A) We calculate its mass-to-light ratio.
B) We apply Newton's version of Kepler's third law to the orbits of globular clusters in the galaxy's halo.
C) We count the number of stars we can see at different distances from the galaxy's center.
D) We construct its rotation curve by measuring Doppler shifts from gas clouds at different distances from the galaxy's center.
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A) We infer its existence by observing its gravitational effects on the galaxy motions.
B) The hot gas shows up as bright pink in visible-light photos of galaxy clusters.
C) We can observe the frictional effects of the hot gas in slowing the speeds of galaxies in the clusters.
D) We detect this gas with X-ray telescopes.
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A) the higher amount of mass relative to light (higher mass-to-light ratio) ,the lower the proportion of dark matter.
B) the higher the amount of mass relative to light (higher mass-to-light ratio) ,the greater the proportion of dark matter.
C) the amount of light should be at least one solar luminosity for each solar mass of matter (mass-to-light ratio less than or equal to 1) .
D) the higher the amount of mass relative to light (higher mass-to-light ratio) ,the older the galaxy or cluster.
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A) They travel at speeds close to the speed of light.
B) They are subatomic particles.
C) They can neither emit nor absorb light.
D) They tend to orbit at large distances from the galactic center.
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A) the energy associated with dark matter through E=mc²
B) any unknown force that opposes gravity
C) the repulsive agent causing the universal expansion to accelerate
D) highly energetic particles that are believed to constitute dark matter
E) the total energy in the Universe after the Big Bang but before the first stars
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A) We observe clouds of atomic hydrogen far from the galactic center orbiting the galaxy at unexpectedly high speeds,higher speeds than they would have if they felt only the gravitational attraction from objects that we can see.
B) We see many lanes of dark material blocking out the light of stars behind them along the band of the Milky Way.
C) We see many dark voids between the stars in the halo of the Milky Way.
D) When we observe in different wavelengths,such as infrared or radio,we see objects that don't appear in visible-light observations.
E) When we look at the galactic center,we are able to observe a large black hole that is composed of dark matter.
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A) We are not measuring the orbital velocities of atomic clouds and stars properly.
B) We are not measuring the distances to atomic clouds and stars properly.
C) We are not attributing enough mass to the visible or "bright" matter.
D) We are not observing all the visible or "bright" matter in the galaxy.
E) Our understanding of gravity is not correct for galaxy-size scales.
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A) The light that they emit is so weak that it is undetectable to our telescopes.
B) They are only weakly bound by gravity,which means they can fly off and escape from galaxies quite easily.
C) They respond to the weak force but not to the electromagnetic force,which means they cannot emit light.
D) They interact with other matter only through the weak force and not through gravity or any other force.
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A) a precise description of the shape of a star's orbit around the center of the Milky Way Galaxy
B) a graph showing how orbital velocity depends on distance from the center for a spiral galaxy
C) a curve used to decide whether a star's orbit places it in the disk or the halo of a spiral galaxy
D) a graph that shows a galaxy's mass on the vertical axis and size on the horizontal axis
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1.Recollapsing universe: the density of ...View Answer
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A) the Big Crunch
B) Stars will expand away from each other and galaxies effectively "evaporate."
C) All matter decays to a low-density sea of photons and subatomic particles.
D) All matter eventually ends up in massive black holes.
E) Individual stars die but their gas is recycled through the interstellar medium and new stars form in a never-ending process.
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A) velocities perpendicular to our line of sight
B) velocities directly along our line of sight
C) velocities that we cannot explain by only the force of gravity
D) velocities caused by the expansion of the universe
E) velocities of distant objects that are not caused by the expansion of the universe
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A) electrons
B) neutrinos
C) protons
D) quarks
E) photons
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A) It is a name given to whatever is causing the expansion of the universe to accelerate with time.
B) It is the energy contained in dark matter.
C) It is the energy of black holes.
D) It is a type of energy that is associated with the "dark side" of The Force that rules the cosmos.
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A) measuring the orbital velocities of galaxies in a cluster
B) measuring the temperature of X-ray gas in the intracluster medium
C) measuring the amount of distortion caused by a gravitational lens
D) none of the above
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A) The orbital speeds of stars far from the galactic center are surprisingly high,suggesting that these stars are feeling gravitational effects from unseen matter in the halo.
B) Although dark matter emits no visible light,it can be seen with radio wavelengths,and such observations confirm that the halo is full of this material.
C) Theoretical models of galaxy formation suggest that a galaxy cannot form unless it has at least 10 times as much matter as we see in the Milky Way disk,suggesting that the halo is full of dark matter.
D) Our view of distant galaxies is sometimes obscured by dark blotches in the sky,and we believe these blotches are dark matter located in the halo.
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A) The so-called dark matter is really just ordinary stars that are enshrouded in clouds of dust.
B) There could be something wrong or incomplete with our understanding of how gravity operates on galaxy-size scales.
C) There could be something wrong with our understanding of how atoms produce light.
D) We could just be having a hard time understanding the observations because they involve very distant galaxies.
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A) It emits no visible light.
B) We cannot detect the type of radiation that it emits.
C) It emits no or very little radiation of any wavelength.
D) It blocks out the light of stars in a galaxy.
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A) approximately spherical and about the same size as the galaxy halo.
B) approximately spherical and about ten times the size of the galaxy halo.
C) flattened in a disk and about the same size as the stellar disk.
D) flattened in a disk but about ten times larger than the stellar disk.
E) predominantly concentrated in the spiral arms.
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