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reddening (interstellar) : the reddening of starlight passing through interstellar dust because dust scatters blue light more effectively than red | https://openstax.org/books/astronomy-2e/pages/20-key-terms |
About 15% of the visible matter in the Galaxy is in the form of gas and dust, serving as the raw material for new stars. About 99% of this interstellar matter is in the form of gasâindividual atoms or molecules. The most abundant elements in the interstellar gas are hydrogen and helium. About 1% of the interstellar m... | https://openstax.org/books/astronomy-2e/pages/20-summary |
Interstellar gas may be hot or cold. Gas found near hot stars emits light by fluorescence, that is, light is emitted when an electron is captured by an ion and cascades down to lower-energy levels. Glowing clouds (nebulae) of ionized hydrogen are called H II regions and have temperatures of about 10,000 K. Most hydroge... | https://openstax.org/books/astronomy-2e/pages/20-summary |
Interstellar dust can be detected: (1) when it blocks the light of stars behind it, (2) when it scatters the light from nearby stars, and (3) because it makes distant stars look both redder and fainter. These effects are called interstellar extinction and reddening. Dust can also be detected in the infrared because it ... | https://openstax.org/books/astronomy-2e/pages/20-summary |
Cosmic rays are particles that travel through interstellar space at a typical speed of 90% of the speed of light. The most abundant elements in cosmic rays are the nuclei of hydrogen and helium, but electrons and positrons are also found. It is likely that many cosmic rays are produced in supernova shocks. | https://openstax.org/books/astronomy-2e/pages/20-summary |
Interstellar matter is constantly flowing through the Galaxy and changing from one phase to another. At the same time, gas is constantly being added to the Galaxy by accretion from extragalactic space, while mass is removed from the interstellar medium by being locked in stars. Some of the mass in stars is, in turn, re... | https://openstax.org/books/astronomy-2e/pages/20-summary |
The Sun is located at the edge of a low-density cloud called the Local Fluff. The Sun and this cloud are located within the Local Bubble, a region extending to at least 300 light-years from the Sun, within which the density of interstellar material is extremely low. Astronomers think this bubble was blown by some nearb... | https://openstax.org/books/astronomy-2e/pages/20-summary |
exoplanet : a planet orbiting a star other than our Sun | https://openstax.org/books/astronomy-2e/pages/21-key-terms |
giant molecular clouds : large, cold interstellar clouds with diameters of dozens of light-years and typical masses of 105solar masses; found in the spiral arms of galaxies, these clouds are where stars form | https://openstax.org/books/astronomy-2e/pages/21-key-terms |
Herbig-Haro (HH) object : luminous knots of gas in an area of star formation that are set to glow by jets of material from a protostar | https://openstax.org/books/astronomy-2e/pages/21-key-terms |
mini-Neptune : a planet that is intermediate between the largest terrestrial planet in our solar system (Earth) and the smallest jovian planet (Neptune); generally, mini-Neptunes have sizes between 2.8 and 4 times Earthâs size | https://openstax.org/books/astronomy-2e/pages/21-key-terms |
protostar : a very young star still in the process of formation, before nuclear fusion begins | https://openstax.org/books/astronomy-2e/pages/21-key-terms |
stellar wind : the outflow of gas, sometimes at speeds as high as hundreds of kilometers per second, from a star | https://openstax.org/books/astronomy-2e/pages/21-key-terms |
super-Earth : a planet larger than Earth, generally between 1.4 and 2.8 times the size of our planet | https://openstax.org/books/astronomy-2e/pages/21-key-terms |
transit : when one astronomical object moves in front of another | https://openstax.org/books/astronomy-2e/pages/21-key-terms |
Most stars form in giant molecular clouds with masses as large as 3 Ã 106solar masses. The most well-studied molecular cloud is Orion, where star formation is currently taking place. Molecular clouds typically contain regions of higher density called clumps, which in turn contain several even-denser cores of gas and d... | https://openstax.org/books/astronomy-2e/pages/21-summary |
The evolution of a star can be described in terms of changes in its temperature and luminosity, which can best be followed by plotting them on an HâR diagram. Protostars generate energy (and internal heat) through gravitational contraction that typically continues for millions of years, until the star reaches the mai... | https://openstax.org/books/astronomy-2e/pages/21-summary |
Observational evidence shows that most protostars are surrounded by disks with large-enough diameters and enough mass (as much as 10% that of the Sun) to form planets. After a few million years, the inner part of the disk is cleared of dust, and the disk is then shaped like a donut with the protostar centered in the ho... | https://openstax.org/books/astronomy-2e/pages/21-summary |
Several observational techniques have successfully detected planets orbiting other stars. These techniques fall into two general categoriesâdirect and indirect detection. The Doppler and transit techniques are our most powerful indirect tools for finding exoplanets. Some planets are also being found by direct imaging... | https://openstax.org/books/astronomy-2e/pages/21-summary |
The Kepler mission found thousands of new exoplanets, although these were limited to orbital periods of less than 400 days and sizes larger than Mars. Other missions are now continuing the search for exoplanets. Still, we can use the Kepler discoveries to extrapolate the size distribution of planets in our Galaxy. The ... | https://openstax.org/books/astronomy-2e/pages/21-summary |
The ensemble of exoplanets is incredibly diverse and has led to a revision in our understanding of planet formation that includes the possibility of vigorous, chaotic interactions, with planet migration and scattering. It is possible that the solar system is unusual (and not representative) in how its planets are arran... | https://openstax.org/books/astronomy-2e/pages/21-summary |
association : a loose group of young stars whose spectral types, motions, and positions in the sky indicate a common origin | https://openstax.org/books/astronomy-2e/pages/22-key-terms |
globular cluster : one of about 150 large, spherical star clusters (each with hundreds of thousands of stars) that form a spherical halo around the center of our Galaxy | https://openstax.org/books/astronomy-2e/pages/22-key-terms |
helium flash : a nearly explosive ignition of helium in the triple-alpha process in the dense core of a red giant star | https://openstax.org/books/astronomy-2e/pages/22-key-terms |
main-sequence turnoff : location in the HâR diagram where stars begin to leave the main sequence | https://openstax.org/books/astronomy-2e/pages/22-key-terms |
nucleosynthesis : the building up of heavy elements from lighter ones by nuclear fusion | https://openstax.org/books/astronomy-2e/pages/22-key-terms |
open cluster : a comparatively loose cluster of stars, containing from a few dozen to a few thousand members, located in the spiral arms or disk of our Galaxy; sometimes referred to as a galactic cluster | https://openstax.org/books/astronomy-2e/pages/22-key-terms |
planetary nebula : a shell of gas ejected by and expanding away from an extremely hot low-mass star that is nearing the end of its life (the nebulae glow because of the ultra-violet energy of the central star) | https://openstax.org/books/astronomy-2e/pages/22-key-terms |
triple-alpha process : a nuclear reaction by which three helium nuclei are built up (fused) into one carbon nucleus | https://openstax.org/books/astronomy-2e/pages/22-key-terms |
zero-age main sequence : a line denoting the main sequence on the HâR diagram for a system of stars that have completed their contraction from interstellar matter and are now deriving all their energy from nuclear reactions, but whose chemical composition has not yet been altered substantially by nuclear reactions | https://openstax.org/books/astronomy-2e/pages/22-key-terms |
When stars first begin to fuse hydrogen to helium, they lie on the zero-age main sequence. The amount of time a star spends in the main-sequence stage depends on its mass. More massive stars complete each stage of evolution more quickly than lower-mass stars. The fusion of hydrogen to form helium changes the interior c... | https://openstax.org/books/astronomy-2e/pages/22-summary |
Star clusters provide one of the best tests of our calculations of what happens as stars age. The stars in a given cluster were formed at about the same time and have the same composition, so they differ mainly in mass, and thus, in their life stage. There are three types of star clusters: globular, open, and associati... | https://openstax.org/books/astronomy-2e/pages/22-summary |
The HâR diagram of stars in a cluster changes systematically as the cluster grows older. The most massive stars evolve most rapidly. In the youngest clusters and associations, highly luminous blue stars are on the main sequence; the stars with the lowest masses lie to the right of the main sequence and are still cont... | https://openstax.org/books/astronomy-2e/pages/22-summary |
After stars become red giants, their cores eventually become hot enough to produce energy by fusing helium to form carbon (and sometimes a bit of oxygen.) The fusion of three helium nuclei produces carbon through the triple-alpha process. The rapid onset of helium fusion in the core of a low-mass star is called the hel... | https://openstax.org/books/astronomy-2e/pages/22-summary |
In stars with masses higher than about 8 solar masses, nuclear reactions involving carbon, oxygen, and still heavier elements can build up nuclei as heavy as iron. The creation of new chemical elements is called nucleosynthesis. The late stages of evolution occur very quickly. Ultimately, all stars must use up all of t... | https://openstax.org/books/astronomy-2e/pages/22-summary |
Chandrasekhar limit : the upper limit to the mass of a white dwarf (equals 1.4 times the mass of the Sun) | https://openstax.org/books/astronomy-2e/pages/23-key-terms |
degenerate gas : a gas that resists further compression because no two electrons can be in the same place at the same time doing the same thing (Pauli exclusion principle) | https://openstax.org/books/astronomy-2e/pages/23-key-terms |
millisecond pulsar : a pulsar that rotates so quickly that it can give off hundreds of pulses per second (and its period is therefore measured in milliseconds) | https://openstax.org/books/astronomy-2e/pages/23-key-terms |
neutron star : a compact object of extremely high density composed almost entirely of neutrons | https://openstax.org/books/astronomy-2e/pages/23-key-terms |
nova : the cataclysmic explosion produced in a binary system, temporarily increasing its luminosity by hundreds to thousands of times | https://openstax.org/books/astronomy-2e/pages/23-key-terms |
pulsar : a variable radio source of small physical size that emits very rapid radio pulses in very regular periods that range from fractions of a second to several seconds; now understood to be a rotating, magnetic neutron star that is energetic enough to produce a detectable beam of radiation and particles | https://openstax.org/books/astronomy-2e/pages/23-key-terms |
type II supernova : a stellar explosion produced at the endpoint of the evolution of stars whose mass exceeds roughly 10 times the mass of the Sun | https://openstax.org/books/astronomy-2e/pages/23-key-terms |
During the course of their evolution, stars shed their outer layers and lose a significant fraction of their initial mass. Stars with masses of 8MSunor less can lose enough mass to become white dwarfs, which have masses less than the Chandrasekhar limit (about 1.4MSun). The pressure exerted by degenerate electrons keep... | https://openstax.org/books/astronomy-2e/pages/23-summary |
In a massive star, hydrogen fusion in the core is followed by several other fusion reactions involving heavier elements. Just before it exhausts all sources of energy, a massive star has an iron core surrounded by shells of silicon, sulfur, oxygen, neon, carbon, helium, and hydrogen. The fusion of iron requires energy ... | https://openstax.org/books/astronomy-2e/pages/23-summary |
A supernova occurs on average once every 25 to 100 years in the Milky Way Galaxy. Despite the odds, no supernova in our Galaxy has been observed from Earth since the invention of the telescope. However, one nearby supernova (SN 1987A) has been observed in a neighboring galaxy, the Large Magellanic Cloud. The star that ... | https://openstax.org/books/astronomy-2e/pages/23-summary |
At least some supernovae leave behind a highly magnetic, rapidly rotating neutron star, which can be observed as a pulsar if its beam of escaping particles and focused radiation is pointing toward us. Pulsars emit rapid pulses of radiation at regular intervals; their periods are in the range of 0.001 to 10 seconds. The... | https://openstax.org/books/astronomy-2e/pages/23-summary |
When a white dwarf or neutron star is a member of a close binary star system, its companion star can transfer mass to it. Material fallinggraduallyonto a white dwarf can explode in a sudden burst of fusion and make a nova. If material fallsrapidlyonto a white dwarf, it can push it over the Chandrasekhar limit and cause... | https://openstax.org/books/astronomy-2e/pages/23-summary |
Gamma-ray bursts last from a fraction of a second to a few minutes. They come from all directions and are now known to be associated with very distant objects. The energy is most likely beamed, and, for the ones we can detect, Earth lies in the direction of the beam. Long-duration bursts (lasting more than a few second... | https://openstax.org/books/astronomy-2e/pages/23-summary |
accretion disk : the disk of gas and dust found orbiting newborn stars, as well as compact stellar remnants such as white dwarfs, neutron stars, and black holes when they are in binary systems and are sufficiently close to their binary companions to draw off material | https://openstax.org/books/astronomy-2e/pages/24-key-terms |
black hole : a region in spacetime where gravity is so strong that nothingânot even lightâcan escape | https://openstax.org/books/astronomy-2e/pages/24-key-terms |
equivalence principle : concept that a gravitational force and a suitable acceleration are indistinguishable within a sufficiently local environment | https://openstax.org/books/astronomy-2e/pages/24-key-terms |
event horizon : a boundary in spacetime such that events inside the boundary can have no effect on the world outside itâthat is, the boundary of the region around a black hole where the curvature of spacetime no longer provides any way out | https://openstax.org/books/astronomy-2e/pages/24-key-terms |
general theory of relativity : Einsteinâs theory relating gravity and the structure (geometry) of space and time | https://openstax.org/books/astronomy-2e/pages/24-key-terms |
gravitational redshift : an increase in wavelength of an electromagnetic wave (light) when propagating from or near a massive object | https://openstax.org/books/astronomy-2e/pages/24-key-terms |
gravitational wave : a disturbance in the curvature of spacetime caused by changes in how matter is distributed; gravitational waves propagate at (or near) the speed of light. | https://openstax.org/books/astronomy-2e/pages/24-key-terms |
singularity : the point of zero volume and infinite density to which any object that becomes a black hole must collapse, according to the general theory of relativity | https://openstax.org/books/astronomy-2e/pages/24-key-terms |
spacetime : system of one time and three space coordinates, with respect to which the time and place of an event can be specified | https://openstax.org/books/astronomy-2e/pages/24-key-terms |
Einstein proposed the equivalence principle as the foundation of the general theory of relativity. According to this principle, there is no way that anyone or any experiment in a sealed environment can distinguish between free fall and the absence of gravity. | https://openstax.org/books/astronomy-2e/pages/24-summary |
By considering the consequences of the equivalence principle, Einstein concluded that we live in a curved spacetime. The distribution of matter determines the curvature of spacetime; other objects (and even light) entering a region of spacetime must follow its curvature. Light must change its path near a massive object... | https://openstax.org/books/astronomy-2e/pages/24-summary |
In weak gravitational fields, the predictions of general relativity agree with the predictions of Newtonâs law of gravity. However, in the stronger gravity of the Sun, general relativity makes predictions that differ from Newtonian physics and can be tested. For example, general relativity predicts that light or radi... | https://openstax.org/books/astronomy-2e/pages/24-summary |
General relativity predicts that the stronger the gravity, the more slowly time must run. Experiments on Earth and with spacecraft have confirmed this prediction with remarkable accuracy. When light or other radiation emerges from a compact smaller remnant, such as a white dwarf or neutron star, it shows a gravitationa... | https://openstax.org/books/astronomy-2e/pages/24-summary |
Theory suggests that stars with stellar cores more massive than three times the mass of the Sun at the time they exhaust their nuclear fuel will collapse to become black holes. The surface surrounding a black hole, where the escape velocity equals the speed of light, is called the event horizon, and the radius of the s... | https://openstax.org/books/astronomy-2e/pages/24-summary |
The best evidence of stellar-mass black holes comes from binary star systems in which (1) one star of the pair is not visible, (2) the flickering X-ray emission is characteristic of an accretion disk around a compact object, and (3) the orbit and characteristics of the visible star indicate that the mass of its invisib... | https://openstax.org/books/astronomy-2e/pages/24-summary |
General relativity predicts that the rearrangement of matter in space should produce gravitational waves. The existence of such waves was first confirmed in observations of a pulsar in orbit around another neutron star whose orbits were spiraling closer and losing energy in the form of gravitational waves. In 2015, LIG... | https://openstax.org/books/astronomy-2e/pages/24-summary |
central bulge : (or nuclear bulge) the central (round) part of the Milky Way or a similar galaxy | https://openstax.org/books/astronomy-2e/pages/25-key-terms |
dark matter : nonluminous mass, whose presence can be inferred only because of its gravitational influence on luminous matter; the composition of the dark matter is not known | https://openstax.org/books/astronomy-2e/pages/25-key-terms |
dark matter halo : the mass in the Milky Way that extends well beyond the boundary of the luminous stars to a distance of at least 200,000 light-years from the center of the Galaxy; although we deduce its existence from its gravity, the composition of this matter remains a mystery | https://openstax.org/books/astronomy-2e/pages/25-key-terms |
differential galactic rotation : the idea that different parts of the Galaxy turn at different rates, since the parts of the Galaxy follow Keplerâs third law: more distant objects take longer to complete one full orbit around the center of the Galaxy | https://openstax.org/books/astronomy-2e/pages/25-key-terms |
halo : the outermost extent of our Galaxy (or another galaxy), containing a sparse distribution of stars and globular clusters in a more or less spherical distribution | https://openstax.org/books/astronomy-2e/pages/25-key-terms |
Milky Way Galaxy : the band of light encircling the sky, which is due to the many stars and diffuse nebulae lying near the plane of the Milky Way Galaxy | https://openstax.org/books/astronomy-2e/pages/25-key-terms |
population I star : a star containing heavy elements; typically young and found in the disk | https://openstax.org/books/astronomy-2e/pages/25-key-terms |
population II star : a star with very low abundance of heavy elements; found throughout the Galaxy | https://openstax.org/books/astronomy-2e/pages/25-key-terms |
spiral arm : a spiral-shaped region, characterized by relatively dense interstellar material and young stars, that is observed in the disks of spiral galaxies | https://openstax.org/books/astronomy-2e/pages/25-key-terms |
supermassive black hole : the object in the center of most large galaxies that is so massive and compact that light cannot escape from it; the Milky Wayâs supermassive black hole contains 4.6 millions of Sunsâ worth of mass | https://openstax.org/books/astronomy-2e/pages/25-key-terms |
The Milky Way Galaxy consists of a thin disk containing dust, gas, and young and old stars; a spherical halo containing populations of very old stars, including RR Lyrae variable stars and globular star clusters; a thick, more diffuse disk with stars that have properties intermediate between those in the thin disk and ... | https://openstax.org/books/astronomy-2e/pages/25-summary |
The gaseous distribution in the Galaxyâs disk has two main spiral arms that emerge from the ends of the central bar, along with several fainter arms and short spurs; the Sun is located in one of those spurs. Measurements show that the Galaxy does not rotate as a solid body, but instead its stars and gas follow differ... | https://openstax.org/books/astronomy-2e/pages/25-summary |
The Sun revolves completely around the galactic center in about 225 million years (a galactic year). The mass of the Galaxy can be determined by measuring the orbital velocities of stars and interstellar matter. The total mass of the Galaxy is about2Ã10122Ã1012MSun.As much as 95% of this mass consists of dark matter ... | https://openstax.org/books/astronomy-2e/pages/25-summary |
A supermassive black hole is located at the center of the Galaxy. Measurements of the velocities of stars located within a few light-days of the center show that the mass inside their orbits around the center is about 4.6 millionMSun. Radio observations show that this mass is concentrated in a volume with a diameter si... | https://openstax.org/books/astronomy-2e/pages/25-summary |
We can roughly divide the stars in the Galaxy into two categories. Old stars with few heavy elements are referred to as population II stars and are found in the halo and in globular clusters. Population I stars contain more heavy elements than globular cluster and halo stars, are typically younger and found in the disk... | https://openstax.org/books/astronomy-2e/pages/25-summary |
The Galaxy began forming a little more than 13 billion years ago. Models suggest that the stars in the halo and globular clusters formed first, while the Galaxy was spherical. The gas, somewhat enriched in heavy elements by the first generation of stars, then collapsed from a spherical distribution to a rotating disk-s... | https://openstax.org/books/astronomy-2e/pages/25-summary |
elliptical galaxy : a galaxy whose shape is an ellipse and that contains no conspicuous interstellar material | https://openstax.org/books/astronomy-2e/pages/26-key-terms |
Hubble constant : a constant of proportionality in the law relating the velocities of remote galaxies to their distances | https://openstax.org/books/astronomy-2e/pages/26-key-terms |
Hubbleâs law : a rule that the radial velocities of remote galaxies are proportional to their distances from us | https://openstax.org/books/astronomy-2e/pages/26-key-terms |
irregular galaxy : a galaxy without any clear symmetry or pattern; neither a spiral nor an elliptical galaxy | https://openstax.org/books/astronomy-2e/pages/26-key-terms |
mass-to-light ratio : the ratio of the total mass of a galaxy to its total luminosity, usually expressed in units of solar mass and solar luminosity; the mass-to-light ratio gives a rough indication of the types of stars contained within a galaxy and whether or not substantial quantities of dark matter are present | https://openstax.org/books/astronomy-2e/pages/26-key-terms |
redshift : when lines in the spectra are displaced toward longer wavelengths (toward the red end of the visible spectrum) | https://openstax.org/books/astronomy-2e/pages/26-key-terms |
spiral galaxy : a flattened, rotating galaxy with pinwheel-like arms of interstellar material and young stars, winding out from its central bulge | https://openstax.org/books/astronomy-2e/pages/26-key-terms |
type Ia supernova : a supernova formed by the explosion of a white dwarf in a binary system and reach a luminosity of about 4.5 Ã 109LSun; can be used to determine distances to galaxies on a large scale | https://openstax.org/books/astronomy-2e/pages/26-key-terms |
Faint star clusters, clouds of glowing gas, and galaxies all appeared as faint patches of light (or nebulae) in the telescopes available at the beginning of the twentieth century. It was only when Hubble measured the distance to the Andromeda galaxy using cepheid variables with the giant 2.5-meter reflector on Mount Wi... | https://openstax.org/books/astronomy-2e/pages/26-summary |
The majority of bright galaxies are either spirals or ellipticals. Spiral galaxies contain both old and young stars, as well as interstellar matter, and have typical masses in the range of 109to 1012MSun. Our own Galaxy is a large spiral. Ellipticals are spheroidal or slightly elongated systems that consist almost enti... | https://openstax.org/books/astronomy-2e/pages/26-summary |
The masses of spiral galaxies are determined from measurements of their rates of rotation. The masses of elliptical galaxies are estimated from analyses of the motions of the stars within them. Galaxies can be characterized by their mass-to-light ratios. The luminous parts of galaxies with active star formation typical... | https://openstax.org/books/astronomy-2e/pages/26-summary |
Astronomers determine the distances to galaxies using a variety of methods, including the period-luminosity relationship for cepheid variables; objects such as type Ia supernovae, which appear to be standard bulbs; and the Tully-Fisher relation, which connects the line broadening of 21-cm radiation to the luminosity of... | https://openstax.org/books/astronomy-2e/pages/26-summary |
The universe is expanding. Observations show that the spectral lines of distant galaxies are redshifted, and that their recession velocities are proportional to their distances from us, a relationship known as Hubbleâs law. The rate of recession, called the Hubble constant, is approximately 22 kilometers per second p... | https://openstax.org/books/astronomy-2e/pages/26-summary |
active galactic nuclei (AGN) : galaxies that are almost as luminous as quasars and share many of their properties, although to a less spectacular degree; abnormal amounts of energy are produced in their centers | https://openstax.org/books/astronomy-2e/pages/27-key-terms |
active galaxies : galaxies that house active galactic nuclei | https://openstax.org/books/astronomy-2e/pages/27-key-terms |
quasar : an object of very high redshift that looks like a star but is extragalactic and highly luminous; also called a quasi-stellar object, or QSO | https://openstax.org/books/astronomy-2e/pages/27-key-terms |
The first quasars discovered looked like stars but had strong radio emission. Their visible-light spectra at first seemed confusing, but then astronomers realized that they had much larger redshifts than stars. The quasar spectra obtained so far show redshifts ranging from 15% to more than 96% the speed of light. Obser... | https://openstax.org/books/astronomy-2e/pages/27-summary |
Both active galactic nuclei and quasars derive their energy from material falling toward, and forming a hot accretion disk around, a massive black hole. This model can account for the large amount of energy emitted and for the fact that the energy is produced in a relatively small volume of space. It can also explain w... | https://openstax.org/books/astronomy-2e/pages/27-summary |
Quasars and galaxies affect each other: the galaxy supplies fuel to the black hole, and the quasar heats and disrupts the gas clouds in the galaxy. The balance between these two processes probably helps explain why the black hole seems always to be about 1/200 the mass of the spherical bulge of stars that surrounds the... | https://openstax.org/books/astronomy-2e/pages/27-summary |
Quasars were much more common billions of years ago than they are now, and astronomers speculate that they mark an early stage in the formation of galaxies. Quasars were more likely to be active when the universe was young and fuel for their accretion disk was more available. | https://openstax.org/books/astronomy-2e/pages/27-summary |
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