Astronomy Exam 2 Chapters 7-15

Asteroid

A "minor planet", a non-luminous chunk of rock smaller than planetsize but larger than a meteoroid, in orbit around a star.

Hertzsprung-Russeel Diagram

A graph of temperature (or equivalent) vs. luminosity (or equivalent) for a group of stars

Kuiper Belt

A reservoir of perhaps hundreds of thousands of solar-system objects each tens or hundreds of kilometers in diameter, orbiting the sun outside the orbit of Neptune. It is the source of many short period comets

11.3 A star's intrinsic brightness, or power, is its luminosity (Sec. 11.3). Absolute magnitudes are the magnitudes that stars would appear to have if they were at a standard distance equivalent to about 32.6 light‐years from us. The apparent brightness of a star decreases with increasing distance from it, following the inverse square law.

A star's intrinsic brightness, or power, is its luminosity (Sec. 11.3). Absolute magnitudes are the magnitudes that stars would appear to have if they were at a standard distance equivalent to about 32.6 light‐years from us. The apparent brightness of a star decreases with increasing distance from it, following the inverse square law.

Cepheid Variable

A type of super giant star that oscillates in brightness in a manner similar to the stars delta Cephei. The periods of Cepheid variables are between 1 and 100 days- linked to the average luminosity of the stars by known relationships; this allows the distances to be found

15.5 Although mysterious gamma‐ray bursts were at one time considered to be in our Galaxy, we now know that most of them originate billions of light‐years away, probably from the formation of stellarmass black holes (Section 15.6). NASA has sent the Swift satellite aloft to study them. The gamma rays and x‐rays throughout our Galaxy and elsewhere are studied as part of high‐energy astrophysics.

Although mysterious gamma‐ray bursts were at one time considered to be in our Galaxy, we now know that most of them originate billions of light‐years away, probably from the formation of stellarmass black holes (Section 15.6). NASA has sent the Swift satellite aloft to study them. The gamma rays and x‐rays throughout our Galaxy and elsewhere are studied as part of high‐energy astrophysics.

11.8 Among variable stars, stars that change considerably in brightness over time, longperiod variables, like Mira, are common (Sec. 11.7). Cepheid variables are rare but valuable; the period of a Cepheid variable's variation is linked to its average intrinsic brightness (luminosity), so studying its variation shows its luminosity. Measuring luminosity allows Cepheid variable stars to be used as standard candles. Comparing intrinsic brightness with apparent brightness gives the distances of stars, and they are valuable tools for determining distances to other galaxies. RR Lyrae variables also can be used to find distances, particularly to globular clusters in our Milky Way Galaxy.

Among variable stars, stars that change considerably in brightness over time, longperiod variables, like Mira, are common (Sec. 11.7). Cepheid variables are rare but valuable; the period of a Cepheid variable's variation is linked to its average intrinsic brightness (luminosity), so studying its variation shows its luminosity. Measuring luminosity allows Cepheid variable stars to be used as standard candles. Comparing intrinsic brightness with apparent brightness gives the distances of stars, and they are valuable tools for determining distances to other galaxies. RR Lyrae variables also can be used to find distances, particularly to globular clusters in our Milky Way Galaxy.

9.6 Another very successful technique for finding exoplanet candidates (called the transit method or blink method) is by looking for the very slight drop in a star's light as an exoplanet transits across the face of its parent star (Sec. 9.2d). These candidates must then be verified with the Dopplerwobble technique, and most of them end up being genuine exoplanets. For these systems, we know that the inclination of the orbital plane is close to 90 degrees, so the mass of the exoplanet can be accurately determined. With the launch of the CoRoT spacecraft and especially the Kepler mission, plus some observations from ground‐based telescopes, thousands of exoplanet candidates have now been observed. In a few cases, the transit method has revealed, through spectroscopy, sodium and other gases in the exoplanet's atmosphere.

Another very successful technique for finding exoplanet candidates (called the transit method or blink method) is by looking for the very slight drop in a star's light as an exoplanet transits across the face of its parent star (Sec. 9.2d). These candidates must then be verified with the Dopplerwobble technique, and most of them end up being genuine exoplanets. For these systems, we know that the inclination of the orbital plane is close to 90 degrees, so the mass of the exoplanet can be accurately determined. With the launch of the CoRoT spacecraft and especially the Kepler mission, plus some observations from ground‐based telescopes, thousands of exoplanet candidates have now been observed. In a few cases, the transit method has revealed, through spectroscopy, sodium and other gases in the exoplanet's atmosphere.

8.7 Asteroids are minor planets (Sec. 8.5). Most asteroids are in the asteroid belt between the orbits of Mars and Jupiter. They were prevented from forming a planet by Jupiter's gravitational tugs. Asteroids range up to about 1,000 km across (Sec. 8.5a), but the vast majority are much smaller, down to about 50 m. Most of them are stony, while some contain carbon and others consist largely of iron and nickel. The Galileo, Hyabusa, Rosetta, Dawn, and Near Earth Asteroid Rendezvous (renamed NEAR Shoemaker) missions have studied asteroids up close, and NEAR Shoemaker landed on the asteroid Eros (Sec. 8.5b). It found a solid object of primitive composition, in contrast with a rubble pile like the asteroid Mathilde.

Asteroids are minor planets (Sec. 8.5). Most asteroids are in the asteroid belt between the orbits of Mars and Jupiter. They were prevented from forming a planet by Jupiter's gravitational tugs. Asteroids range up to about 1,000 km across (Sec. 8.5a), but the vast majority are much smaller, down to about 50 m. Most of them are stony, while some contain carbon and others consist largely of iron and nickel. The Galileo, Hyabusa, Rosetta, Dawn, and Near Earth Asteroid Rendezvous (renamed NEAR Shoemaker) missions have studied asteroids up close, and NEAR Shoemaker landed on the asteroid Eros (Sec. 8.5b). It found a solid object of primitive composition, in contrast with a rubble pile like the asteroid Mathilde.

9.9 Astronomers are now intensively looking for Goldilocks planets - that is, exoplanets in the habitable zone, in which water can exist in the liquid state, being neither too hot nor too cold (Sec. 9.4). Exoplanets in or near the habitable zone have already been found, and a few are even small enough to have rocky surfaces.

Astronomers are now intensively looking for Goldilocks planets - that is, exoplanets in the habitable zone, in which water can exist in the liquid state, being neither too hot nor too cold (Sec. 9.4). Exoplanets in or near the habitable zone have already been found, and a few are even small enough to have rocky surfaces.

14.11 Astronomers conclude that supermassive black holes, with millions or billions of times the mass of the Sun, exist in the centers of quasars and most galaxies (Section 14.8). Measurements of stars and rotating gas disks in the central regions of these objects reveal very high speeds, necessitating strong gravitational fields in very small volumes. Even our own Milky Way Galaxy contains a central black hole having several million solar masses. The largest known black holes have the equivalent of about 10 billion Suns.

Astronomers conclude that supermassive black holes, with millions or billions of times the mass of the Sun, exist in the centers of quasars and most galaxies (Section 14.8). Measurements of stars and rotating gas disks in the central regions of these objects reveal very high speeds, necessitating strong gravitational fields in very small volumes. Even our own Milky Way Galaxy contains a central black hole having several million solar masses. The largest known black holes have the equivalent of about 10 billion Suns.

14.9 At first glance, it appears as though black holes provide passageways to distant parts of our Universe, or even to other universes (Section 14.6). Such wormholes do not work, even in principle, for nonrotating black holes; one would need to travel faster than the speed of light to traverse them. In the case of a rotating black hole, on the other hand, travel through the wormhole at speeds slower than that of light, avoiding the singularity, initially seems feasible. But actually, the gravity of any object attempting to traverse a wormhole destroys the passage, precluding a safe journey.

At first glance, it appears as though black holes provide passageways to distant parts of our Universe, or even to other universes (Section 14.6). Such wormholes do not work, even in principle, for nonrotating black holes; one would need to travel faster than the speed of light to traverse them. In the case of a rotating black hole, on the other hand, travel through the wormhole at speeds slower than that of light, avoiding the singularity, initially seems feasible. But actually, the gravity of any object attempting to traverse a wormhole destroys the passage, precluding a safe journey.

12.3 Atoms that have lost electrons are ions (Sec. 12.3a). Each element is defined by the number of protons in its nucleus. Varieties of the same element with different numbers of neutrons are isotopes (Sec. 12.3b). Isotopes that decay spontaneously are radioactive (Sec. 12.3c). Neutrinos are extremely light, neutral particles given off in some radioactive decays as well as during nuclear fusion in stars. They are so elusive that they escape from a star immediately.

Atoms that have lost electrons are ions (Sec. 12.3a). Each element is defined by the number of protons in its nucleus. Varieties of the same element with different numbers of neutrons are isotopes (Sec. 12.3b). Isotopes that decay spontaneously are radioactive (Sec. 12.3c). Neutrinos are extremely light, neutral particles given off in some radioactive decays as well as during nuclear fusion in stars. They are so elusive that they escape from a star immediately.

13.10 Binary‐star systems consisting of a pair of neutron stars, at least one of which is a pulsar, are proving very useful for testing the general theory of relativity (Sec. 13.3f ). Their orbital periods are getting shorter at a rate that agrees with the idea that energy in the form of gravitational waves is given off. Observational facilities have been built to directly detect gravitational waves, so far without success.

Binary‐star systems consisting of a pair of neutron stars, at least one of which is a pulsar, are proving very useful for testing the general theory of relativity (Sec. 13.3f ). Their orbital periods are getting shorter at a rate that agrees with the idea that energy in the form of gravitational waves is given off. Observational facilities have been built to directly detect gravitational waves, so far without success.

14.10 Black holes can be detected through their gravitational effects on their surroundings (Section 14.7). We look for black holes from collapsed stars in regions where flickering x‐rays come from an accretion disk (Section 14.7a). We think a black hole is present where, as for Cygnus X‐1, we find a visible object that is in an orbit (detected via a periodically varying Doppler shift in its spectrum) around an invisible object that is too massive and too faint to be anything other than a black hole (Section 14.7b). More than two dozen probable stellar‐mass black holes in our Galaxy have been identified and measured (Section 14.7c). The most massive known stellar‐mass black hole is in another nearby galaxy and has a mass of 23 to 33 Suns. Astronomers are now searching for signs of event horizons in these systems. A peculiar x‐ray binary known as SS433, now thought to contain a black hole, shoots out jets of matter reaching 25 per cent the speed of light (Section 14.7d).

Black holes can be detected through their gravitational effects on their surroundings (Section 14.7). We look for black holes from collapsed stars in regions where flickering x‐rays come from an accretion disk (Section 14.7a). We think a black hole is present where, as for Cygnus X‐1, we find a visible object that is in an orbit (detected via a periodically varying Doppler shift in its spectrum) around an invisible object that is too massive and too faint to be anything other than a black hole (Section 14.7b). More than two dozen probable stellar‐mass black holes in our Galaxy have been identified and measured (Section 14.7c). The most massive known stellar‐mass black hole is in another nearby galaxy and has a mass of 23 to 33 Suns. Astronomers are now searching for signs of event horizons in these systems. A peculiar x‐ray binary known as SS433, now thought to contain a black hole, shoots out jets of matter reaching 25 per cent the speed of light (Section 14.7d).

14.1 Black holes result when too much mass is present in a collapsed star to stop at the neutron‐star stage (introductory section). The strong gravitational field of a stellar‐mass black hole bends light, according to Einstein's general theory of relativity (Section 14.1). Only light within an exit cone escapes (Section 14.2). Light on the edge of the exit cone orbits in the photon sphere.

Black holes result when too much mass is present in a collapsed star to stop at the neutron‐star stage (introductory section). The strong gravitational field of a stellar‐mass black hole bends light, according to Einstein's general theory of relativity (Section 14.1). Only light within an exit cone escapes (Section 14.2). Light on the edge of the exit cone orbits in the photon sphere.

12.6 Brown dwarfs are sometimes called "failed stars," with insufficient mass to begin nuclear fusion of protons, ordinary hydrogen (Sec. 12.6). However, they do fuse deuterium (a rare, heavy form of hydrogen) to helium for a short time, so in a sense they do act like stars. Brown dwarfs have a mass range of 13 to 75 Jupiter masses (which is 1.3 to 7.5 per cent of the Sun's mass). Being very cool and dim, brown dwarfs were not discovered until 1995, but new infrared surveys of the sky have revealed large numbers of them.

Brown dwarfs are sometimes called "failed stars," with insufficient mass to begin nuclear fusion of protons, ordinary hydrogen (Sec. 12.6). However, they do fuse deuterium (a rare, heavy form of hydrogen) to helium for a short time, so in a sense they do act like stars. Brown dwarfs have a mass range of 13 to 75 Jupiter masses (which is 1.3 to 7.5 per cent of the Sun's mass). Being very cool and dim, brown dwarfs were not discovered until 1995, but new infrared surveys of the sky have revealed large numbers of them.

15.3 By studying the distribution of globular star clusters in the sky, Harlow Shapley deduced that we are not at the center of our Galaxy (Section 15.4). However, he overestimated our distance from the center because he didn't know about interstellar extinction, the dimming of starlight by dust. We now can measure this effect by noticing that it also reddens starlight: Dust preferentially scatters (reflects) or absorbs the violet and blue light, while the longer wavelengths pass through more easily.

By studying the distribution of globular star clusters in the sky, Harlow Shapley deduced that we are not at the center of our Galaxy (Section 15.4). However, he overestimated our distance from the center because he didn't know about interstellar extinction, the dimming of starlight by dust. We now can measure this effect by noticing that it also reddens starlight: Dust preferentially scatters (reflects) or absorbs the violet and blue light, while the longer wavelengths pass through more easily.

Methods to Measure: The Composition of a Star

Chemical composition detailed analysis of the stars spectral lines.

11.9 Clusters, or physical groups of stars, come in two basic varieties. Open clusters contain up to a few thousand stars irregularly spread in a small region of sky, generally in the plane of our Galaxy (Sec. 11.8a). They are loosely bound by gravity, but gradually dissipate away as individual stars escape. Globular clusters, consisting of 10,000 to a million stars more strongly bound by gravity than in open clusters, have spherical shapes and form a halo around the center of our Galaxy. They contain low abundances of heavy elements, so they are very old.

Clusters, or physical groups of stars, come in two basic varieties. Open clusters contain up to a few thousand stars irregularly spread in a small region of sky, generally in the plane of our Galaxy (Sec. 11.8a). They are loosely bound by gravity, but gradually dissipate away as individual stars escape. Globular clusters, consisting of 10,000 to a million stars more strongly bound by gravity than in open clusters, have spherical shapes and form a halo around the center of our Galaxy. They contain low abundances of heavy elements, so they are very old.

14.8 Determining the spin of a black hole is difficult, but there are now two methods for doing so by measuring the properties of the accretion disk of gas surrounding the black hole (Section 14.5b). The accretion disk's inner edge is set by a relativistic phenomenon known as frame‐dragging. Stable circular orbits in the same direction as the black‐hole spin can exist closer to rapidly spinning black holes than to slowly spinning ones. However, the last stable orbit is farther out if the black hole's spin is opposite in direction to that of the accretion disk.

Determining the spin of a black hole is difficult, but there are now two methods for doing so by measuring the properties of the accretion disk of gas surrounding the black hole (Section 14.5b). The accretion disk's inner edge is set by a relativistic phenomenon known as frame‐dragging. Stable circular orbits in the same direction as the black‐hole spin can exist closer to rapidly spinning black holes than to slowly spinning ones. However, the last stable orbit is farther out if the black hole's spin is opposite in direction to that of the accretion disk.

15.2 Emission nebulae glow because ultraviolet radiation from hot stars ionizes the gas, and electrons jumping down to lower energy levels emit light (Section 15.3). Absorption nebulae (dark nebulae) block radiation that comes from behind them. Reflection nebulae, like those near the stars of the Pleiades, reflect radiation; they often look blue, for the same reason that the sky is blue.

Emission nebulae glow because ultraviolet radiation from hot stars ionizes the gas, and electrons jumping down to lower energy levels emit light (Section 15.3). Absorption nebulae (dark nebulae) block radiation that comes from behind them. Reflection nebulae, like those near the stars of the Pleiades, reflect radiation; they often look blue, for the same reason that the sky is blue.

14.12 Evidence for intermediate‐mass black holes (containing about 100 to 100,000 solar masses) has been found (Section 14.9). Such black holes may have resulted from a merger of perhaps hundreds or thousands of stellar‐mass black holes in a star cluster, or perhaps they are nuclei of small galaxies being consumed by a larger galaxy.

Evidence for intermediate‐mass black holes (containing about 100 to 100,000 solar masses) has been found (Section 14.9). Such black holes may have resulted from a merger of perhaps hundreds or thousands of stellar‐mass black holes in a star cluster, or perhaps they are nuclei of small galaxies being consumed by a larger galaxy.

Methods to Measure: The Mass of a Star or Planet

From studies of the orbit of binary stars

Methods to Measure: The Power Output (luminosity) of a Start

From the star's apparent brightness, gives its distance

14.14 Gamma‐ray bursts may be the birth cries of certain types of stellar‐mass black holes (Section 14.10b). Specifically, bursts of relatively long duration (typically a few tens of seconds) appear to be produced when the core of a very massive star deficient in hydrogen and usually even helium collapses to form a black hole, with the remaining material ejected as an unusually energetic Type Ib or (more frequently) Type Ic supernova. Those of short duration (typically less than 2 s) are less well understood, but probably result from either the merging of two neutron stars to form a black hole or the merging of a neutron star and a black hole.

Gamma‐ray bursts may be the birth cries of certain types of stellar‐mass black holes (Section 14.10b). Specifically, bursts of relatively long duration (typically a few tens of seconds) appear to be produced when the core of a very massive star deficient in hydrogen and usually even helium collapses to form a black hole, with the remaining material ejected as an unusually energetic Type Ib or (more frequently) Type Ic supernova. Those of short duration (typically less than 2 s) are less well understood, but probably result from either the merging of two neutron stars to form a black hole or the merging of a neutron star and a black hole.

15.1 Gas and dust (small particles of matter) are present to some extent throughout a galaxy, between the stars; a nebula (plural nebulae) is a substantial cloud of such gas and dust (introductory paragraph). At night, from a good observing location, we see a band of stars, nebulae, gas, and dust - the Milky Way - stretch across the sky (Section 15.1). It is our internal, edge‐on view of our Galaxy, the Milky Way Galaxy. With the unaided eye, we cannot see more than a few thousand light‐years through the Milky Way because of the large amount of dust; thus, it appears that we are close to the center of our Galaxy, but this is only an illusion (Section 15.2).

Gas and dust (small particles of matter) are present to some extent throughout a galaxy, between the stars; a nebula (plural nebulae) is a substantial cloud of such gas and dust (introductory paragraph). At night, from a good observing location, we see a band of stars, nebulae, gas, and dust - the Milky Way - stretch across the sky (Section 15.1). It is our internal, edge‐on view of our Galaxy, the Milky Way Galaxy. With the unaided eye, we cannot see more than a few thousand light‐years through the Milky Way because of the large amount of dust; thus, it appears that we are close to the center of our Galaxy, but this is only an illusion (Section 15.2).

15.9 Giant molecular clouds, containing a hundred thousand to a million times the mass of the Sun, are fundamental building blocks of our Galaxy; they are the locations at which new stars form (Section 15.13). Infrared satellites and radio telescopes have permitted mapping of the Orion Molecular Cloud and others. The Atacama Large Millimeter/submillimeter Array is a major international project to provide high‐resolution images of giant molecular clouds and other objects at radio wavelengths (Section 15.14).

Giant molecular clouds, containing a hundred thousand to a million times the mass of the Sun, are fundamental building blocks of our Galaxy; they are the locations at which new stars form (Section 15.13). Infrared satellites and radio telescopes have permitted mapping of the Orion Molecular Cloud and others. The Atacama Large Millimeter/submillimeter Array is a major international project to provide high‐resolution images of giant molecular clouds and other objects at radio wavelengths (Section 15.14).

11.1 Heating dense, opaque matter causes it to grow much brighter and to have the maximum in its continuous spectrum shift to shorter wavelengths (Sec. 11.1a). The continuous radiation approximately follows a blackbody curve, also called a Planck curve. Classifying stars by their spectra led to O B A F G K M L T Y, with spectral‐type A stars having the strongest hydrogen lines (Sec. 11.1b). The spectral sequence is one of progressively decreasing surface temperature, with Otype stars being the hottest. Some L‐type objects, and most of the even cooler T‐type and Y‐type objects, are actually brown dwarfs

Heating dense, opaque matter causes it to grow much brighter and to have the maximum in its continuous spectrum shift to shorter wavelengths (Sec. 11.1a). The continuous radiation approximately follows a blackbody curve, also called a Planck curve. Classifying stars by their spectra led to O B A F G K M L T Y, with spectral‐type A stars having the strongest hydrogen lines (Sec. 11.1b). The spectral sequence is one of progressively decreasing surface temperature, with Otype stars being the hottest. Some L‐type objects, and most of the even cooler T‐type and Y‐type objects, are actually brown dwarfs

13.3 Heavyweight stars, having more than 8 to 10 times the Sun's mass, become red supergiants (Sec. 13.2a). Heavy elements build up in layers inside these stars. When the innermost core builds up enough iron, it collapses and the surrounding layers rebound (explode); the result is a Type II supernova (plural supernovae), a type of core‐collapse supernova that has hydrogen in its spectrum. The remaining compact object is a very dense neutron star, consisting almost entirely of neutrons. If the evolved massive star had previously lost its outer envelope of gases, it would explode in a similar manner, but hydrogen lines would be absent in its spectrum; these are known as Type Ib (helium present) and Type Ic (no helium) supernovae.

Heavyweight stars, having more than 8 to 10 times the Sun's mass, become red supergiants (Sec. 13.2a). Heavy elements build up in layers inside these stars. When the innermost core builds up enough iron, it collapses and the surrounding layers rebound (explode); the result is a Type II supernova (plural supernovae), a type of core‐collapse supernova that has hydrogen in its spectrum. The remaining compact object is a very dense neutron star, consisting almost entirely of neutrons. If the evolved massive star had previously lost its outer envelope of gases, it would explode in a similar manner, but hydrogen lines would be absent in its spectrum; these are known as Type Ib (helium present) and Type Ic (no helium) supernovae.

13.4 Hydrogen‐deficient Type Ia supernovae, in contrast, occur when a carbon‐oxygen white dwarf in a binary star system receives too much mass from its companion to remain in that state and undergoes a nuclear runaway that completely destroys the white dwarf (Sec. 13.2b). This happens a little bit below the Chandrasekhar limit, the theoretical maximum mass of a white dwarf (about 1.4 solar masses). In some of these white‐dwarf supernovae, the companion appears to be a relatively normal star, but in others it may be a second white dwarf that merges with the first one. In both scenarios for white‐dwarf supernovae, the explosion does not produce a compact remnant (neutron star or black hole), unlike the case in core‐collapse supernovae.

Hydrogen‐deficient Type Ia supernovae, in contrast, occur when a carbon‐oxygen white dwarf in a binary star system receives too much mass from its companion to remain in that state and undergoes a nuclear runaway that completely destroys the white dwarf (Sec. 13.2b). This happens a little bit below the Chandrasekhar limit, the theoretical maximum mass of a white dwarf (about 1.4 solar masses). In some of these white‐dwarf supernovae, the companion appears to be a relatively normal star, but in others it may be a second white dwarf that merges with the first one. In both scenarios for white‐dwarf supernovae, the explosion does not produce a compact remnant (neutron star or black hole), unlike the case in core‐collapse supernovae.

13.2 If a star is in a binary system, its evolution can be sped up by the transfer of matter from a companion star filling its Roche lobe, the region in which the companion's gravity dominates (Sec. 13.1e). The flowing gas forms an accretion disk around the recipient star because of the rotation of the system. Matter falling onto a white dwarf from a companion star can release gravitational energy suddenly, or even undergo rapid nuclear fusion on the white dwarf's surface, flaring up to form a nova (plural novae).

If a star is in a binary system, its evolution can be sped up by the transfer of matter from a companion star filling its Roche lobe, the region in which the companion's gravity dominates (Sec. 13.1e). The flowing gas forms an accretion disk around the recipient star because of the rotation of the system. Matter falling onto a white dwarf from a companion star can release gravitational energy suddenly, or even undergo rapid nuclear fusion on the white dwarf's surface, flaring up to form a nova (plural novae).

14.5 If you were far from a black hole, watching clocks fall toward it, you would see them run progressively more slowly, an effect known as time dilation (Section 14.4). Indeed, it would take an infinite amount of time (as measured by your clock) for them to reach the horizon. From the falling clocks' perspective, in contrast, no such time dilation occurs. On the other hand, if the falling clocks were to approach the event horizon and subsequently escape from the vicinity of the black hole, less time would have passed for them than for you; they would have aged less.

If you were far from a black hole, watching clocks fall toward it, you would see them run progressively more slowly, an effect known as time dilation (Section 14.4). Indeed, it would take an infinite amount of time (as measured by your clock) for them to reach the horizon. From the falling clocks' perspective, in contrast, no such time dilation occurs. On the other hand, if the falling clocks were to approach the event horizon and subsequently escape from the vicinity of the black hole, less time would have passed for them than for you; they would have aged less.

10.8 In 1905, Albert Einstein published five very important papers, including two on the "special theory of relativity," which assumes that the speed of light is an important constant that cannot be exceeded by real objects moving through space (Sec. 10.3). In 1916, Einstein published his new theory of gravity, the general theory of relativity; mass and energy are assumed to warp (curve) space and time, and objects move freely within curved space‐time. Some basic tests of this theory involved the large mass of the Sun. Specifically, it was verified that the Sun's mass affects the orbit of Mercury; the perihelion (point of closest approach to the Sun) rotates, or precesses, slowly with time, showing the changing orientation of Mercury's elliptical orbit. Observational verification that the Sun bends starlight passing near it, as predicted by Einstein, instantly made him world famous.

In 1905, Albert Einstein published five very important papers, including two on the "special theory of relativity," which assumes that the speed of light is an important constant that cannot be exceeded by real objects moving through space (Sec. 10.3). In 1916, Einstein published his new theory of gravity, the general theory of relativity; mass and energy are assumed to warp (curve) space and time, and objects move freely within curved space‐time. Some basic tests of this theory involved the large mass of the Sun. Specifically, it was verified that the Sun's mass affects the orbit of Mercury; the perihelion (point of closest approach to the Sun) rotates, or precesses, slowly with time, showing the changing orientation of Mercury's elliptical orbit. Observational verification that the Sun bends starlight passing near it, as predicted by Einstein, instantly made him world famous.

8.3 In a comet, a long tail that points away from the Sun may extend from a bright head (Sec. 8.3). The head consists of the nucleus, which is like a dirty snowball, and the gases of the coma (Sec. 8.3a). The dust tail contains dust particles released from the dirty ices of the nucleus. The gas tail, also known as an ion tail, is blown out behind the comet by the solar wind. Comets we see come from a huge spherical Oort comet cloud far beyond Pluto's orbit or from a more flattened belt of icy objects called the scattered disk, somewhat beyond Neptune's orbit (Sec. 8.3b)

In a comet, a long tail that points away from the Sun may extend from a bright head (Sec. 8.3). The head consists of the nucleus, which is like a dirty snowball, and the gases of the coma (Sec. 8.3a). The dust tail contains dust particles released from the dirty ices of the nucleus. The gas tail, also known as an ion tail, is blown out behind the comet by the solar wind. Comets we see come from a huge spherical Oort comet cloud far beyond Pluto's orbit or from a more flattened belt of icy objects called the scattered disk, somewhat beyond Neptune's orbit (Sec. 8.3b)

14.6 In equilibrium, a black hole "has no hair" - in other words, its only measurable properties are mass, angular momentum (loosely, the rate at which it is spinning), and total electric charge (Section 14.5).

In equilibrium, a black hole "has no hair" - in other words, its only measurable properties are mass, angular momentum (loosely, the rate at which it is spinning), and total electric charge (Section 14.5).

12.4 In the hot cores of stars, collisions have stripped electrons away from the atomic nuclei, leaving the atoms fully ionized (Sec. 12.4). Nuclear fusion requires very high temperatures (millions of kelvins); only at high speeds can some of the protons overcome their electrical repulsion and get sufficiently close to each other for the "strong nuclear force" (which holds protons and neutrons together in nuclei) to be effective. The greater a star's mass, the hotter its core becomes before it generates enough pressure to counteract gravity, so the star is more luminous.

In the hot cores of stars, collisions have stripped electrons away from the atomic nuclei, leaving the atoms fully ionized (Sec. 12.4). Nuclear fusion requires very high temperatures (millions of kelvins); only at high speeds can some of the protons overcome their electrical repulsion and get sufficiently close to each other for the "strong nuclear force" (which holds protons and neutrons together in nuclei) to be effective. The greater a star's mass, the hotter its core becomes before it generates enough pressure to counteract gravity, so the star is more luminous.

Galilean satellites

Io, Europa, Ganymede, and Callistro: the four major satellites of Jupiter discovered in 1610

8.8 It is plausible that an impact of a large asteroid or comet threw so much dust into the Earth's atmosphere that it led to the extinction of most living species, including dinosaurs, 65 million years ago (Sec. 8.5b and A Closer Look 8.4). We worry about a Near‐ Earth Object (an asteroid or a comet) hitting the Earth, which could lead to the demise of civilization, or of the more likely collision with the Earth of a smaller object, which could cause substantial damage, killing thousands or millions of people.

It is plausible that an impact of a large asteroid or comet threw so much dust into the Earth's atmosphere that it led to the extinction of most living species, including dinosaurs, 65 million years ago (Sec. 8.5b and A Closer Look 8.4). We worry about a Near‐ Earth Object (an asteroid or a comet) hitting the Earth, which could lead to the demise of civilization, or of the more likely collision with the Earth of a smaller object, which could cause substantial damage, killing thousands or millions of people.

7. 6 Its atmosphere appears very bland (Sec. 7.3a); chemical reactions are more limited than on Jupiter and Saturn because it is colder. Uranus's extensive clouds form relatively deep in the atmosphere. Methane is a major constituent of Uranus's atmosphere, including the clouds, and it accounts for the planet's greenish blue color. Uranus has several thin rings, discovered during an occultation of a star observed from Earth (Sec. 7.3b). The magnetic field of Uranus is greatly tipped and offset from the planet's center (Sec. 7.3c).

Its atmosphere appears very bland (Sec. 7.3a); chemical reactions are more limited than on Jupiter and Saturn because it is colder. Uranus's extensive clouds form relatively deep in the atmosphere. Methane is a major constituent of Uranus's atmosphere, including the clouds, and it accounts for the planet's greenish blue color. Uranus has several thin rings, discovered during an occultation of a star observed from Earth (Sec. 7.3b). The magnetic field of Uranus is greatly tipped and offset from the planet's center (Sec. 7.3c).

7. 1 Jupiter (318 times more massive than Earth), Saturn (95 times more massive), Uranus (15 times more massive), and Neptune (17 times more massive) are giant planets, also known as jovian planets (introductory section). They have extensive gaseous atmospheres, liquid (Jupiter, Saturn) or icy (Uranus, Neptune) interiors, and rocky cores. Each of them has many moons and at least one ring. All were visited by at least one Voyager spacecraft.

Jupiter (318 times more massive than Earth), Saturn (95 times more massive), Uranus (15 times more massive), and Neptune (17 times more massive) are giant planets, also known as jovian planets (introductory section). They have extensive gaseous atmospheres, liquid (Jupiter, Saturn) or icy (Uranus, Neptune) interiors, and rocky cores. Each of them has many moons and at least one ring. All were visited by at least one Voyager spacecraft.

7. 3 Jupiter's Galilean satellites (Sec. 7.1g) are comparable in size to our Moon. They were studied in detail from the Voyagers and the Galileo spacecraft. Io has over 100 volcanoes, many of which are erupting at any given time (Sec. 7.1g(i)). Europa is covered with smooth ice, perhaps over an ocean (Sec. 7.1g(ii)). Ganymede (Sec. 7.1g(iii)) and Callisto (Sec. 7.1g(iv)) are heavily cratered. Jupiter has dozens of additional moons (Sec. 7.1g(v)), making it look like a miniature planetary system of its own.

Jupiter's Galilean satellites (Sec. 7.1g) are comparable in size to our Moon. They were studied in detail from the Voyagers and the Galileo spacecraft. Io has over 100 volcanoes, many of which are erupting at any given time (Sec. 7.1g(i)). Europa is covered with smooth ice, perhaps over an ocean (Sec. 7.1g(ii)). Ganymede (Sec. 7.1g(iii)) and Callisto (Sec. 7.1g(iv)) are heavily cratered. Jupiter has dozens of additional moons (Sec. 7.1g(v)), making it look like a miniature planetary system of its own.

7.2 Jupiter's diameter is 11 times that of Earth (Sec. 7.1). We learned a tremendous amount about the planet and its moons from the two Voyager (both in 1979) and Galileo (1995-2003) spacecraft (Sec. 7.1a). Jupiter's Great Red Spot (Sec. 7.1b), a giant circulating storm, is a few times larger than Earth. Jupiter's atmosphere shows bright and dark bands, which are different cloud layers (Sec. 7.1c). A probe dropped from the Galileo spacecraft measured the atmosphere's composition, density, and other properties. Jupiter has an internal source of energy, perhaps the energy remaining from its contraction (Sec. 7.1d). Its magnetic field is very strong (Sec. 7.1e), and it has a thin ring (Sec. 7.1f ).

Jupiter's diameter is 11 times that of Earth (Sec. 7.1). We learned a tremendous amount about the planet and its moons from the two Voyager (both in 1979) and Galileo (1995-2003) spacecraft (Sec. 7.1a). Jupiter's Great Red Spot (Sec. 7.1b), a giant circulating storm, is a few times larger than Earth. Jupiter's atmosphere shows bright and dark bands, which are different cloud layers (Sec. 7.1c). A probe dropped from the Galileo spacecraft measured the atmosphere's composition, density, and other properties. Jupiter has an internal source of energy, perhaps the energy remaining from its contraction (Sec. 7.1d). Its magnetic field is very strong (Sec. 7.1e), and it has a thin ring (Sec. 7.1f ).

Methods to Measure: Whether or not a Star has a Planet Orbiting it

Look at the way and direction which the star moves.

8.6 Many meteors are seen in showers, which occur when the Earth crosses the path of a defunct or disintegrating comet (Sec. 8.4b); these meteors appear to come from a single radiant in the sky. Sporadic meteors are not associated with showers and appear at the rate of a few each hour.

Many meteors are seen in showers, which occur when the Earth crosses the path of a defunct or disintegrating comet (Sec. 8.4b); these meteors appear to come from a single radiant in the sky. Sporadic meteors are not associated with showers and appear at the rate of a few each hour.

8.5 Meteoroids are chunks of rock in space up to about 50 m across, but most are small, like grains of sand (Sec. 8.4). When one hits the Earth's atmosphere, we see the streak of light from its vaporization as a meteor, often incorrectly called a shooting star or a falling star. A fragment that survives and reaches Earth's surface is a meteorite. Bits of space dust are micrometeorites (Sec. 8.4a). Most meteorites that are found are made of an iron-nickel alloy, but when meteorites are seen to fall, they are most often stony. The meteorites bring us primordial material to study and occasionally pieces of the Moon or Mars.

Meteoroids are chunks of rock in space up to about 50 m across, but most are small, like grains of sand (Sec. 8.4). When one hits the Earth's atmosphere, we see the streak of light from its vaporization as a meteor, often incorrectly called a shooting star or a falling star. A fragment that survives and reaches Earth's surface is a meteorite. Bits of space dust are micrometeorites (Sec. 8.4a). Most meteorites that are found are made of an iron-nickel alloy, but when meteorites are seen to fall, they are most often stony. The meteorites bring us primordial material to study and occasionally pieces of the Moon or Mars.

14.15 Mini black holes may have formed in the early Universe, but there is no direct evidence for them (Section 14.11). If they were indeed present, they should gradually evaporate by a quantum process, finally resulting in a violent burst of gamma rays. They are not, however, the origin of observed gamma‐ray bursts.

Mini black holes may have formed in the early Universe, but there is no direct evidence for them (Section 14.11). If they were indeed present, they should gradually evaporate by a quantum process, finally resulting in a violent burst of gamma rays. They are not, however, the origin of observed gamma‐ray bursts.

9.4 Most exoplanets discovered with ground‐based telescopes since 1995 were found with the radialvelocity method, also known as the Doppler‐wobble method (Sec. 9.2c), in which periodic changes in the Doppler shift (radial velocity) of a star (the subtle "reflex motion" caused by the orbiting exoplanet) are measured with high‐quality spectrographs. But one limitation of this method is that we generally don't know the angle of the plane in which the planets are orbiting their parent stars; thus, only the minimum mass of each planet can be derived. This problem left the nagging question of whether the objects were really planets or merely lowmass companion stars.

Most exoplanets discovered with ground‐based telescopes since 1995 were found with the radialvelocity method, also known as the Doppler‐wobble method (Sec. 9.2c), in which periodic changes in the Doppler shift (radial velocity) of a star (the subtle "reflex motion" caused by the orbiting exoplanet) are measured with high‐quality spectrographs. But one limitation of this method is that we generally don't know the angle of the plane in which the planets are orbiting their parent stars; thus, only the minimum mass of each planet can be derived. This problem left the nagging question of whether the objects were really planets or merely lowmass companion stars.

13.9 Most pulsars slow down as they radiate energy (Sec. 13.3e). Some pulsars, however, spin very rapidly, up to a few hundred times per second. They were probably spun up by accretion of gas from a companion star.

Most pulsars slow down as they radiate energy (Sec. 13.3e). Some pulsars, however, spin very rapidly, up to a few hundred times per second. They were probably spun up by accretion of gas from a companion star

14.3 Near a stellar‐mass black hole, or on the surface of a star collapsing to form a black hole, the tidal force can be immense, pulling a local observer apart (Section 14.3b). This effect occurs because of the large difference in gravity between his head and feet: the force on the latter is much larger than that on the former. The tidal force on an observer would be smaller near a more massive black hole than near a less massive black hole.

Near a stellar‐mass black hole, or on the surface of a star collapsing to form a black hole, the tidal force can be immense, pulling a local observer apart (Section 14.3b). This effect occurs because of the large difference in gravity between his head and feet: the force on the latter is much larger than that on the former. The tidal force on an observer would be smaller near a more massive black hole than near a less massive black hole.

7. 7 Neptune was discovered in 1846, after mathematicians predicted its existence based on discrepancies between the observed and expected positions of Uranus in the sky (Sec. 7.4). Apparently, Galileo saw it in 1613, but did not recognize it as a new planet. Like Uranus, Neptune is an ice giant and appears greenish blue in a telescope because of its atmospheric methane (Sec. 7.4a). Methane‐crystal clouds also cover Neptune. When Voyager 2 flew by in 1989, Neptune's surface showed a Great Dark Spot, a high‐pressure region analogous to Jupiter's Great Red Spot. It disappeared between the Voyager visit and a later Hubble Space Telescope view. Neptune has a substantial core, as does Uranus; unlike Uranus, however, Neptune has an internal energy source (Sec. 7.4b). Neptune's magnetic field is strange, resembling that of Uranus. These magnetic fields are thus probably formed in electrically conducting shells of gas. Neptune's rings are clumpy (Sec. 7.4c). The clumps are dynamic, fading faster than had been expected.

Neptune was discovered in 1846, after mathematicians predicted its existence based on discrepancies between the observed and expected positions of Uranus in the sky (Sec. 7.4). Apparently, Galileo saw it in 1613, but did not recognize it as a new planet. Like Uranus, Neptune is an ice giant and appears greenish blue in a telescope because of its atmospheric methane (Sec. 7.4a). Methane‐crystal clouds also cover Neptune. When Voyager 2 flew by in 1989, Neptune's surface showed a Great Dark Spot, a high‐pressure region analogous to Jupiter's Great Red Spot. It disappeared between the Voyager visit and a later Hubble Space Telescope view. Neptune has a substantial core, as does Uranus; unlike Uranus, however, Neptune has an internal energy source (Sec. 7.4b). Neptune's magnetic field is strange, resembling that of Uranus. These magnetic fields are thus probably formed in electrically conducting shells of gas. Neptune's rings are clumpy (Sec. 7.4c). The clumps are dynamic, fading faster than had been expected.

7. 8 Neptune's moon Triton is very large (Sec. 7.4d). Voyager 2 took high‐resolution images of Triton that showed a varied surface with plumes from ice volcanoes. Triton has been very geologically active. Occultation studies show that it is gradually warming.

Neptune's moon Triton is very large (Sec. 7.4d). Voyager 2 took high‐resolution images of Triton that showed a varied surface with plumes from ice volcanoes. Triton has been very geologically active. Occultation studies show that it is gradually warming.

15.8 Observations of our Galaxy at radio wavelengths have been very important (Section 15.10). Specifically, the observed 21‐cm line comes from the spin‐flip transition of hydrogen atoms, when the spin of the electron changes relative to that of the proton. Studies of the 21‐cm line have enabled us to map our Galaxy by finding the distances to H I regions (Section 15.11). Observations of interstellar molecules, primarily carbon monoxide, have also been valuable in this regard (Section 15.12).

Observations of our Galaxy at radio wavelengths have been very important (Section 15.10). Specifically, the observed 21‐cm line comes from the spin‐flip transition of hydrogen atoms, when the spin of the electron changes relative to that of the proton. Studies of the 21‐cm line have enabled us to map our Galaxy by finding the distances to H I regions (Section 15.11). Observations of interstellar molecules, primarily carbon monoxide, have also been valuable in this regard (Section 15.12).

14.4 Once a collapsing star passes inside its event horizon, it continues to contract, according to the general theory of relativity. Nothing can ever stop its contraction. In fact, the classical mathematical theory predicts that it will contract to zero radius - it will reach a singularity (Section 14.3b). (But quantum effects probably prevent it from actually reaching exactly zero radius.) The event horizon, however, does not change in size as long as the amount of mass inside is constant.

Once a collapsing star passes inside its event horizon, it continues to contract, according to the general theory of relativity. Nothing can ever stop its contraction. In fact, the classical mathematical theory predicts that it will contract to zero radius - it will reach a singularity (Section 14.3b). (But quantum effects probably prevent it from actually reaching exactly zero radius.) The event horizon, however, does not change in size as long as the amount of mass inside is constant.

14.13 One of the most exciting new astronomical fields is the study of gamma‐ray bursts, brief flashes of extremely short‐wavelength radiation (Section 14.10). Although their distances were for many years highly controversial, most of them are now known to come from galaxies billions of light‐years away (Section 14.10a). Their radiation is highly beamed, like that of pulsars; it cannot be emitted uniformly over the sky (Section 14.10b). A leading hypothesis is known as the "fireball model," in which some sort of compact engine releases blobs of material at speeds close to that of light along a narrow "jet."

One of the most exciting new astronomical fields is the study of gamma‐ray bursts, brief flashes of extremely short‐wavelength radiation (Section 14.10). Although their distances were for many years highly controversial, most of them are now known to come from galaxies billions of light‐years away (Section 14.10a). Their radiation is highly beamed, like that of pulsars; it cannot be emitted uniformly over the sky (Section 14.10b). A leading hypothesis is known as the "fireball model," in which some sort of compact engine releases blobs of material at speeds close to that of light along a narrow "jet."

11.6 Optical doubles appear close together by chance (Sec. 11.6a). Most stars are in multiple‐star systems, binary stars, bound by gravity. Visual binaries are revolving around each other and can be detected as double from Earth (Sec. 11.6a). Spectroscopic binaries show their double status from spectra (Sec. 11.6a); one or two sets of spectral lines show periodically changing Doppler shifts. Eclipsing binaries pass in front of each other, as shown in their plots of brightness over time, or "light curves" (Sec. 11.6a). Since astrometry is the study of the positions and motions of stars, astrometric binaries can be detected by their wobbling from side to side in their paths across the sky (Sec. 11.6a).

Optical doubles appear close together by chance (Sec. 11.6a). Most stars are in multiple‐star systems, binary stars, bound by gravity. Visual binaries are revolving around each other and can be detected as double from Earth (Sec. 11.6a). Spectroscopic binaries show their double status from spectra (Sec. 11.6a); one or two sets of spectral lines show periodically changing Doppler shifts. Eclipsing binaries pass in front of each other, as shown in their plots of brightness over time, or "light curves" (Sec. 11.6a). Since astrometry is the study of the positions and motions of stars, astrometric binaries can be detected by their wobbling from side to side in their paths across the sky (Sec. 11.6a).

15.4 Our Galaxy has a nuclear bulge centered on the nucleus, and surrounded by a flat disk that contains spiral arms (Section 15.4). A spherical halo includes the globular clusters, and has a much greater diameter than the disk. We can detect the very center of our Galaxy in infrared light, radio waves, x‐rays, or gamma rays that penetrate the dust between us and it (Section 15.5). The Galactic center is a bright infrared source and almost certainly contains a very massive black hole, about 4 million solar masses.

Our Galaxy has a nuclear bulge centered on the nucleus, and surrounded by a flat disk that contains spiral arms (Section 15.4). A spherical halo includes the globular clusters, and has a much greater diameter than the disk. We can detect the very center of our Galaxy in infrared light, radio waves, x‐rays, or gamma rays that penetrate the dust between us and it (Section 15.5). The Galactic center is a bright infrared source and almost certainly contains a very massive black hole, about 4 million solar masses.

15.6 Our Galaxy is a pinwheel‐shaped spiral galaxy, with spiral arms marked by massive stars, open clusters, and nebulae (Section 15.7). However, from our vantage point inside the Galaxy, it is difficult to accurately trace out the arms. These spiral arms appear to be caused by a slowly rotating spiral density wave (Section 15.8).

Our Galaxy is a pinwheel‐shaped spiral galaxy, with spiral arms marked by massive stars, open clusters, and nebulae (Section 15.7). However, from our vantage point inside the Galaxy, it is difficult to accurately trace out the arms. These spiral arms appear to be caused by a slowly rotating spiral density wave (Section 15.8).

Methods to Measure: The Distance of a Star of a Galaxy

Parallax- the visual effect produced when nearby objects appear to shift positions. Rely on supernovae explosions to gauge distances

11.4 Plotting temperature (often as spectral type) on the horizontal axis and luminosity (intrinsic brightness) on the vertical axis gives a temperatureluminosity diagram (Sec. 11.4), also known as a temperature‐magnitude diagram or a Hertzsprung‐Russell diagram. Most stars appear in a band known as the main sequence, which goes from hot, bluish, luminous stars to cool, reddish, dim stars. Stars on the main sequence are known as dwarfs. Some brighter stars exist and are giants or even supergiants. Stars less luminous than the main sequence for a given temperature are white dwarfs

Plotting temperature (often as spectral type) on the horizontal axis and luminosity (intrinsic brightness) on the vertical axis gives a temperatureluminosity diagram (Sec. 11.4), also known as a temperature‐magnitude diagram or a Hertzsprung‐Russell diagram. Most stars appear in a band known as the main sequence, which goes from hot, bluish, luminous stars to cool, reddish, dim stars. Stars on the main sequence are known as dwarfs. Some brighter stars exist and are giants or even supergiants. Stars less luminous than the main sequence for a given temperature are white dwarfs

8.1 Pluto, which emerged in 1999 from inside Neptune's orbit, is so small and so far away that we know little about it (Sec. 8.1). The discovery of its moon, Charon, allowed us to calculate that Pluto contains only 1/500 the mass of the Earth (Sec. 8.1a). Mutual occultations of Pluto and Charon revealed sizes and surface structures of each. Pluto has a very thin nitrogen atmosphere, at least when it is closest to the Sun in its orbit (Sec. 8.1b).

Pluto, which emerged in 1999 from inside Neptune's orbit, is so small and so far away that we know little about it (Sec. 8.1). The discovery of its moon, Charon, allowed us to calculate that Pluto contains only 1/500 the mass of the Earth (Sec. 8.1a). Mutual occultations of Pluto and Charon revealed sizes and surface structures of each. Pluto has a very thin nitrogen atmosphere, at least when it is closest to the Sun in its orbit (Sec. 8.1b).

8.2 Pluto, which emerged in 1999 from inside Neptune's orbit, is so small and so far away that we know little about it (Sec. 8.1). The discovery of its moon, Charon, allowed us to calculate that Pluto contains only 1/500 the mass of the Earth (Sec. 8.1a). Mutual occultations of Pluto and Charon revealed sizes and surface structures of each. Pluto has a very thin nitrogen atmosphere, at least when it is closest to the Sun in its orbit (Sec. 8.1b).

Pluto, which emerged in 1999 from inside Neptune's orbit, is so small and so far away that we know little about it (Sec. 8.1). The discovery of its moon, Charon, allowed us to calculate that Pluto contains only 1/500 the mass of the Earth (Sec. 8.1a). Mutual occultations of Pluto and Charon revealed sizes and surface structures of each. Pluto has a very thin nitrogen atmosphere, at least when it is closest to the Sun in its orbit (Sec. 8.1b).

7. 4 Saturn has a very low density, lower even than water (Sec. 7.2). Saturn's rings were produced by tidal forces that prevented the formation of a moon, and are within Saturn's Roche limit (Sec. 7.2a). The rings are so thin that stars can be seen through them. Cassini's division is the major break between inner and outer rings. Saturn's atmosphere shows fewer features than Jupiter's, probably because it is colder (Sec. 7.2b). Saturn has an internal source of heat, perhaps both energy from its collapse and energy from sinking helium (Sec. 7.2c). Saturn's moon Titan has a thick, smoggy atmosphere (Sec. 7.2d). The Cassini spacecraft, in 2004, arrived at the Saturn system, which it is still studying in detail. It dropped the Huygens probe down to Titan's surface. On the way down, it photographed channels that were cut by running liquid, undoubtedly methane or ethane. Saturn has many additional satellites, some of which were imaged by the Cassini spacecraft (Sec. 7.2e).

Saturn has a very low density, lower even than water (Sec. 7.2). Saturn's rings were produced by tidal forces that prevented the formation of a moon, and are within Saturn's Roche limit (Sec. 7.2a). The rings are so thin that stars can be seen through them. Cassini's division is the major break between inner and outer rings. Saturn's atmosphere shows fewer features than Jupiter's, probably because it is colder (Sec. 7.2b). Saturn has an internal source of heat, perhaps both energy from its collapse and energy from sinking helium (Sec. 7.2c). Saturn's moon Titan has a thick, smoggy atmosphere (Sec. 7.2d). The Cassini spacecraft, in 2004, arrived at the Saturn system, which it is still studying in detail. It dropped the Huygens probe down to Titan's surface. On the way down, it photographed channels that were cut by running liquid, undoubtedly methane or ethane. Saturn has many additional satellites, some of which were imaged by the Cassini spacecraft (Sec. 7.2e).

10.7 Solar flares and coronal mass ejections are eruptions of tremendous amounts of energy (Sec. 10.2c). Electromagnetic radiation and particles from flares can disrupt radio communications or produce electrical surges, affecting satellites and leading to blackouts of electricity. Prominences are pinkish protrusions off the edge of the Sun and can be seen during total solar eclipses; when viewed in silhouette on the face of the Sun, they appear as relatively dark filaments (Sec. 10.2d).

Solar flares and coronal mass ejections are eruptions of tremendous amounts of energy (Sec. 10.2c). Electromagnetic radiation and particles from flares can disrupt radio communications or produce electrical surges, affecting satellites and leading to blackouts of electricity. Prominences are pinkish protrusions off the edge of the Sun and can be seen during total solar eclipses; when viewed in silhouette on the face of the Sun, they appear as relatively dark filaments (Sec. 10.2d).

9.10 Some of the objects found with the Doppler‐shift technique might actually be brown dwarfs, which are sometimes called "failed stars" because they don't fuse ordinary hydrogen into helium (Sec. 9.5). They can be thought of as links between planets and normal stars. Planetary companions to some brown dwarfs have been imaged at infrared wavelengths.

Some of the objects found with the Doppler‐shift technique might actually be brown dwarfs, which are sometimes called "failed stars" because they don't fuse ordinary hydrogen into helium (Sec. 9.5). They can be thought of as links between planets and normal stars. Planetary companions to some brown dwarfs have been imaged at infrared wavelengths.

8.4 Spacecraft imaged the nucleus of Halley's Comet up close in 1986 and showed it to be about 16 km long and 8 km across (Sec. 8.3c). The impact of the tidally disrupted Comet Shoemaker‐Levy 9 with Jupiter in 1994 produced temporary Earth‐sized scars in Jupiter's atmosphere (Sec. 8.3d). Comet Hale‐Bopp in 1997 was not only spectacular for the general public but also allowed many scientific studies (Sec. 8.3e). The Deep Impact mission crashed into a comet in 2005 (Sec. 8.3f ), providing information on its composition

Spacecraft imaged the nucleus of Halley's Comet up close in 1986 and showed it to be about 16 km long and 8 km across (Sec. 8.3c). The impact of the tidally disrupted Comet Shoemaker‐Levy 9 with Jupiter in 1994 produced temporary Earth‐sized scars in Jupiter's atmosphere (Sec. 8.3d). Comet Hale‐Bopp in 1997 was not only spectacular for the general public but also allowed many scientific studies (Sec. 8.3e). The Deep Impact mission crashed into a comet in 2005 (Sec. 8.3f ), providing information on its composition

12.1 Stars are formed from dense regions in nebulae, "clouds" of gas and dust that are best observed at infrared and radio wavelengths (Sec. 12.1). The collapsing cloud becomes a cluster of protostars and subsequently luminous pre‐main‐sequence stars, which are powered by gradual gravitational contraction (Sec. 12.1a). The dust heats up and becomes visible in the infrared. Some planetary systems in formation may have been discovered around young stars (Sec. 12.1b).

Stars are formed from dense regions in nebulae, "clouds" of gas and dust that are best observed at infrared and radio wavelengths (Sec. 12.1). The collapsing cloud becomes a cluster of protostars and subsequently luminous pre‐main‐sequence stars, which are powered by gradual gravitational contraction (Sec. 12.1a). The dust heats up and becomes visible in the infrared. Some planetary systems in formation may have been discovered around young stars (Sec. 12.1b).

12.2 Stars get their energy from nuclear fusion (Sec. 12.2), keeping the interior gas hot. The outward force from the thermal pressure balances the inward force of gravity, a condition known as "hydrostatic equilibrium." The basic fusion process in the Sun and other main‐sequence stars is a merger (fusion) of four hydrogen nuclei (protons) into one helium nucleus (two protons and two neutrons; two of the protons turned into neutrons during the process), with the difference in mass between the original four protons and the helium nucleus (the "binding energy" of the helium nucleus) transformed to energy according to Einstein's famous equation, E = mc2.

Stars get their energy from nuclear fusion (Sec. 12.2), keeping the interior gas hot. The outward force from the thermal pressure balances the inward force of gravity, a condition known as "hydrostatic equilibrium." The basic fusion process in the Sun and other main‐sequence stars is a merger (fusion) of four hydrogen nuclei (protons) into one helium nucleus (two protons and two neutrons; two of the protons turned into neutrons during the process), with the difference in mass between the original four protons and the helium nucleus (the "binding energy" of the helium nucleus) transformed to energy according to Einstein's famous equation, E = mc2.

13.1 Stars have different fates depending on the mass with which they were born. Lightweight stars are those with mass up to about 10 (but perhaps only 8) times the Sun's mass, while heavyweight stars have larger masses (introductory section). When the Sun and other low‐mass stars exhaust their central hydrogen, they will swell and become luminous red giants as their cores contract and the outer layers expand (Sec. 13.1a). The outer layers subsequently drift off as planetary nebulae, which glow because their gases are ionized by radiation from the hot dying star (Sec. 13.1b). The remaining core continues to contract until electrons won't be compressed further (they become "degenerate"), and the core becomes a white dwarf (Sec. 13.1c). The entire postmain‐ sequence evolution of the Sun can be conveniently traced on a temperature‐luminosity (Hertzsprung‐Russell) diagram (Sec. 13.1d). The evolution of other solitary (single) stars is similar, but differs in detail.

Stars have different fates depending on the mass with which they were born. Lightweight stars are those with mass up to about 10 (but perhaps only 8) times the Sun's mass, while heavyweight stars have larger masses (introductory section). When the Sun and other low‐mass stars exhaust their central hydrogen, they will swell and become luminous red giants as their cores contract and the outer layers expand (Sec. 13.1a). The outer layers subsequently drift off as planetary nebulae, which glow because their gases are ionized by radiation from the hot dying star (Sec. 13.1b). The remaining core continues to contract until electrons won't be compressed further (they become "degenerate"), and the core becomes a white dwarf (Sec. 13.1c). The entire postmain‐ sequence evolution of the Sun can be conveniently traced on a temperature‐luminosity (Hertzsprung‐Russell) diagram (Sec. 13.1d). The evolution of other solitary (single) stars is similar, but differs in detail.

12.8 Stars have several different endpoints to their evolution after the main sequence (Sec. 12.8). These will be discussed in the next two chapters.

Stars have several different endpoints to their evolution after the main sequence (Sec. 12.8). These will be discussed in the next two chapters.

11.5 Stars have small motions across the sky, known as proper motions (Sec. 11.5a). Proper motion is an angular movement of stars across the sky, relative to each other; to get the physical velocity (in km/s) of each star in the plane of the sky, perpendicular to our line of sight, we need to also know its distance. Studies of their Doppler shifts show their radial velocities (motions toward or away from us along the line of sight) as blueshifts or redshifts (Sec. 11.5b). The larger the blueshift or redshift, the proportionally larger is the object's speed toward or away from you.

Stars have small motions across the sky, known as proper motions (Sec. 11.5a). Proper motion is an angular movement of stars across the sky, relative to each other; to get the physical velocity (in km/s) of each star in the plane of the sky, perpendicular to our line of sight, we need to also know its distance. Studies of their Doppler shifts show their radial velocities (motions toward or away from us along the line of sight) as blueshifts or redshifts (Sec. 11.5b). The larger the blueshift or redshift, the proportionally larger is the object's speed toward or away from you.

13.11 Strange planets have been discovered around one pulsar, but they must have formed after the supernova explosion (Sec. 13.3g). Neutron stars in binary systems can be studied in a way other than their existence as radio pulsars: They give off x‐rays emitted by hot gas in an accretion disk fed by the companion star (Sec. 13.3h).

Strange planets have been discovered around one pulsar, but they must have formed after the supernova explosion (Sec. 13.3g). Neutron stars in binary systems can be studied in a way other than their existence as radio pulsars: They give off x‐rays emitted by hot gas in an accretion disk fed by the companion star (Sec. 13.3h).

10.5 Sunspots are regions in the photosphere with strong, tangled magnetic fields and are cooler than the surrounding photosphere; the magnetic fields inhibit the rise of hot, ionized gases from beneath the sunspot (Sec. 10.2a). Each sunspot has a dark umbra surrounded by a lighter penumbra. Sunspots usually appear in pairs having a north pole, a south pole, and magnetic‐field lines linking them; these pairs are often in larger groups.

Sunspots are regions in the photosphere with strong, tangled magnetic fields and are cooler than the surrounding photosphere; the magnetic fields inhibit the rise of hot, ionized gases from beneath the sunspot (Sec. 10.2a). Each sunspot has a dark umbra surrounded by a lighter penumbra. Sunspots usually appear in pairs having a north pole, a south pole, and magnetic‐field lines linking them; these pairs are often in larger groups.

13.6 Supernovae in our Galaxy have left detectable supernova remnants, the expanding debris of the explosions. The gases ejected by supernovae are enriched in heavy elements produced prior to, and during, the explosions (Sec. 13.2d). We owe our existence to supernovae, being made of their debris.

Supernovae in our Galaxy have left detectable supernova remnants, the expanding debris of the explosions. The gases ejected by supernovae are enriched in heavy elements produced prior to, and during, the explosions (Sec. 13.2d). We owe our existence to supernovae, being made of their debris.

Methods to Measure: Temperature of a Star

Surface temperature from the stars spectral lines or from its blackbody curve, which can be found accurately enough by measuring the brightness through different filters such as blue, yellow, and red (the color of the star)

10.3 The Sun's chromosphere is a very thin layer, somewhat hotter than the photosphere below it (Sec. 10.1b). It consists of many spicules, which are jets of gas that rise and fall, and it shows an emissionline spectrum rather than absorption lines, since the gas is silhouetted against the dark sky rather than against the bright photosphere.

The Sun's chromosphere is a very thin layer, somewhat hotter than the photosphere below it (Sec. 10.1b). It consists of many spicules, which are jets of gas that rise and fall, and it shows an emissionline spectrum rather than absorption lines, since the gas is silhouetted against the dark sky rather than against the bright photosphere.

10.2 The Sun's photosphere, observed in all the visible light together (which is known as white light), is covered with tiny granulation (Sec. 10.1a). Each granule is a pocket of rising hot gas or falling cool gas driven by the process of convection, which is similar to water boiling. Oscillations (vibrations) of the surface reveal conditions in the solar interior; the study of the Sun in this manner is called helioseismology by analogy with Earth seismology. The spectrum of the photosphere shows a multitude of Fraunhofer (absorption) lines; as with other normal stars, it does not produce emission lines.

The Sun's photosphere, observed in all the visible light together (which is known as white light), is covered with tiny granulation (Sec. 10.1a). Each granule is a pocket of rising hot gas or falling cool gas driven by the process of convection, which is similar to water boiling. Oscillations (vibrations) of the surface reveal conditions in the solar interior; the study of the Sun in this manner is called helioseismology by analogy with Earth seismology. The spectrum of the photosphere shows a multitude of Fraunhofer (absorption) lines; as with other normal stars, it does not produce emission lines.

13.7 The Type II Supernova 1987A, in a satellite galaxy known as the Large Magellanic Cloud, was the brightest supernova since 1604 and provided many valuable insights (Sec. 13.2e). The star that exploded was a massive, evolved star, as predicted, although its exact nature (blue, instead of red, supergiant) was a surprise. Neutrinos from the supernova, as well as the detection of radioactive heavy elements, show that we had the basic ideas of the theory of Type II supernovae correct. Cosmic rays are particles accelerated to high energy, in many cases from supernova explosions (Sec. 13.2f).

The Type II Supernova 1987A, in a satellite galaxy known as the Large Magellanic Cloud, was the brightest supernova since 1604 and provided many valuable insights (Sec. 13.2e). The star that exploded was a massive, evolved star, as predicted, although its exact nature (blue, instead of red, supergiant) was a surprise. Neutrinos from the supernova, as well as the detection of radioactive heavy elements, show that we had the basic ideas of the theory of Type II supernovae correct. Cosmic rays are particles accelerated to high energy, in many cases from supernova explosions (Sec. 13.2f).

9.3 The astrometric method depends on astrometry, the precise measurement of stellar positions and motions; the presence of an unseen planet is deduced from the star's observed wobble in the sky (Sec. 9.2a). However, so far no exoplanets have been discovered this way. Some planets were finally found in 1992 by timing the radio pulses from a "pulsar," a weird kind of collapsed star to be discussed in Chapter 13, but these are very unusual planets that have little to do with normal planets orbiting Sun‐like stars (Sec. 9.2b).

The astrometric method depends on astrometry, the precise measurement of stellar positions and motions; the presence of an unseen planet is deduced from the star's observed wobble in the sky (Sec. 9.2a). However, so far no exoplanets have been discovered this way. Some planets were finally found in 1992 by timing the radio pulses from a "pulsar," a weird kind of collapsed star to be discussed in Chapter 13, but these are very unusual planets that have little to do with normal planets orbiting Sun‐like stars (Sec. 9.2b).

9.1 The conservation of angular momentum explains why planetary systems in formation contract into a disk (Sec. 9.1a). We think that small clumps of dust joined to make planetesimals, and planetesimals combined to make protoplanets orbiting the protosun. In one of the main models for the formation of the Solar System's outer planets (Sec. 9.1b), silicates (rocks) and clumps of various ices that had formed beyond the "frost line" collided and grew. These planetesimals subsequently came together to form a solid core resembling a terrestrial planet, but more massive, and its gravity then attracted large amounts of hydrogen and helium. Terrestrial planets, on the other hand, formed much later from a debris disk that consisted of rocks but not ice grains, since temperatures were too high inside the frost line. Also, by that time, the powerful young Sun had already blown away the gas in the disk.

The conservation of angular momentum explains why planetary systems in formation contract into a disk (Sec. 9.1a). We think that small clumps of dust joined to make planetesimals, and planetesimals combined to make protoplanets orbiting the protosun. In one of the main models for the formation of the Solar System's outer planets (Sec. 9.1b), silicates (rocks) and clumps of various ices that had formed beyond the "frost line" collided and grew. These planetesimals subsequently came together to form a solid core resembling a terrestrial planet, but more massive, and its gravity then attracted large amounts of hydrogen and helium. Terrestrial planets, on the other hand, formed much later from a debris disk that consisted of rocks but not ice grains, since temperatures were too high inside the frost line. Also, by that time, the powerful young Sun had already blown away the gas in the disk.

13.8 The cores of massive stars, after the supernova explosions, consist of degenerate neutrons that cannot be compressed further; they are called neutron stars (Sec. 13.3a). Some give off beams of radio radiation (and sometimes other electromagnetic waves as well) as they rotate like a lighthouse, and we detect pulses in their brightness. We now know of thousands of these pulsars (Sec. 13.3b). They are explained by the lighthouse model, in which two oppositely directed beams along the magnetic axis are seen only when they rotate into our line of sight (Sec. 13.3c). The discovery of a very rapid pulsar in the Crab Nebula, a young supernova remnant, provided strong support for the hypothesis that pulsars are rotating, magnetized neutron stars (Sec. 13.3d).

The cores of massive stars, after the supernova explosions, consist of degenerate neutrons that cannot be compressed further; they are called neutron stars (Sec. 13.3a). Some give off beams of radio radiation (and sometimes other electromagnetic waves as well) as they rotate like a lighthouse, and we detect pulses in their brightness. We now know of thousands of these pulsars (Sec. 13.3b). They are explained by the lighthouse model, in which two oppositely directed beams along the magnetic axis are seen only when they rotate into our line of sight (Sec. 13.3c). The discovery of a very rapid pulsar in the Crab Nebula, a young supernova remnant, provided strong support for the hypothesis that pulsars are rotating, magnetized neutron stars (Sec. 13.3d).

10.4 The corona, best seen at total solar eclipses, contains gas with a temperature of 2 million kelvins (Sec. 10.1c); thus, it is actually a plasma, consisting of positively and negatively charged particles, and it produces emission lines. The magnetic field shapes the coronal gas into streamers. Regions where the corona is less dense and cooler than average are coronal holes. The slow solar wind is a general coronal expansion, and the fast solar wind comes from coronal holes. Although instruments on spacecraft can be used to block the photosphere, some chromospheric and coronal phenomena are still best observed from the ground during solar eclipses (Sec. 10.1d).

The corona, best seen at total solar eclipses, contains gas with a temperature of 2 million kelvins (Sec. 10.1c); thus, it is actually a plasma, consisting of positively and negatively charged particles, and it produces emission lines. The magnetic field shapes the coronal gas into streamers. Regions where the corona is less dense and cooler than average are coronal holes. The slow solar wind is a general coronal expansion, and the fast solar wind comes from coronal holes. Although instruments on spacecraft can be used to block the photosphere, some chromospheric and coronal phenomena are still best observed from the ground during solar eclipses (Sec. 10.1d).

9.5 The discovery of a three‐planet system helped dispel these concerns, as did the large observed gap in mass between the purported exoplanets and the lowest‐mass stars. Now many such planetary systems are known. Moreover, we have found many "super‐Earths" in which the planet appears to be rocky (rather than a gas giant) and has a mass fewer than 10 Earth masses (down to just a few Earth masses).

The discovery of a three‐planet system helped dispel these concerns, as did the large observed gap in mass between the purported exoplanets and the lowest‐mass stars. Now many such planetary systems are known. Moreover, we have found many "super‐Earths" in which the planet appears to be rocky (rather than a gas giant) and has a mass fewer than 10 Earth masses (down to just a few Earth masses).

9.8 The first exoplanets discovered were giant planets, though lessmassive, terrestrial planets (with rocky surfaces) are being increasingly found in those planetary systems. Exoplanets are often in quite eccentric (elongated) orbits, perhaps as a result of previous gravitational encounters with other planets or the protoplanetary disk (Sec. 9.3). Some of the planets are in circular orbits very close to their parent stars, which probably means (according to the Nice model) that they were formed elsewhere in the systems and later drifted inward.

The first exoplanets discovered were giant planets, though lessmassive, terrestrial planets (with rocky surfaces) are being increasingly found in those planetary systems. Exoplanets are often in quite eccentric (elongated) orbits, perhaps as a result of previous gravitational encounters with other planets or the protoplanetary disk (Sec. 9.3). Some of the planets are in circular orbits very close to their parent stars, which probably means (according to the Nice model) that they were formed elsewhere in the systems and later drifted inward.

7. 9 The jovian planets are much larger than the terrestrial planets because they formed beyond the frost line (or snow line) in the Solar System, accumulating nearby grains and clumps of ices that had condensed out of the gas (Sec. 7.5). After reaching a substantial size, they were able to gravitationally attract very large amounts of hydrogen and helium, unlike the terrestrial planets.

The jovian planets are much larger than the terrestrial planets because they formed beyond the frost line (or snow line) in the Solar System, accumulating nearby grains and clumps of ices that had condensed out of the gas (Sec. 7.5). After reaching a substantial size, they were able to gravitationally attract very large amounts of hydrogen and helium, unlike the terrestrial planets.

13.5 The last bright supernovae to have been seen in our Galaxy were in 1572 and 1604 (Sec. 13.2c). Many supernovae are discovered each year in other galaxies, however.

The last bright supernovae to have been seen in our Galaxy were in 1572 and 1604 (Sec. 13.2c). Many supernovae are discovered each year in other galaxies, however.

15.7 The matter between the stars, the interstellar medium, is mainly hydrogen gas (Section 15.9). Emission nebulae are mostly H II regions, regions of ionized hydrogen. Clouds of higher density in which the atoms of hydrogen are predominantly neutral are called H I regions. Hydrogen molecules (H2) are very difficult to detect, but we think they are plentiful in regions where they are protected by dust from ultraviolet radiation.

The matter between the stars, the interstellar medium, is mainly hydrogen gas (Section 15.9). Emission nebulae are mostly H II regions, regions of ionized hydrogen. Clouds of higher density in which the atoms of hydrogen are predominantly neutral are called H I regions. Hydrogen molecules (H2) are very difficult to detect, but we think they are plentiful in regions where they are protected by dust from ultraviolet radiation.

Maunder minimum

The period 1645-1715 when there were very few sunspots and no periodicity visible.

11.7 The primary use of binary stars is the determination of stellar masses (Sec. 11.6b). In the case of main‐sequence stars, it is found that massive stars are much more luminous (intrinsically bright) than low‐mass stars (Sec. 11.6c). Thus, massive stars use up their fuel faster, and have shorter lives, than low‐mass stars.

The primary use of binary stars is the determination of stellar masses (Sec. 11.6b). In the case of main‐sequence stars, it is found that massive stars are much more luminous (intrinsically bright) than low‐mass stars (Sec. 11.6c). Thus, massive stars use up their fuel faster, and have shorter lives, than low‐mass stars.

12.5 The proton‐proton chain is the basic fusion process in the Sun, while the carbon‐nitrogen‐oxygen cycle dominates in hotter, more massive stars (Sec. 12.5). At still higher temperatures, when stars have left the main sequence, the triple‐alpha process is dominant, where an alpha particle is a helium nucleus. Element building processes are what is meant by nucleosynthesis.

The proton‐proton chain is the basic fusion process in the Sun, while the carbon‐nitrogen‐oxygen cycle dominates in hotter, more massive stars (Sec. 12.5). At still higher temperatures, when stars have left the main sequence, the triple‐alpha process is dominant, where an alpha particle is a helium nucleus. Element building processes are what is meant by nucleosynthesis.

9.2 The search for planets around stars other than the Sun, known as extra‐solar planets or exoplanets, has been going on for many decades (Sec. 9.2). With current technology, they are extremely difficult to see in the glare of their "parent star" (or "host star," the star they orbit), though a few have been directly imaged. Instead, we watch for motions in the star that are caused by something orbiting it, or we look for planetary transits

The search for planets around stars other than the Sun, known as extra‐solar planets or exoplanets, has been going on for many decades (Sec. 9.2). With current technology, they are extremely difficult to see in the glare of their "parent star" (or "host star," the star they orbit), though a few have been directly imaged. Instead, we watch for motions in the star that are caused by something orbiting it, or we look for planetary transits

Methods to Measure: The Speed with which a Star is Moving Towards or Away from Earth

The shifts in the star's spectral lines toward the blue or red end of the spectrum.

10.6 The solar‐activity cycle, including the sunspot cycle, lasts about 11 years (or about 22 years, if magnetic polarity is included); the average number of sunspots periodically rises and falls (Sec. 10.2b). The Maunder minimum was a 17th‐ and 18th‐century period when sunspots were essentially absent. The next solar maximum is expected in late 2013 and will probably be weaker than average, but there is some evidence that the following sunspot cycle might not occur at all.

The solar‐activity cycle, including the sunspot cycle, lasts about 11 years (or about 22 years, if magnetic polarity is included); the average number of sunspots periodically rises and falls (Sec. 10.2b). The Maunder minimum was a 17th‐ and 18th‐century period when sunspots were essentially absent. The next solar maximum is expected in late 2013 and will probably be weaker than average, but there is some evidence that the following sunspot cycle might not occur at all.

14.7 The spherical event horizon of a rotating black hole is smaller than that of a nonrotating black hole of the same mass (Section 14.5a). Moreover, rotating black holes have a stationary limit within which no particles can remain at rest, even though they are outside the event horizon. The space between the stationary limit and the event horizon is the ergosphere, from which energy (work) can be extracted. If a black hole was to rotate faster than a certain value, its event horizon would vanish, revealing a naked singularity from which distant observers could receive signals. Most theorists assume that all singularities are clothed by an event horizon.

The spherical event horizon of a rotating black hole is smaller than that of a nonrotating black hole of the same mass (Section 14.5a). Moreover, rotating black holes have a stationary limit within which no particles can remain at rest, even though they are outside the event horizon. The space between the stationary limit and the event horizon is the ergosphere, from which energy (work) can be extracted. If a black hole was to rotate faster than a certain value, its event horizon would vanish, revealing a naked singularity from which distant observers could receive signals. Most theorists assume that all singularities are clothed by an event horizon.

11.10 The temperature‐luminosity diagrams for open clusters show the ages of the clusters by the length of the cluster's main sequence; massive, short‐lived stars leave the main sequence faster than the low‐mass, long‐lived stars (Sec. 11.8b). The temperature‐luminosity diagrams for globular clusters in our Galaxy are all very similar, with short main sequences, indicating that the globular clusters are all about the same age - very old - providing a minimum value for the age of our Universe.

The temperature‐luminosity diagrams for open clusters show the ages of the clusters by the length of the cluster's main sequence; massive, short‐lived stars leave the main sequence faster than the low‐mass, long‐lived stars (Sec. 11.8b). The temperature‐luminosity diagrams for globular clusters in our Galaxy are all very similar, with short main sequences, indicating that the globular clusters are all about the same age - very old - providing a minimum value for the age of our Universe.

12.7 Theorists predict a certain number of neutrinos that should result from fusion in the Sun. Careful experiments had generally found only about one‐third to one‐half that number (Sec. 12.7a). To explain this deficit, physicists suggested that most of the neutrinos change from one type to several different types on their journey from the Sun to the Earth. Recently, measurements at the Sudbury Neutrino Observatory showed that the predicted number of neutrinos is indeed being produced by the Sun, and that they change type as they travel (Sec. 12.7b).

Theorists predict a certain number of neutrinos that should result from fusion in the Sun. Careful experiments had generally found only about one‐third to one‐half that number (Sec. 12.7a). To explain this deficit, physicists suggested that most of the neutrinos change from one type to several different types on their journey from the Sun to the Earth. Recently, measurements at the Sudbury Neutrino Observatory showed that the predicted number of neutrinos is indeed being produced by the Sun, and that they change type as they travel (Sec. 12.7b).

7. 5 Uranus was the first planet to be discovered that had not been known to the ancients (Sec. 7.3). It consists largely of hydrogen and helium, although its rocky and icy core makes up a more substantial part of the planet than in the case of Jupiter or Saturn. Surrounding the core is an extensive layer of various compressed ices, primarily water, methane, and ammonia; thus, Uranus can be thought of as an "ice giant," while Jupiter and Saturn are often called "gas giants" or "gas/liquid giants." There is no internal energy source. Uranus rotates on its side, so it is heated by the Sun in a strange way over an 84‐year period of revolution.

Uranus was the first planet to be discovered that had not been known to the ancients (Sec. 7.3). It consists largely of hydrogen and helium, although its rocky and icy core makes up a more substantial part of the planet than in the case of Jupiter or Saturn. Surrounding the core is an extensive layer of various compressed ices, primarily water, methane, and ammonia; thus, Uranus can be thought of as an "ice giant," while Jupiter and Saturn are often called "gas giants" or "gas/liquid giants." There is no internal energy source. Uranus rotates on its side, so it is heated by the Sun in a strange way over an 84‐year period of revolution.

10.1 We are increasingly finding signs that planetary systems are forming around other stars (Sec. 9.6). A probable disk around the star β (beta) Pictoris and apparent protoplanetary disks observed with the Hubble Space Telescope seem to be direct observations of planetary systems in formation.

We are increasingly finding signs that planetary systems are forming around other stars (Sec. 9.6). A probable disk around the star β (beta) Pictoris and apparent protoplanetary disks observed with the Hubble Space Telescope seem to be direct observations of planetary systems in formation.

9.11 We are increasingly finding signs that planetary systems are forming around other stars (Sec. 9.6). A probable disk around the star β (beta) Pictoris and apparent protoplanetary disks observed with the Hubble Space Telescope seem to be direct observations of planetary systems in formation.

We are increasingly finding signs that planetary systems are forming around other stars (Sec. 9.6). A probable disk around the star β (beta) Pictoris and apparent protoplanetary disks observed with the Hubble Space Telescope seem to be direct observations of planetary systems in formation.

11.2 We find the distance to the nearest stars by triangulation, or "trigonometric parallax" (Sec. 11.2), a kind of binocular vision obtained by taking advantage of our location on a moving platform (the Earth). The more distant a star, the smaller is its parallax. Measuring the positions and motions of stars is known as astrometry. The Hipparcos spacecraft has greatly improved the accuracy of the astrometry for a hundred thousand stars, and with a lesser improvement for about a million more, but only within about 1 per cent of the diameter of our Galaxy.

We find the distance to the nearest stars by triangulation, or "trigonometric parallax" (Sec. 11.2), a kind of binocular vision obtained by taking advantage of our location on a moving platform (the Earth). The more distant a star, the smaller is its parallax. Measuring the positions and motions of stars is known as astrometry. The Hipparcos spacecraft has greatly improved the accuracy of the astrometry for a hundred thousand stars, and with a lesser improvement for about a million more, but only within about 1 per cent of the diameter of our Galaxy.

14.2 When the radius of the star becomes equal to the Schwarzschild radius, it is so compact that the exit cone closes, and no light escapes; the star is entirely within its event horizon (Section 14.3), and it is called a black hole. The Schwarzschild radius, only 3 km for a black hole having the mass of the Sun, is directly proportional to the mass. In the Newtonian view, when a star's size decreases to the Schwarzschild radius, the escape velocity from the star's surface reaches the speed of light (Section 14.3a).

When the radius of the star becomes equal to the Schwarzschild radius, it is so compact that the exit cone closes, and no light escapes; the star is entirely within its event horizon (Section 14.3), and it is called a black hole. The Schwarzschild radius, only 3 km for a black hole having the mass of the Sun, is directly proportional to the mass. In the Newtonian view, when a star's size decreases to the Schwarzschild radius, the escape velocity from the star's surface reaches the speed of light (Section 14.3a).

9.7 With sophisticated equipment, we are now beginning to obtain direct images of exoplanets, especially young systems viewed at infrared wavelengths (Sec. 9.2e). When the images of a given system are separated by a few years, the orbital motion of exoplanets can even be detected. Another method for finding exoplanets involves gravitational microlensing (Sec. 9.2f), though it is extremely difficult to conduct additional follow‐up studies of them.

With sophisticated equipment, we are now beginning to obtain direct images of exoplanets, especially young systems viewed at infrared wavelengths (Sec. 9.2e). When the images of a given system are separated by a few years, the orbital motion of exoplanets can even be detected. Another method for finding exoplanets involves gravitational microlensing (Sec. 9.2f), though it is extremely difficult to conduct additional follow‐up studies of them.

Eclipsing Binaries

a binary star in which one member periodically hides the other.

Exoplanet

a planet orbiting a star other than the sun, aka extra solar planet

Globular cluster

a spherically symmetric, gravitational bound collection of typically hundreds of thousands of stars that form from the same cloud of gas. Generally found in the halo of our galaxy and other galaxies.

7.1 Jupiter radiates more energy than it receives from the Sun. The most likely source of this excess energy is a) gravitational contraction b) hydrogen fusion c) the magnetic field d) radioactive decay e) chemical reactions

a) gravitational contraction

14.10 Which one of the following observations is one of the pieces of evidence used by astronomers to detect an object that could potentially be a black hole? Select one: a. Flickering x-rays from an accretion disk around the black hole. b. A very black area in the universe with no stars visible nearby. c. Hydrogen spectral lines that are greatly Doppler shifted. d. A worm hole leading to another universe. e. A visible star pulling on an apparently empty region of space.

a. Flickering x-rays from an accretion disk around the black hole.

7.2 Violently volcanic, with perhaps the youngest surface in the solar system is the satellite a. Io. b. Europa. c. Ganymede. d. Callisto. e. Amalthea.

a. Io.

14.3 Matter from a companion star in orbit around a black hole forms a/an Select one: a. accretion disk. b. ergosphere. c. event horizon. d. singularity. e. bipolar flow.

a. accretion disk.

13.1 After a star enters its red giant phase its core will heat up enough for the triple-alpha process to begin to produce Select one: a. carbon. b. helium. c. hydrogen. d. nitrogen. e. oxygen.

a. carbon.

15.4 The "spiral arms" of the Galaxy are embedded in the Select one: a. disk. b. galactic corona. c. halo. d. nuclear bulge. e. galactic nucleus.

a. disk.

11.5 A binary star system which varies in brightness as one star passes in front of the other is a/an Select one: a. eclipsing binary. b. optical double. c. astrometric binary. d. spectroscopic binary. e. visual binary.

a. eclipsing binary.

7.9 Because of conditions on Titan, the role water plays on Earth may be played by liquid a. ethane. b. ammonia. c. hydrogen. d. nitrogen. e. carbon dioxide.

a. ethane.

15.8 Gaseous emission nebulae in the Milky Way Galaxy look red because Select one: a. many electrons are jumping from the third to the second energy levels of hydrogen, producing H-alpha emission. b. they are moving away from us, so that their light is redshifted. c. they absorb red light from their surroundings. d. they have temperatures of only about 100 K, and Wein's law tells us that the light they emit is therefore red. e. they are made mostly of iron compounds, like rust.

a. many electrons are jumping from the third to the second energy levels of hydrogen, producing H-alpha emission.

12.8 The amount of energy released in a nuclear fusion reaction is directly proportional to the Select one: a. mass difference between the initial reactants and final products. b. mass of the initial reactants. c. mass of the final products. d. mass of the proton. e. mass of the quarks in the products.

a. mass difference between the initial reactants and final products.

12.5 Compared with low-mass stars, high-mass stars have main-sequence lifetimes that are Select one: a. much shorter. b. much longer. c. about the same. d. slightly shorter. e. slightly longer.

a. much shorter.

10.1 The light we receive from the Sun comes from the layer of the Sun's atmosphere called the a. photosphere. b. solar corona. c. solar wind. d. scattering of sunlight. e. sunspots.

a. photosphere.

13.10 When a main-sequence star runs out of hydrogen fuel in its core, Select one: a. the core contracts and thus heats up. b. the core expands and thus heats up. c. the core expands and thus cools down. d. the core contracts and thus cools down. e. the core remains about the same size, but heats up as the fusion of helium to carbon begins immediately after the hydrogen fuel is gone.

a. the core contracts and thus heats up.

10.9 Of the regions of the Sun listed below, which has the highest temperature? Select one: a. the core. b. the corona. c. the photosphere. d. a typical sunspot. e. a prominence.

a. the core.

11.10 A way of determining that a globular cluster is old is by charting the population of stars on a temperature-luminosity diagram and noticing that Select one: a. the massive, hot stars are missing from the chart. b. most of the stars on the chart are bluish in color. c. the cluster has started to expand in size. d. the spectra of many of the stars are red-shifted. e. most of the stars on the chart are red giants

a. the massive, hot stars are missing from the chart.

8/9.4 Most of the extrasolar planets detected since 1995 are unlikely to harbor life as we know it because a. they are gas giants, without solid surfaces. b. they are too far from their stars. c. their orbits are too inclined relative to the equators of their stars. d. they are too small. e. their atmospheres do not contain sufficient oxygen.

a. they are gas giants, without solid surfaces.

10.7 Which of the following is NOT influenced by the Sun's magnetic field? Select one: a. Solar prominences. b. The Sun's blackbody spectrum. c. Solar flares. d. Solar filaments. e. Sunspots.

b. The Sun's blackbody spectrum.

10.10 Einstein's General Theory of Relativity published in 1916 was verified through which solar event in 1919? Select one: a. a coronal mass ejection (CME). b. a total solar eclipse. c. a solar prominence. d. the existence of sunspots. e. a total lunar eclipse.

b. a total solar eclipse.

11.7 One can determine the radius of a nearby star (that is not necessarily on the main sequence) knowing only its Select one: a. radial velocity and luminosity. b. apparent brightness, parallax, and surface temperature. c. apparent brightness, parallax, and radial velocity. d. chemical composition, distance, and surface temperature. e. apparent brightness and distance.

b. apparent brightness, parallax, and surface temperature.

14.1 If, after a supernova, the stellar core retains more than 2 or 3 solar masses, the result will be a Select one: a. black dwarf. b. black hole. c. neutron star. d. supermassive star. e. white dwarf.

b. black hole.

14.8 If a massive star (more than ten times the mass of the Sun) should form a black hole, a companion star that had been orbiting this now black-hole-star at a distance of one light-year would Select one: a. be pulled into the newly formed black hole by its intense gravity. b. continue to orbit in its original orbit. c. be torn apart by tidal forces from the newly formed black hole. d. be thrown into a much larger orbit by the energy given off by the black hole. e. likely become a black-hole itself.

b. continue to orbit in its original orbit.

13.8 High-energy charged particles believed to be produced most of the time by supernovae are called Select one: a. neutrinos. b. cosmic rays. c. pulsar rays. d. novae. e. gamma rays.

b. cosmic rays.

7.7 The Roche limit of a planet is the a. largest mass that a planet can have before it would fall into the Sun. b. distance from the planet outside which gas and dust could coalesce without being torn apart by tidal forces from the planet. c. distance of a planet's smallest moon from that planet. d. maximum distance from the planet that gas and dust could coalesce to form a moon for that planet. e. closest that a spacecraft could come to that planet without being torn apart by tidal forces from the planet.

b. distance from the planet outside which gas and dust could coalesce without being torn apart by tidal forces from the planet.

15.9 At the present time, stars in our Galaxy tend to form most readily in Select one: a. the Galactic halo. b. giant molecular clouds in spiral arms. c. the central supermassive black hole. d. the Galactic bulge. e. globular clusters.

b. giant molecular clouds in spiral arms.

15.3 The large (60,000 light-year radius) sphere of older stars and globular clusters around the central region of the Galaxy is the Select one: a. disk. b. halo. c. nuclear bulge. d. galactic nucleus. e. galactic black hole.

b. halo.

12.2 The fusion process which produces energy in most stars forms helium from the nuclei of Select one: a. carbon. b. hydrogen. c. iron. d. oxygen. e. nitrogen.

b. hydrogen.

12.9 Brown dwarfs emit electromagnetic radiation primarily as which of the following types of light? Select one: a. microwave radiation. b. infrared radiation. c. ultraviolet radiation. d. gamma radiation. e. x-ray radiation.

b. infrared radiation.

11.4 A temperature-luminosity diagram of stars usually includes a diagonal band called the Select one: a. H-R line. b. main sequence. c. main line. d. color index. e. star diagonal.

b. main sequence.

8/9.5 Modern theories of solar system formation posit that planets formed through aggregation of many smaller bodies, each perhaps only hundreds of kilometers in size, called a. planetoids. b. planetesimals. c. protoplanets. d. nebulae. e. satellites.

b. planetesimals.

8.9/2 The largest component of a comet is its a. coma. b. tail. c. head. d. hydrogen cloud. e. nucleus.

b. tail.

8/9.7 Jupiter's chemical composition is most similar to that of a. Earth. b. the Sun. c. Pluto. d. Venus. e. meteorites.

b. the Sun.

14.5 The presence of a black hole in a galaxy core can be inferred from Select one: a. the mass of the galaxy. b. the velocities of stars near the core. c. the color index of the galaxy. d. the distance of the galaxy from the Milky Way. e. the visible light luminosity of the galaxy.

b. the velocities of stars near the core.

13.6 The Chandrasekhar limit is the Select one: a. fastest speed with which light can travel in empty space. b. theoretical maximum mass which a white dwarf can have without exploding as a nova. c. maximum mass that a pulsar can have without slowing down. d. minimum mass that a pulsar can have and still emit radio waves. e. theoretical maximum mass a black hole can have.

b. theoretical maximum mass which a white dwarf can have without exploding as a nova.

12.7 In a stable star, the gravitational contraction forces are balanced by Select one: a. gravitational forces acting in the opposite direction. b. thermal pressure. c. the strong nuclear force. d. chemical reactions. e. electromagnetic forces.

b. thermal pressure.

15.5 To trace out the spiral structure of our Galaxy we should look at Select one: a. very old objects. b. very young objects. c. neutron stars. d. dark nebulae. e. stars with more than one planet.

b. very young objects.

11.3 A star that is 40 parsecs from the Sun, if moved to a distance of 10 parsecs, would be Select one: a. 4 times brighter. b. 4 magnitudes brighter. c. 16 times brighter. d. 16 times dimmer. e. the same brightness.

c. 16 times brighter.

10.5 The orientation of the Sun's magnetic field changes, repeating a full cycle about every Select one: a. year. b. 11 years. c. 22 years. d. 33 years. e. 44 years.

c. 22 years.

12.3 The number of neutrons in a neutral atom of 614C is (atomic number=6; atomic mass=14) Select one: a. 2. b. 6. c. 8. d. 14. e. 20.

c. 8.

11.6 Star Albert appears to have the same brightness through red and blue filters. Star Bohr appears brighter in the red than in the blue. Star Curie appears brighter in the blue than in the red. Which of the following orderings of these stars has them ordered from coolest to hottest? Select one: a. Bohr, Curie, Albert b. Albert, Curie, Bohr c. Bohr, Albert, Curie d. Curie, Albert, Bohr e. Albert, Bohr, Curie

c. Bohr, Albert, Curie

7.10 The name of the space probe that arrived at Saturn in 2004, and has been making scientific observations of Saturn and its moons since then is named a. Titan b. Huygens c. Cassini d. Roche e. Apollo

c. Cassini

15.7 The astronomer who is credited with first arguing in 1917 that Earth is not at the center of our galaxy is Select one: a. Edwin Hubble b. Galileo Galilei c. Harlow Shapley d. Stephen Hawking e. Nicholas Copernicus

c. Harlow Shapley

7.6 The planet that rotates 'on its side' as it revolves around the Sun is a. Jupiter. b. Saturn. c. Uranus. d. Neptune. e. Pluto.

c. Uranus.

8/9.1 Which of the following is NOT a possible description of Pluto? a. an escaped satellite of Neptune b. the 9th major planet c. a brown dwarf d. a Kuiper belt object e. a trans-Neptunian object

c. a brown dwarf

12.10 The so-called neutrino problem of the 1980s and 90s was concerned with Select one: a. the absence of any neutrinos from our Sun. b. the death of dinosaurs from increased neutrino radiation from our Sun about 65 million years ago. c. detection of only about one-third as many neutrinos from the Sun as were expected. d. constructing the Hubble telescope to detect these very important particles from supernovae. e. destruction of Earth's ozone from neutrinos originating in the center of our galaxy.

c. detection of only about one-third as many neutrinos from the Sun as were expected.

15.6 The spectral line that has a wavelength of 21 cm is believed to be produced by Select one: a. black holes. b. a Type II supernova. c. hydrogen gas. d. interstellar dust. e. a Type Ia supernova.

c. hydrogen gas.

14.7 Gamma-ray bursts have been found to be have sources Select one: a. in our solar system. b. mostly in our galaxy, the Milky Way. c. in galaxies billions of light years away. d. in supernovae in galaxies close to our galaxy e. in T-Tauri stars in distant galaxies.

c. in galaxies billions of light years away.

7.3 Compared with the other giant planets, Uranus is evidently unique in having no a. cloud structure. b. equator. c. internal heat source. d. rings. e. small satellites.

c. internal heat source.

8/9.6 Using ground-based telescopes, most massive exoplanets orbiting other stars have been found by a. looking at a massive planet's Doppler-shifted light as it orbits the star. b. taking spectra of the star to look for contamination from the light of the massive planet. c. looking for a periodic Doppler shift in the star's spectrum as a massive planet causes the star to "wobble" slightly. d. pointing the telescope to a spot about 5 to 10 au away from the star, because massive planets can only orbit near that distance. e. measuring the Doppler shift of the star, and seeing whether it keeps on increasing with time, as would be expected if a massive planet were pulling on it.

c. looking for a periodic Doppler shift in the star's spectrum as a massive planet causes the star to "wobble" slightly.

13.7 Typical novae occur when Select one: a. a red-giant star ejects a planetary nebula. b. two neutron stars merge, forming a more massive neutron star. c. matter accreted from a companion star unstably ignites on the surface of a white dwarf. d. an extremely massive star collapses, and also ejects its outer atmosphere. e. a neutron star's magnetic field becomes strong enough to produce two oppositely directed jets of rapidly moving particles.

c. matter accreted from a companion star unstably ignites on the surface of a white dwarf.

13.5 If, after a Type II supernova, the stellar core is less than 2 solar masses, the result will probably be a Select one: a. black dwarf. b. black hole. c. neutron star. d. supermassive star. e. white dwarf.

c. neutron star.

14.4 Supermassive black holes probably exist in Select one: a. the outer solar system. b. pulsars. c. the centers of most galaxies. d. novae. e. the halo of the Milky Way Galaxy.

c. the centers of most galaxies.

8/9.9 Most known comets do not reappear in our sky very often because a. comets are not gravitationally bound to our Solar System. b. comets burn quite rapidly, using up all their fuel in a few decades. c. the orbits of comets are highly eccentric and their semimajor axes are large. d. most comets only pass close to the Sun two or three times, and then they disintegrate. e. it is impossible to tell when most comets will return close to the Sun.

c. the orbits of comets are highly eccentric and their semimajor axes are large.

15.2 The symbol H II would denote a hydrogen atom with a charge of Select one: a. -2. b. -1. c. 0. d. +1. e. +2.

d. +1.

10.8 Solar filaments that project into space at the Sun's visible edge are called Select one: a. the corona. b. a sunspot. c. the solar wind. d. a prominence. e. supergranulations.

d. a prominence.

7.4 Neptune's characteristic color is caused by absorption by atmospheric a. carbon dioxide. b. sulfuric acid. c. nitrogen. d. methane. e. oxygen

d. methane.

12.4 The process of building heavier atoms from lighter ones is called Select one: a. atom building. b. ionization. c. isotope production. d. nucleosynthesis. e. nuclear mitosis.

d. nucleosynthesis.

8/9.8 The giant (jovian) planets are large compared with the terrestrial planets because the giant planets a. have higher densities. b. formed by the same processes as the Sun, and can be considered miniature stars. c. have many moons. d. obtained and retained more gas and ice because of their large distance from the Sun. e. did not suffer as many major destructive collisions as the terrestrial planets.

d. obtained and retained more gas and ice because of their large distance from the Sun.

11.2 The angle at which a star appears to move when observed from the ends of a baseline of 1 A.U. is the star's Select one: a. proper motion. b. radial velocity. c. space velocity. d. parallax. e. speckle.

d. parallax.

13.4 Oscillating stars were ruled out as an explanation for pulsars because they would not have the correct Select one: a. energy. b. intensity. c. magnitude. d. period. e. luminosity.

d. period.

14.2 The spherical surface around a collapsed star in which light can orbit is the Select one: a. ergosphere. b. event horizon. c. exit cone. d. photon sphere. e. Schwarzschild radius.

d. photon sphere.

13.2 When a star becomes unstable, it may eject shells of gas into space forming a/an Select one: a. asteroid belt. b. dust cloud. c. ring. d. planetary nebula. e. protostar.

d. planetary nebula.

12.1 A stellar sized hot body that derives its energy from free-falling gravitational collapse is a Select one: a. brown dwarf. b. circumstellar disk . c. main-sequence star. d. protostar. e. white dwarf.

d. protostar.

11.8 The Doppler shift of a star depends only on the star's Select one: a. mass. b. distance. c. temperature. d. radial velocity. e. parallax.

d. radial velocity.

15.1 A cloud of dust and gas that scatters the light of nearby stars is a/an Select one: a. dark nebula. b. emission nebula. c. nebula. d. reflection nebula. e. sky nebula.

d. reflection nebula.

10.4 Areas of the Sun appearing relatively dark when seen in white light are Select one: a. coronal holes. b. supergranulations. c. spicules. d. sunspots. e. chromospheres.

d. sunspots.

10.6 Sunspots appear dark to us on Earth because Select one: a. they are rocks protruding from under the Sun's photosphere. b. they are remnants of black holes that formed as the Sun was forming. c. clouds in the Sun's atmosphere are blocking the light from those areas. d. these areas are cooler and not as bright as the rest of the photosphere. e. large coronal mass ejections heading toward Earth block light from the Sun.

d. these areas are cooler and not as bright as the rest of the photosphere.

7.8 Although supposedly discovered in 1846 after computing the location of an unknown planet that was perturbing the orbit of Uranus, it is likely that Neptune was actually observed by a. Aristotle. b. Tycho Brahe. c. Johannes Kepler. d. Isaac Newton. e. Galileo Galilei

e. Galileo Galilei

15.10 Which one of the following statements about our Milky Way Galaxy is FALSE? Select one: a. New stars generally form in the spiral arms. b. Nebulae such as the Orion Nebula are stellar nurseries - regions where new stars are forming, or recently formed. c. It is difficult to see the central regions of our Galaxy in optical (visible) light because intervening dust absorbs and scatters the light. d. Rapid motions of stars near the center suggest that it harbors a black hole, millions of times the mass of our Sun. e. Globular star clusters reside in the halo, and contain main-sequence stars spanning all spectral types, from O through M.

e. Globular star clusters reside in the halo, and contain main-sequence stars spanning all spectral types, from O through M.

14.6 What three quantities completely define the physical characteristics of a black hole? These characteristics can be used to discriminate one black hole from another black hole. Select one: a. Color (black), speed, composition. b. Percentage of iron, diameter, speed. c. Mass, Color (Black), radius of event horizon. d. Nuclear charge, direction of spin, age. e. Mass, spin rate, electric charge.

e. Mass, spin rate, electric charge.

13.9 Which of the following statements about pulsars is true? Select one: a. All neutron stars are pulsars, if viewed from the right direction. b. Pulsars could very likely be signals from intelligent aliens. c. Once a pulsar forms, it typically lasts for billions of years. d. If we observe a Type II supernova and do not see a pulsar afterward at the same location, we know that a black hole must be present there. e. Pulsars cannot be rotating white dwarfs because their rotation rate is so high that a white dwarf would be ripped apart.

e. Pulsars cannot be rotating white dwarfs because their rotation rate is so high that a white dwarf would be ripped apart.

12.6 As a protostar is formed from a cloud of gas and becomes a pre-main-sequence star, which one of the following events does NOT occur? Select one: a. The cloud of gas contracts due to gravity. b. The density of the gas increases. c. The pressure of the gas increases. d. The temperature of the gas increases. e. The protons in the gas begin to fuse.

e. The protons in the gas begin to fuse.

14.9 Astronomers believe that when our Sun becomes a black hole far into the future Select one: a. the outer layers of the Sun will extend at least as far as the current orbit of Jupiter incinerating all the terrestrial planets. b. all the major and minor planets in our Solar System will be pulled into this black hole. c. only the inner terrestrial planets will be pulled into this black hole. d. life would continue as usual on Earth. e. Wait! Our Sun is too small to ever form a black hole.

e. Wait! Our Sun is too small to ever form a black hole.

8/9.10 The major piece of evidence that Pluto has an atmosphere has been obtained from a. samples of its atmospheric gases returned via the New Horizons spacecraft. b. spectroscopic measurement of its atmosphere from a Voyager I flyby. c. noticing the frictional drag on its moon Charon that is causing it to slow down in its orbit. d. using a spectrometer to observe the spectral lines of the Sun as Pluto passed between it and Earth. e. measuring the gradual decrease, then increase in the light of a known star as Pluto passed in front of (occulted) that star.

e. measuring the gradual decrease, then increase in the light of a known star as Pluto passed in front of (occulted) that star.

8/9.3 When a fragment of interplanetary matter survives its trip through an atmosphere to reach the surface of an astronomical body it is called a a. bolide. b. fireball. c. meteor. d. meteoroid. e. meteorite.

e. meteorite.

10.3 As the solar atmosphere expands outward into interplanetary space it becomes the Select one: a. chromosphere. b. core. c. corona. d. photosphere. e. solar wind.

e. solar wind.

11.1 Categories of stars based on their respective spectra are called Select one: a. absorption groups. b. absorption classes. c. line types. d. spectral groups. e. spectral types.

e. spectral types.

11.9 Cephid variables are useful to astronomers for measuring distance to other galaxies because Select one: a. the period of their variation in brightness is correlated to their distance from Earth. b. their radial velocity is correlated with their distance from Earth. c. their temperature on the H-R Diagram is correlated to their distance from Earth. d. their composition is correlated to their average intrinsic luminosity. e. the period of their variation in brightness is correlated to their average intrinsic luminosity.

e. the period of their variation in brightness is correlated to their average intrinsic luminosity.

7.5 The low average densities of Jupiter and Saturn compared with the density of Earth suggest that a. they are hollow. b. their gravitational attraction has squeezed material out of their cores. c. they consist primarily of water. d. volcanic eruptions have ejected all the iron that was originally in their cores. e. they contain large quantities of light elements, such as hydrogen and helium.

e. they contain large quantities of light elements, such as hydrogen and helium.

13.3 Stars retaining less than 1.4 solar masses after their unstable phase will become Select one: a. neutron stars. b. protostars. c. red giants. d. variable. e. white dwarfs.

e. white dwarfs.

10.2 The solar corona is so hot it emits mainly Select one: a. infrared radiation. b. radio waves. c. ultraviolet radiation. d. visible light. e. x-rays.

e. x-rays.

Albedo

the fraction of incident light reflected by a body

Ionosphere

the highest region of the Earths atmosphere

Cassini's division

the major division in the rings of saturn

Corona/solar or steller

the outermost region of the sun, characterized by temperatures of millions of kelvins

Chromosphere

the part of the atmosphere of the sun (or another star) between the photosphere and the corona. It is probably entirely composed of spicules and probably roughly corresponds to the region in which mechanical energy is deposited