spectral... |
White dwarf stars have their own class that begins with the letter D. This is further sub @-@ divided into the classes DA , DB , DC , DO , DZ , and DQ , depending on the types of prominent lines found in the spectrum . This is followed by a numerical value that indicates the temperature .
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= = Variable stars = =
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Variable stars have periodic or random changes in luminosity because of intrinsic or extrinsic properties . Of the intrinsically variable stars , the primary types can be subdivided into three principal groups .
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During their stellar evolution , some stars pass through phases where they can become pulsating variables . Pulsating variable stars vary in radius and luminosity over time , expanding and contracting with periods ranging from minutes to years , depending on the size of the star . This category includes Cepheid and Ce... |
Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events . This group includes protostars , Wolf @-@ Rayet stars , and flare stars , as well as giant and supergiant stars .
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Cataclysmic or explosive variable stars are those that undergo a dramatic change in their properties . This group includes novae and supernovae . A binary star system that includes a nearby white dwarf can produce certain types of these spectacular stellar explosions , including the nova and a Type 1a supernova . The ... |
Stars can also vary in luminosity because of extrinsic factors , such as eclipsing binaries , as well as rotating stars that produce extreme starspots . A notable example of an eclipsing binary is Algol , which regularly varies in magnitude from 2 @.@ 3 to 3 @.@ 5 over a period of 2 @.@ 87 days .
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= = Structure = =
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The interior of a stable star is in a state of hydrostatic equilibrium : the forces on any small volume almost exactly counterbalance each other . The balanced forces are inward gravitational force and an outward force due to the pressure gradient within the star . The pressure gradient is established by the temperatu... |
As atomic nuclei are fused in the core , they emit energy in the form of gamma rays . These photons interact with the surrounding plasma , adding to the thermal energy at the core . Stars on the main sequence convert hydrogen into helium , creating a slowly but steadily increasing proportion of helium in the core . Ev... |
In addition to hydrostatic equilibrium , the interior of a stable star will also maintain an energy balance of thermal equilibrium . There is a radial temperature gradient throughout the interior that results in a flux of energy flowing toward the exterior . The outgoing flux of energy leaving any layer within the sta... |
The radiation zone is the region of the stellar interior where the flux of energy outward is dependent on radiative heat transfer , since convective heat transfer is inefficient in that zone . In this region the plasma will not be perturbed , and any mass motions will die out . If this is not the case , however , then... |
The occurrence of convection in the outer envelope of a main sequence star depends on the star 's mass . Stars with several times the mass of the Sun have a convection zone deep within the interior and a radiative zone in the outer layers . Smaller stars such as the Sun are just the opposite , with the convective zone... |
The photosphere is that portion of a star that is visible to an observer . This is the layer at which the plasma of the star becomes transparent to photons of light . From here , the energy generated at the core becomes free to propagate into space . It is within the photosphere that sun spots , regions of lower than ... |
Above the level of the photosphere is the stellar atmosphere . In a main sequence star such as the Sun , the lowest level of the atmosphere , just above the photosphere , is the thin chromosphere region , where spicules appear and stellar flares begin . Above this is the transition region , where the temperature rapid... |
From the corona , a stellar wind of plasma particles expands outward from the star , until it interacts with the interstellar medium . For the Sun , the influence of its solar wind extends throughout a bubble @-@ shaped region called the heliosphere .
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= = Nuclear fusion reaction pathways = =
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A variety of nuclear fusion reactions take place in the cores of stars , that depend upon their mass and composition . When nuclei fuse , the mass of the fused product is less than the mass of the original parts . This lost mass is converted to electromagnetic energy , according to the mass @-@ energy equivalence rela... |
The hydrogen fusion process is temperature @-@ sensitive , so a moderate increase in the core temperature will result in a significant increase in the fusion rate . As a result , the core temperature of main sequence stars only varies from 4 million kelvin for a small M @-@ class star to 40 million kelvin for a massiv... |
In the Sun , with a 10 @-@ million @-@ kelvin core , hydrogen fuses to form helium in the proton @-@ proton chain reaction :
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41H β 22H + 2e + + 2Ξ½e ( 2 x 0 @.@ 4 MeV )
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2e + + 2e- β 2Ξ³ ( 2 x 1 @.@ 0 MeV )
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21H + 22H β 23He + 2Ξ³ ( 2 x 5 @.@ 5 MeV )
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23He β 4He + 21H ( 12 @.@ 9 MeV )
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These reactions result in the overall reaction :
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41H β 4He + 2e + + 2Ξ³ + 2Ξ½e ( 26 @.@ 7 MeV )
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where e + is a positron , Ξ³ is a gamma ray photon , Ξ½e is a neutrino , and H and He are isotopes of hydrogen and helium , respectively . The energy released by this reaction is in millions of electron volts , which is actually only a tiny amount of energy . However enormous numbers of these reactions occur constantly ... |
In more massive stars , helium is produced in a cycle of reactions catalyzed by carbon called the carbon @-@ nitrogen @-@ oxygen cycle .
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In evolved stars with cores at 100 million kelvin and masses between 0 @.@ 5 and 10 M β , helium can be transformed into carbon in the triple @-@ alpha process that uses the intermediate element beryllium :
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4He + 4He + 92 keV β 8 * Be
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4He + 8 * Be + 67 keV β 12 * C
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12 * C β 12C + Ξ³ + 7 @.@ 4 MeV
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For an overall reaction of :
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34He β 12C + Ξ³ + 7 @.@ 2 MeV
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In massive stars , heavier elements can also be burned in a contracting core through the neon burning process and oxygen burning process . The final stage in the stellar nucleosynthesis process is the silicon burning process that results in the production of the stable isotope iron @-@ 56 , an endothermic process that... |
The example below shows the amount of time required for a star of 20 M β to consume all of its nuclear fuel . As an O @-@ class main sequence star , it would be 8 times the solar radius and 62 @,@ 000 times the Sun 's luminosity .
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= Perry the Platypus =
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Perry the Platypus , also known as Agent P or simply Perry , is an anthropomorphic platypus from the animated series Phineas and Ferb . Perry was created by the series ' co @-@ founders , Dan Povenmire and Jeff " Swampy " Marsh . He first appeared along with the majority of the main cast in the pilot episode " Rollerc... |
Perry is the pet platypus of the Flynn @-@ Fletcher family , and is perceived as mindless and domesticated . In secret , however , he lives a double life as a member of an all @-@ animal espionage organization referred to as O.W.C.A. ( The Organization Without a Cool Acronym ) . Many secret entrances to his undergroun... |
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