A) The number of quasars has remained approximately constant over time, with new quasars being formed to replace those which deplete their fuel and die out.
B) Quasars were much more numerous 11 or 12 billion years ago, but since then the supermassive black holes in their centers have depleted their available gas and dust and their accretion disks have disappeared and they are no longer quasars.
C) It takes billions of years to accumulate enough material to form a supermassive black hole, the heart of a quasar. Thus, the number of quasars has grown steadily as more and more supermassive black holes are formed.
D) The number of quasars has decreased steadily as they have evolved into other types of AGN like radio galaxies and BL Lacertae objects.
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A) spiral collection of stars, dust, and gas 200,000 ly across.
B) extension of the Milky Way.
C) vortex surrounding a black hole.
D) gaseous nebula extending for 6° across Earth's sky.
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A) very much fainter than at present because neighboring stars would be obscured by dense dust and gas clouds.
B) about the same as it is now since neighboring stars would still be relatively far away.
C) about twice as bright as at present since neighboring stars would be mostly bright, young blue stars in about the same numbers as the present, older, and less bright red giant neighbors to the Sun.
D) extremely intense from the dense field of stars, equivalent to about 200 full Moons.
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A) measuring the speed of stars in their orbits around a galactic center
B) gravitational lensing of very distant galaxies by a nearby galaxy (or cluster) and its accompanying dark matter
C) observing the aftermath of a collision such as that in galaxy cluster 1E0657-56
D) observing X rays being emitted from the space between galaxies in rich clusters
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A) 100 kpc.
B) 31 kpc.
C) 3.1 kpc.
D) 2 kpc.
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A) their emission, which is mostly in the infrared and radio range
B) their starlike appearance on photographs, showing no structure
C) rapid fluctuations in output, often in less than 1 day
D) very high redshift of their light
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A) 10,000 to 20,000 K
B) 1 to 10 billion K
C) 10 to 100 K
D) 10 to 100 million K
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A) globular clusters.
B) dwarf elliptical galaxies.
C) the Magellanic Clouds.
D) individual (field) stars.
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A) 1000
B) 4
C) 10,000
D) 10²
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A) hot, young O and B stars, via their UV radiation
B) numerous old red giant K and M stars, via their IR heat radiation
C) very hot white dwarf stars, the remnants of planetary nebulae in the gas clouds
D) many nova and supernova explosions of stars within the gas and dust clouds
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A) lenticular
B) large spiral, like the Milky Way
C) irregular
D) elliptical
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A) 10⁷ ly.
B) 10⁵ ly.
C) 1 ly.
D) 100 ly.
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A) The rotation curve of these galaxies shows no decrease in orbital velocity of stars as the radius of orbit increases out to the observable limit of the galaxy, indicating an unusual source of gravity.
B) Spectroscopic observations of stars near the centers of these galaxies show extremely fast orbital velocities, indicating the presence of a large mass to keep stars in orbit.
C) Extreme redshift of light from stars near the centers of these galaxies is caused by gravitational redshift from a very massive object.
D) These galaxies are rushing rapidly toward each other (and in at least one case, toward the Milky Way!) because of the gravitational attraction among them.
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A) gravitational radiation emitted by stars as they are swallowed by the black hole.
B) powerful magnetic fields in the huge filaments arching away from (or toward) the center.
C) the number of globular clusters that concentrate toward the galactic center.
D) the very high orbital speed of ionized gas clouds close to the galactic center.
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A) are more common in nearby clusters of galaxies and less common in distant clusters of galaxies.
B) are rare in the Local Group, with only one or two examples.
C) increase in number as redshift increases, a relationship that persists to the highest redshifts we can measure.
D) peaked in number about 2 billion years after the Big Bang.
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A) two oppositely directed jets of matter, ejected from a small source
B) two radio galaxies orbiting each other much like two binary stars
C) a radio-bright galaxy with a dark absorbing disk edge-on to Earth, splitting the source into two as seen from Earth
D) two black holes orbiting around a small but massive galactic nucleus
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A) generally between 10 and 100 times larger.
B) about the same.
C) significantly less.
D) several hundred to several thousand times larger.
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A) distributed uniformly, out to the farthest distances.
B) clustered together in several high-density centers with very little matter linking them together.
C) concentrated around one position in space, presumably the original site of the Big Bang.
D) concentrated on the surface of huge open spaces or voids, like soap bubbles.
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A) a Seyfert galaxy
B) a BL Lacertae object
C) a quasar
D) a barred spiral
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A) small central bulge with loosely wound spiral arms.
B) disk and central bulge with a smooth light distribution and no spiral arms.
C) large central bulge with tightly wound spiral arms.
D) round or spherical shape with a smooth light distribution and no disk or central bulge.
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