Chapter 16: The Milky Way Galaxy

Section: 16-1

1. The twentieth century opened with a major unresolved question among astronomers. This was the subject of the famous Shapley–Curtis debate. What was this question?

A) How old is the Milky Way Galaxy?

B) What is the nature of dark matter?

C) Is general relativity true?

D) Is the Milky Way Galaxy the whole universe, or only one of many galaxies?

2. Who was the first person to observe the spiral structure of distant galaxies with a telescope?

A) Sir Isaac Newton

B) Sir William Herschel

C) William Parsons, Earl of Rosse

D) Galileo Galilei

3. The Milky Way Galaxy

A) is one of many billions of galaxies in the universe.

B) is unique in the universe in showing definite spiral structure.

C) contains the whole universe; everything observable is within its volume.

D) is one of only a few spiral galaxies; most other galaxies in the universe are amorphous collections of stars shaped like ellipsoids.

4. The famous nineteenth-century observational astronomer Lord Rosse built the largest telescope of his time (and discovered the spiral nature of many so-called “nebulae”) in which country?

A) England

B) United States

C) Ireland

D) Germany

5. In the mid-nineteenth century, Lord Rosse built the world’s largest telescope, the “Leviathan of Parsonstown.” What major disadvantage did Rosse have in using his telescope at this time?

A) The mirror, although large, was not made using modern techniques. It had so much distortion that deep sky objects like galaxies could not be seen clearly.

B) No catalogs of deep sky objects were available, so Rosse did not know where to locate anything of interest.

C) The telescope used an objective lens rather than a mirror and thus suffered from distortion due to sagging and chromatic aberration.

D) Photography was not available to record the images Rosse observed.

6. The first suggestion that there were collections of stars beyond the Milky Way in the universe was made by

A) Edwin Hubble in 1923.

B) Immanuel Kant in 1755.

C) William Parsons, Earl of Rosse, in 1845.

D) Sir Isaac Newton in 1690.

7. The idea that some of the nebulae that are observed in the sky might be “island universes,” immense collections of stars far beyond the Milky Way, was first proposed by

A) Immanuel Kant in 1755.

B) Albert Einstein in 1909.

C) Heber Curtis in 1920.

D) Lord Rosse in 1845.

8. Which concept concerning the diffuse objects observed by Herschel was proposed by Kant in the 1700s and further expounded by the Earl of Rosse based on observations made in 1845 with his world-class telescope in Ireland?

A) Many of the diffuse objects in Earth’s sky are moving rapidly away from the Milky Way Galaxy.

B) Many of the nebulae in the sky are separate entities beyond the Milky Way Galaxy.

C) The diffuse objects observed by Herschel and known as planetary nebulae were in fact planets orbiting other stars in the Milky Way Galaxy.

D) All the diffuse nebulae seen in the sky are gas clouds in the Milky Way Galaxy.

9. The famous Shapley–Curtis debate in 1920 concerned which fundamental astronomical question in astronomy?

A) whether all stars were like the Sun or fundamentally different

B) whether the spiral “nebulae” were part of the Milky Way Galaxy or more distant, separate entities

C) whether the universe was expanding outward in all directions

D) whether the Sun was at the center of the Milky Way Galaxy

10. Who won the Shapley–Curtis debate when it concluded in 1920?

A) Shapley won.

B) Curtis won.

C) Both sides won in the sense that each successfully explained part of the data which were being debated.

D) Neither side won because there was no hard, reliable evidence to support either side.

11. An astronomer finds a very old astronomy book, in which the Messier object M31 is referred to as the Andromeda Nebula. The astronomer might suspect that this book was written BEFORE

A) 1780.

B) 1845.

C) 1924.

D) 1954.

12. The event that settled the Shapley–Curtis debate about “spiral nebulae” was

A) Edwin Hubble measuring the distance to the Andromeda Galaxy.

B) Arno Penzias and Robert Wilson detecting the cosmic microwave background radiation.

C) Albert Einstein showing that gravity can bend the path of light.

D) Edwin Hubble showing that the universe was expanding.

13. Variable stars such as Cepheid variables are used in what important measurement in astronomy?

A) measurement of the distances to stars

B) measurement of the rotation speeds of galaxies

C) measurement of the surface temperatures of stars

D) keeping of accurate time

14. In the 1920s Edwin Hubble began answering the question about the nature of spiral “nebulae” when he discovered Cepheid variables in

A) the Milky Way Galaxy.

B) the Whirlpool Galaxy (M51).

C) the Andromeda Galaxy (M31).

D) the Small Magellanic Cloud.

15. What important role do Cepheid variables stars have in astronomy?

A) distance measurements to galaxies

B) determination of stellar luminosities

C) determination of speeds of stars in galactic arms from the Doppler shift of their spectra

D) keeping of accurate time

16. Cepheid variable stars are useful to astronomers as indicators of

A) stars with very high-speed motion.

B) white dwarf star behavior.

C) the existence of black holes.

D) distance, particularly to nearby galaxies.

17. Distances to a nearby galaxy can be determined MOST accurately by

A) measuring the periods of pulsating stars.

B) measuring the chemical compositions of the brightest stars.

C) measuring the shifts of spectral lines from the Milky Way Galaxy.

D) measuring the total amount of energy received from the Milky Way Galaxy.

18. The luminosity of a Cepheid variable star compared with that of the Sun is

A) generally between 10 and 100 times larger.

B) about the same.

C) significantly less.

D) several hundred to several thousand times larger.

19. The period-luminosity relationship for Cepheid variable stars, relating variability to absolute overall brightness, was discovered by

A) Harlow Shapley.

B) Henrietta Leavitt.

C) Edwin Hubble.

D) Sir Isaac Newton.

20. Henrietta Leavitt worked out the period-luminosity relation for the Type I (or classical) Cepheids found in the Small Magellanic Cloud. Later, Cepheids were discovered in globular clusters, and these were assumed to be Type I Cepheids, and the distances to these clusters were calculated accordingly. However, it was later discovered that these were dimmer Type II Cepheids. Refer to Figure 13-28 from the text. Did this require a recalculation of the distances to these Cepheids?

A) No. The distances were still correct.

B) Yes. For a given Cepheid period, the new distances were smaller than those originally calculated.

C) Yes. For a given Cepheid period, the new distances were larger than those originally calculated.

D) Yes. New distances had to be calculated, which could be larger or smaller than the original calculation.

21. Cepheid variable stars are invaluable in astronomy because of the close relationship between

A) the peak wavelength of their spectra and their surface temperatures.

B) their luminosity, or absolute magnitude, and their pulsation period.

C) the redshift of their spectra and their distance from the Sun.

D) their apparent magnitude and their pulsation period.

22. The observational fact about a Cepheid variable star that leads to a measurement of its distance from Earth is that its period of variation is directly related to its

A) surface temperature.

B) position along the spiral arms of a galaxy.

C) apparent magnitude.

D) absolute magnitude.

23. The significant feature of a Cepheid variable is that there is a relationship between two intrinsic parameters, one of which can be easily measured, while knowledge of the other parameter is required. These parameters are

A) period of brightness variation and spectral color.

B) variation of spectral color and distance to the star.

C) period of brightness variation and luminosity.

D) amplitude of brightness variation and luminosity.

24. A Cepheid variable star with a pulsation period of a few days is seen in the spiral arm of a galaxy. Its apparent brightness is measured as 10⁴ times fainter than an equivalent star 1000 ly away from the Sun in the Milky Way Galaxy. Assuming no light absorption between galaxies, how far away is the more distant Cepheid?

A) 10⁷ ly (10⁴ times farther away)

B) 10,000 ly (10 times farther away)

C) 10 ly (100 times closer)

D) 100,000 ly (100 times farther away)

25. A classical Cepheid variable star is seen to vary regularly with a period of 25 days. Approximately how many times brighter than the Sun would this star appear to be if it were to replace the Sun in the solar system? Refer to Figure 16-4 from the text.

A) 1000

B) 4

C) 10,000

D) 10²

26. Hubble carried out the determination of the distance to the Andromeda “nebula” (which finally resolved the Shapley–Curtis debate on the nature of “spiral nebulae”) by observing

A) pulsars.

B) Cepheid variable stars.

C) the Doppler shift of stars in the “nebula.”

D) the main sequence of stars in the “nebula.”

27. The observation by Hubble that demonstrated for the first time that the Andromeda “nebula” was at a very large distance from the Sun and outside the Milky Way Galaxy was that

A) Cepheid variable stars appeared to be very faint in the “nebula.”

B) the “nebula” appeared to be rotating night by night around a center that was not the center of the Milky Way.

C) stars with characteristics similar to those of the Sun appeared to be absent in the “nebula.”

D) globular clusters appeared to be distributed in a halo around the “nebula,” a sure sign of a separate galaxy.

28. What was the implication of the observation by Hubble that the Cepheid variable stars he measured in the Andromeda “nebula” appeared to be very faint compared with what was expected?

A) The observed period was affected by the intense gravitational field of the “nebula” as predicted by general relativity, leading to incorrect luminosity determination.

B) The Andromeda “nebula” was very far away and was in fact a galaxy.

C) Hubble had discovered a new class of intrinsically faint Cepheid variable stars.

D) Dust and gas in the “nebula” had severely reduced the light from these stars.

29. The method Hubble used to determine the distance to the Andromeda Galaxy (M31), thereby establishing the concept of separate and individual galaxies throughout the universe, was the

A) observation of Cepheid variable stars.

B) measurement of the redshift of the whole galaxy.

C) measurement of stellar parallax, or apparent motion, of stars because of Earth’s orbital motion.

D) observation of the brightnesses of novae.

30. The Andromeda Galaxy (M31) is BEST described as a(n)

A) extension of the Milky Way.

B) vortex surrounding a black hole.

C) gaseous nebula extending for 6° across the sky.

D) spiral collection of stars, dust, and gas 2 million ly away.

31. What is it that makes the study of the structure of the Milky Way Galaxy more difficult than that of much more distant spiral galaxies?

A) The galactic center is visible only from the southern hemisphere, where, until recently, no major telescopes were available for the study of galactic structure.

B) The Milky Way appears to be unique, with a structure quite unlike any other galaxy.

C) The Sun is within the Galaxy, and its motion confuses the interpretation of the motion of other parts of the Galaxy.

D) Most of the Milky Way is hidden behind dense gas and dust clouds in the galactic plane.

32. In determining the distance to a galaxy by using observations of a Cepheid variable star, which of these properties is NOT needed?

A) average brightness or apparent magnitude of the Cepheid

B) velocity of the Cepheid via the Doppler effect

C) type of spectrum of the Cepheid—whether it is metal-rich or metal-poor

D) period of variability of the Cepheid

33. A classical Cepheid variable star in a distant galaxy has an apparent magnitude of +20.3. By measuring its period and using the period-luminosity relation, its absolute magnitude is –6.0. Assuming that there is no interstellar absorption, what is the distance to the galaxy in parsecs?

A) 3.6  10⁴ pc

B) 7.2  10³ pc

C) 1.1  10⁵ pc

D) 1.8  10⁶ pc

34.

Suppose a spiral galaxy is a million parsecs away so that its distance modulus is 25. What would be the approximate pulsation period of a Type I classical Cepheid variable star in this galaxy?

A) 1.0 day

B) 5.6 days

C) 25 days

D) The pulsation period cannot be determined from this information alone.

35. Suppose a spiral galaxy is a million parsecs away so that its distance modulus is 25. What would be the approximate luminosity of an RR Lyrae variable star in this galaxy?

A) 25 L_Sun

B) 100 L_Sun

C) 2500 L_Sun

D) The luminosity cannot be determined from this information alone.

36. In 2006, a Type Ia supernova erupted in the spiral galaxy M100. Its peak apparent magnitude was +13.5, and its absolute magnitude was about –17.5. What do these numbers give for its distance?

A) 7 kpc

B) 10 kpc

C) 31 kpc

D) 42 kpc

Section: 16-2

37. In the 1780s, Sir William Herschel tried to measure the Sun’s position in the Milky Way Galaxy by

A) counting the density of stars in different directions along the Milky Way.

B) measuring distances to star clusters and H II regions in the disk of the Galaxy.

C) measuring the locations of globular clusters around the Galaxy.

D) comparing the Milky Way with the Andromeda Galaxy.

38. In 1785, William Herschel proposed that the solar system was at the center of the Milky Way Galaxy. How did he reach this conclusion?

A) Like the ancient Greeks and many of their successors for over a thousand years, Herschel based his conclusion on the philosophical importance of Earth in the cosmos.

B) Herschel’s observations suggested that the stars in the Milky Way Galaxy were distributed uniformly around Earth.

C) Herschel measured distances to globular clusters and determined that they were distributed uniformly around Earth.

D) Herschel determined that Cepheid variable stars in the Milky Way Galaxy were distributed uniformly around Earth.

39. The factor that misled Herschel into concluding that the stars of the Milky Way were distributed with the Sun at the center of the Milky Way Galaxy was

A) hot hydrogen gas in the Galaxy, its emission hiding the more distant stars.

B) interstellar dust that obscured the more distant stars and thereby localized his observations.

C) gravitational bending of light by the mass of the Galaxy, distorting the relative positions of the stars.

D) that most of the “stars” he measured were in fact distant galaxies that are distributed uniformly around the Sun.

40. Why are astronomers on Earth able to see only a relatively small part of the Milky Way Galaxy?

A) There are so many stars in the Milky Way that the more distant ones are hidden behind the nearer ones.

B) Distant stars are obscured by gas in interstellar space.

C) Expansion of the universe has carried the more distant stars out of view.

D) Distant stars are obscured by dust in interstellar space.

41. In the eighteenth century, Sir William Herschel used star counts in different regions of the sky along the Milky Way to estimate the position of the center of the Milky Way. He incorrectly concluded that the Sun was close to the center. The reason for this erroneous conclusion was that

A) the redshift of the more distant stars made them invisible to Herschel.

B) Herschel counted all “stars” in each star field and included many galaxies that were outside the Milky Way, thus confusing the distribution.

C) the large quantity of absorbing dust between stars obscured the more distant regions of the Galaxy.

D) emissions from hot hydrogen gas clouds served to hide the more distant stars, localizing his search.

42. If interstellar dust obscures astronomers’ view of distant regions of space at optical wavelengths, which of these statements is true?

A) The obscuration is very clumpy and random over the whole sky; the individual absorbing dust clouds show no preference for one particular direction or plane.

B) The obscuration is roughly uniform over the whole sky.

C) The obscuration is the least in the plane of the Milky Way Galaxy and is strongest when one looks out into the galactic halo, at right angles in this plane.

D) The obscuration is severe only in the plane of the Milky Way Galaxy.

43. Which component of the Milky Way Galaxy accounts for interstellar extinction, the dimming of light from distant objects?

A) cool hydrogen gas

B) so-called hidden or missing matter, since its absorbing properties render it invisible in the Galaxy

C) molecules such as H₂ and CO, which are strong absorbers, in molecular clouds

D) dust

44. Suppose astronomers could examine a star and determine its apparent magnitude M without any intervening gas and dust. Compared to its apparent magnitude measured with gas and dust between Earth and the star, this “clean” measurement of M would be

A) a larger number.

B) a smaller number.

C) the same number.

D) either larger or smaller depending on the star’s speed (toward or away).

45. Suppose one could examine a star and determine its absolute magnitude M without any intervening gas and dust. Compared to its absolute magnitude measured with gas and dust between Earth and the star, this “clean” measurement of M would be

A) a larger number.

B) a smaller number.

C) the same number.

D) either larger or smaller depending on the star’s speed (toward or away).

46. Trumpler discovered the effect of interstellar dust by comparing two different distance measurements of distant clusters. The first was based on appearance (concentration of stars toward the center, etc.), which was unaffected by dust. The other was spectroscopic parallax (use of the H-R diagram), which was affected by dust. What should such a comparison show?

A) The H-R diagram should always give larger distances.

B) The H-R diagram should always give smaller distances.

C) There should be no difference in the distances.

D) The distances should be different, but sometimes the H-R diagram should give the larger distance and sometimes the smaller distance depending on the actual distance to the cluster.

47. The one component of the material of the Milky Way Galaxy that prevents astronomers from seeing and photographing the galactic center at optical wavelengths is

A) very cold hydrogen gas.

B) interstellar dust.

C) the glare of light from nearby stars.

D) hot hydrogen gas.

48. What useful purpose did variable stars serve for Harlow Shapley in locating the galactic center?

A) Variable stars are important spiral arm tracers and thus defined the shape of the Milky Way Galaxy.

B) The brightness variations of variable stars allowed accurate distances to be measured.

C) Variable stars emit copious amounts of infrared radiation and are thus visible through interstellar dust that obscures visible light.

D) Variable stars are concentrated in the galactic center and so defined its direction.

49. The method used by Harlow Shapley in 1917 to estimate the Sun’s location in the Milky Way Galaxy was the measurement of the

A) locations of globular clusters around the Galaxy.

B) density of stars in different directions along the Milky Way.

C) distances to open star clusters and H II regions in the disk of the Galaxy.

D) structure of the Andromeda Galaxy and a comparison of it with the structure of the Milky Way.

50. In 1917, Harlow Shapley plotted the positions of almost a hundred globular clusters. But the galactic plane is cloaked by gas and dust, and the best way to “see” through this is with long wavelength radio waves. However, in 1917 radio astronomy did not exist. How did Shapley accomplish his task?

A) He could only see globulars that were relatively close to the Sun.

B) He concentrated on using the long-wavelength end of the visible spectrum—red. This worked well because globulars are old and are dominated by red stars.

C) The globulars are mostly in a halo up off the galactic plane, so the dust was not a great problem.

D) In order to form a globular cluster, a great deal of interstellar dust is used up. Thus, globulars create holes in the dust and are relatively easy to see.

51. Harlow Shapley first located the center of the Milky Way Galaxy in 1917 by

A) measuring the positions of supernova explosions throughout the Galaxy.

B) observing the distribution of globular clusters in the galactic halo.

C) measuring redshifts of stars in the galactic plane and disk.

D) observing the distribution of hydrogen gas, measured by 21-cm radio emission.

52. When distances were carefully measured from Earth to globular clusters above and below the Milky Way plane (where the view of them is not obscured by interstellar dust and gas), the distribution of the clusters was found to be

A) spherically symmetric about a point in the constellation Sagittarius and concentrated in that direction.

B) in a relatively flat disk almost perpendicular to the plane of the Galaxy, with a relatively higher density of clusters toward its center.

C) concentrated in the plane of the Milky Way and clustered around the Sun’s position, indicating that the Sun is close to the Galaxy’s center.

D) uniformly distributed throughout space, with no concentration in any area of the Milky Way.

53. Harlow Shapley’s original estimate of the distance from the Sun to the galactic center was incorrect because it did not take scattering by interstellar dust into account. Shapley’s original estimate was

A) too large because light scattering and absorption by dust made nearer stars look dim—as though they were more distant.

B) too large because scattering by dust made stars look redder—as though they were highly Doppler-shifted and thus more distant.

C) too small because gas and dust tend to reflect light into the path of the beam, thus making stars look brighter and nearer.

D) too large because scattering by dust made stars look redder—as though they were red giants of larger luminosity and thus farther away.

54. In which constellation in the Earth’s sky is the center of the Milky Way Galaxy located?

A) Sagittarius

B) Lyra

C) Hercules

D) Ursa Major

Section: 16-3

55. Where is the Sun located in the Milky Way Galaxy?

A) in the Scutum-Centaurus arm

B) in the Sagittarius arm

C) in the Perseus arm

D) between the Sagittarius and Perseus arms

56. The plane of the Milky Way Galaxy is filled with clouds of gas and dust. To receive information from objects embedded in these clouds astronomers use receivers sensitive in these ranges EXCEPT

A) ultraviolet.

B) X-ray.

C) infrared.

D) radio.

57. Why is it that astronomers rely upon the 21-centimeter line associated with the spin flip in hydrogen and not the more familiar hydrogen Balmer lines to gather information from the far side of the galactic plane?

A) The Balmer lines are associated with neutral hydrogen, and most of the hydrogen astronomers need to observe is molecular hydrogen.

B) The Balmer lines are associated with molecular hydrogen, and most of the hydrogen astronomers need to observe is neutral hydrogen.

C) The Balmer lines are in the visible and ultraviolet regions of the spectrum, and these do not penetrate the gas and dust clouds easily.

D) Photons released in the spin-flip transition have much more energy than those that come from the Balmer transitions.

58. Radio waves of 21-cm wavelength originate from which component of the interstellar medium?

A) cool, neutral atomic hydrogen

B) cold molecular hydrogen, H₂

C) cool carbon monoxide, CO

D) hot, ionized atomic hydrogen

59. The spiral-arm structure of the Milky Way Galaxy has been measured and evaluated MOST effectively by observations of

A) globular clusters in the halo of the Galaxy.

B) Balmer emission lines of visible radiation from hydrogen.

C) 21-cm radiation from interstellar hydrogen and the distribution of young stars.

D) Lyman UV radiation from hot hydrogen gas.

60. Which type of radiation has been MOST effective in evaluating the spiral arm structure of the Milky Way Galaxy?

A) 21-cm radio emission from electron spin-flip transitions in cool hydrogen gas

B) Lyman- ultraviolet emission from hot hydrogen gas

C) synchrotron radiation from electrons spiraling in magnetic fields in the spiral arms

D) neutrinos from exploding stars in the spiral arms, since they can easily penetrate dust and gas

61. What happens when the electron in a hydrogen atom flips its direction of spin from parallel to antiparallel to that of the proton?

A) Nothing. This event is a forbidden transition that never occurs.

B) The atom emits a photon of 656.3-nm wavelength (H_) in the red region of the spectrum.

C) The atom emits a photon of 21-cm wavelength in the radio region of the spectrum.

D) The atom emits a photon of 121.5-nm wavelength (L_) in the UV region of the spectrum.

62. When a hydrogen atom is excited by some addition of energy and it responds with electron transitions between electron energy levels, the radiation it emits is usually in which regions of the electromagnetic spectrum?

A) visible and infrared

B) visible and ultraviolet

C) radio

D) microwave

63. The 21-cm line is one of the most important wavelengths in radio astronomy. It is produced mainly in

A) emission nebulae.

B) neutral hydrogen.

C) ionized hydrogen.

D) hot O and B stars.

64. What atomic transition occurs in the atoms of hydrogen gas in the spiral arms of the Milky Way Galaxy to produce the 21-cm radio emission?

A) transition from n = 2 level to n = 1 level in atomic hydrogen

B) change in rotation of the molecule H₂ about an axis perpendicular to the molecular axis

C) change in the vibrational state of the H atoms in the H₂ molecule

D) inversion of the electron spin relative to the proton spin, from parallel to antiparallel

65. What quantum transition occurs inside a hydrogen atom to produce a 21-cm radio photon?

A) An electron falls from the level n = 100 to the level n = 99 in the atom.

B) An electron in the ground atomic state reverses its direction of spin with respect to that of the proton.

C) An electron reverses the direction of its motion in orbit around the proton.

D) The electron combines with the proton in the nucleus to become a neutron, producing energy.

66. The major advantages of the 21-cm radio emission from hydrogen gas for investigating the spiral structure of the Milky Way Galaxy are that

A) it is relatively easily absorbed by hydrogen gas in the Milky Way, so measurements are not confused by emission of this radiation from other galaxies beyond the Milky Way. It originates only from cold hydrogen gas and can be used to map this important component.

B) radio waves easily penetrate the Milky Way dust and gas, and they are a very narrow-wavelength line emission; thus its Doppler shift can be used to measure gas motions.

C) Doppler shift of this narrow-wavelength line emission is caused by the temperature of the hot hydrogen gas and therefore can be used to measure the distribution and temperature of this important component of the Milky Way.

D) this emission can easily penetrate the Milky Way gas and dust and comes only from hot gas, hence its ability to be used to map the distribution of hot hydrogen gas.

67. A neutral hydrogen atom consists of a proton and an electron. The energy of the atom is

A) the same regardless of the relative spin orientations of the two particles.

B) highest when the proton spin and the electron spin are perpendicular to each other.

C) highest when the proton spin and the electron spin directions are parallel.

D) highest when the proton spin and the electron spin directions are antiparallel.

68. If astronomers aim a radio telescope at a distant spiral arm of the Milky Way Galaxy, they will probably observe a 21-cm line. If they point a large optical telescope at this same region, they will probably not be able to detect the neutral hydrogen that gives rise to the 21-cm radio signal. Why not?

A) Neutral hydrogen is incapable of emitting visible radiation.

B) The H_ line—the primary emission of neutral hydrogen—has a much smaller energy than the 21-cm radio wave and thus is harder to detect.

C) There is little energy in the depths of space to excite visible radiation from neutral hydrogen.

D) The visible light emitted by neutral hydrogen is all absorbed by H II gas regions before reaching the solar system.

69. Astronomers aim a radio telescope at a distant region of the Milky Way Galaxy and detect 21-cm radio waves with no Doppler shift. Which one of these possible explanations is NOT correct?

A) The neutral hydrogen in this region is not moving relative to Earth.

B) The neutral hydrogen in this region is moving perpendicularly across the line of sight.

C) The neutral hydrogen in this region is in a circular orbit around the galactic center at the same radius as the solar system.

D) The neutral hydrogen in this region is moving away from Earth instead of toward Earth.

70. When astronomers measure the narrow line emissions of hydrogen at 21-cm radio wavelengths along a particular line of sight through the disk of the Milky Way Galaxy, they can tell the distances to different hydrogen clouds because

A) absorption of extragalactic radiation at this wavelength is greater the farther away the absorber is from the Sun.

B) clouds that are farther away have smaller angular sizes.

C) the emission is weaker from clouds that are farther away.

D) clouds at different distances have different Doppler shifts because of the rotation of the Galaxy.

71. The 21-cm line is one of the most important wavelengths in radio astronomy. The fact that it is a relatively long wavelength is important because long wavelengths

A) are more likely to interact with clouds of gas and dust.

B) are more likely to pass through clouds of gas and dust.

C) carry more energy than short wavelengths of the same energy.

D) can be detected with greater resolution than can short wavelengths.

72. The major features of the Milky Way Galaxy have been found to include all of these EXCEPT

A) more than 100 billion stars.

B) a bar of stars running through the center.

C) a spherical symmetry.

D) spiral arms.

73. Which of these components of the Milky Way Galaxy outlines the spiral arms of the Galaxy?

A) young O and B stars, dust, and gas

B) globular clusters

C) predominantly solar-type stars

D) white dwarf stars

74. Where are bright, young O and B stars MOST likely to be found in the Milky Way Galaxy?

A) inside the spiral arms

B) in globular clusters

C) in the outermost regions of the disk, where much of the “dark matter” is located

D) between the spiral arms, where there is less absorbing material

75. The stellar components of the Milky Way Galaxy that act as tracers for the mapping of the spiral arm structure in the Galaxy are

A) bright Population I stars and emission nebulae surrounding them.

B) globular clusters.

C) supernova explosions, because they are very luminous and can be seen through considerable dust and gas.

D) old, red giant stars and white dwarfs.

76. Which of these statistics is NOT useful for mapping the locations and shapes of the spiral arms of the Milky Way Galaxy?

A) distribution of globular clusters

B) distribution of O and B stars

C) distribution of emission nebulae (H II regions)

D) distribution of giant molecular clouds

77. What is the distribution of giant molecular clouds in the Milky Way Galaxy and other similar galaxies?

A) Giant molecular clouds occur primarily in the spiral arms.

B) Giant molecular clouds are distributed uniformly throughout the disk.

C) Giant molecular clouds are concentrated close to the galactic center.

D) Giant molecular clouds are distributed throughout the halo, with greater density toward the center.

78. When astronomers look out into the plane of the Milky Way Galaxy, how far can they see?

A) infinitely far

B) about 200,000 ly

C) less than 1000 ly

D) about 24,000 ly

79. Recent observations seem to indicate that, rather than being a spiral galaxy, the Milky Way may be

A) a barred spiral with a definite, straight bar across its center.

B) an irregular galaxy in which matter is distributed chaotically.

C) an elliptical galaxy with little structure.

D) two elliptical galaxies colliding with one another, in view of the very active star formation within the galactic plane, brought about by the vastly increased density during the collision.

80. How far away from Earth is the Andromeda Galaxy compared with the diameter of the Milky Way Galaxy?

A) The Andromeda Galaxy is about 3 Milky Way diameters away.

B) The Andromeda Galaxy is about 20 Milky Way diameters away.

C) The Andromeda Galaxy is between 100 and 120 Milky Way diameters away.

D) The Andromeda Galaxy is several million Milky Way diameters away.

81. The dimensions of the disk of the Milky Way Galaxy are diameter _____, thickness _____.

A) 100,000 ly; 2000 ly

B) 10,000 ly; 28,000 ly

C) 2000 ly; 100,000 ly

D) 28,000 ly; 2000 ly

82. The ratio of thickness to diameter of the Milky Way Galaxy is

A) 1/500.

B) 1/5.

C) 1/50.

D) 1/1000.

83. The diameter of the Milky Way Galaxy is about

A) 100 kpc.

B) 31 kpc.

C) 3.1 kpc.

D) 2 kpc.

84. Interstellar matter obscures Earth’s view of the disk of the Milky Way Galaxy

A) most at radio wavelengths, where hydrogen absorbs radio waves efficiently, and least at optical wavelengths.

B) more at optical wavelengths and less or not at all at infrared and radio wavelengths.

C) very little at any wavelength.

D) more or less equally at all wavelengths, from radio waves to light waves.

85. How is cool, neutral hydrogen gas, H I, detected in the spiral arms of galaxies?

A) by the absorption of infrared radiation from extragalactic sources

B) by its Balmer line emissions

C) by its 21-cm line radio emissions

D) by its ultraviolet Lyman- hydrogen line emissions

86. In the Milky Way Galaxy, young stars are found

A) everywhere.

B) only at the galactic center.

C) in the disk and spiral arms.

D) in the globular clusters in the galactic halo.

87. The Milky Way Galaxy appears to have a spiral structure with

A) four separate major arms.

B) a dozen loosely wound arms.

C) one “arm” wound around the nucleus 4 times.

D) two major arms wound twice around the nucleus.

88. The Milky Way is an example of which type of galaxy?

A) spiral

B) irregular

C) lenticular, S0 type

D) elliptical

89. The Milky Way, the galaxy in which the Sun resides, is an example of which type of galaxy?

A) spiral (a regular pattern of spiral arms)

B) elliptical (a smooth star distribution lacking spiral arms)

C) The Milky Way is not a galaxy at all but a large cluster of stars.

D) irregular (possible clumps of stars but no overall pattern)

90. Where is the solar system located in the Milky Way Galaxy?

A) in the galactic disk

B) The solar system is not in a galaxy but in the intergalactic space between galaxies.

C) in the galactic halo

D) in the galactic nucleus

91. The Sun’s position in the Milky Way Galaxy is

A) unknown because Earth’s view is too severely restricted by interstellar dust.

B) in the disk of the Galaxy.

C) in the Galaxy’s central bulge.

D) in the spherical halo, somewhat above and outside the spiral arms.

92. Approximately how far is the Sun from the center of the Milky Way Galaxy?

A) 2 kpc

B) 8 kpc

C) 49 kpc

D) 160 kpc

93. A map of the Milky Way Galaxy deduced from radio observations of the 21-cm line emission from cool hydrogen gas and observations of spiral arm tracers such as giant molecular clouds reveals

A) two spiral arms, one on each side of the Galaxy.

B) a smooth distribution of stars, characteristic of an elliptical galaxy.

C) at least four spiral arms.

D) one spiral arm that wraps around the Galaxy several times.

Section: 16-4

94. Which of these components of the Milky Way was recently mapped by the Planck satellite?

A) the spiral arms

B) the global magnetic field

C) an X-shaped set of stars near the galactic center

D) atomic hydrogen

95. What is the source of the Milky Way’s global magnetic field?

A) supernova explosions

B) the supermassive black hole at the center of the Galaxy

C) mergers with other galaxies

D) The magnetic field’s origin is not yet well-understood.

Section: 16-5

96. Which of these observations was key to estimating the size of Sagittarius A*?

A) an X-ray flare that faded over a few hours

B) synchrotron radiation from the surrounding magnetic fields

C) the Doppler shifts of stars orbiting the black hole

D) the rate gas falls onto the black hole

97. If the Sun were to be at or close to the galactic center of the Milky Way, the intensity of starlight in the nighttime sky on Earth would be

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.

98. Astronomers have observed the central region of the Milky Way Galaxy in a number of wavelength ranges. With which one of the ranges is it MOST difficult to obtain information?

A) infrared

B) radio

C) visible

D) gamma ray

99. Why is the black hole at the center of the Milky Way growing very slowly in comparison to some other black holes?

A) It is much smaller than the black holes in other galaxies.

B) The surrounding gas is too hot.

C) There is no nearby matter that could fall into the black hole.

D) The nearby matter is composed only of positrons, which annihilate with electrons before they can fall onto the black hole.

100. One curious fact about the Milky Way Galaxy, discovered in the past two decades, is that

A) a significant fraction of its mass is in the form of globular clusters, distributed in a spherical halo centered on the galactic center.

B) a large black hole is slowly clearing out the mass near the galactic center, leaving a cold, dark void.

C) enormous amounts of energy are pouring out of a compact but very massive source at its center.

D) most of its mass is in the spiral arms extending out into space.

101. What is the significance of the object Sagittarius A* (“Sagittarius A-star”) in the Milky Way Galaxy?

A) Sagittarius A* appears to be a jet of material ejected from an accretion disk around a supermassive black hole in the galactic nucleus.

B) Sagittarius A* is a globular cluster passing close to the galactic nucleus, and the RR Lyrae stars in it allow the distance to the galactic center to be calculated.

C) Sagittarius A* is a bright, high-speed cloud of gas close to the galactic nucleus that allows the mass of the nucleus to be calculated.

D) Sagittarius A* appears to be the actual nucleus of the Galaxy.

102. Positrons (positively charged electrons) are being produced in the galactic center of the Milky Way. How have the positrons been detected?

A) The positrons create arches of magnetic field that are visible at radio wavelengths.

B) The positrons spiral in magnetic fields, producing a strong source of synchrotron radiation.

C) The positrons annihilate with ordinary electrons, producing gamma rays with a characteristic energy.

D) The positrons reach Earth as cosmic rays from the direction of the galactic center.

103. The positrons near the galactic center can be detected because they annihilate with electrons and emit gamma rays. Two gamma rays are produced in such a situation to conserve both energy and momentum. Suppose the positron and electron, each with energy mc², convert all of this energy into two identical gamma-ray photons. What will be the wavelength of these photons? The mass of an electron is 9.1  10^-31 kg.

A) 1.5  10^–2 nm

B) 2.4  10^–3 nm

C) 3.9  10^–4 nm

D) 7.9  10^–5 nm

104. What type of object has been proposed to explain the tremendous activity detected at the center of the Milky Way Galaxy?

A) giant molecular cloud

B) supermassive black hole

C) supernova explosion

D) rapidly rotating neutron star

105. In which of these sites in the universe has a supermassive black hole been proposed to account for recent observations?

A) center of the Milky Way

B) center of the Ring Nebula, a planetary nebula in Lyra

C) center of the universe, where the Big Bang occurred at the beginning of the universe

D) center of the Crab Nebula, an old supernova remnant

106. What evidence now exists for a supermassive black hole at the center of the Milky Way Galaxy?

A) very bright, steady X-ray emissions from the galactic center

B) observations of intense inflow of matter toward the center of the Galaxy as seen by light, Doppler-shifted toward the red, emitted by this matter

C) very rapid motion of matter close to the nucleus of the Galaxy, requiring a very massive body to hold it in orbit

D) very dark void in an otherwise bright region of space near the galactic center, indicating the presence of a black hole

107. The supermassive black hole at the center of the Milky Way Galaxy is believed to be

4.3  10⁶ solar masses. What is its diameter in light-years?

A) 1.3  10^–6

B) 2.6  10^–6

C) 1.3  10⁷

D) 2.6  10⁷

108. The supermassive black hole at the center of the Galaxy has a diameter of approximately 8 10^–7 pc and is approximately 8000 pc away from Earth. What is the angular size of the black hole?

A) about 2  10⁵ arcseconds

B) about 0.2 arcseconds

C) about 1.25  10^—4 arcseconds

D) about 2  10^–5 arcseconds

109. What is the evidence that indicates to astronomers that a supermassive black hole exists at the center of the Milky Way Galaxy?

A) A Doppler shift of light from stars in the near neighborhood of the galactic center indicates that the stars are falling inward at very high speeds.

B) The Sun’s motion in space shows that, if Kepler’s law holds for its orbit around the galactic center, there must be a very massive object at the center of the Milky Way.

C) No electromagnetic radiation at all comes from the precise position of the galactic center, and it just looks like a dark void in space.

D) Measurement of gas clouds orbiting the galactic center at very high speeds indicates that they would rapidly move out of the Galaxy unless some very massive object was holding them in orbit.

110. The presence of a supermassive black hole at the center of the Milky Way Galaxy has been deduced from

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 stars orbiting the galactic center.

111. Which of these accurately describes the characteristics of the object at the center of the Milky Way Galaxy?

A) 20 solar masses in a volume the size of the Sun

B) several trillion solar masses in a volume 2 ly in diameter

C) 4 million solar masses in a volume smaller than Earth’s orbit

D) 5 billion solar masses in a volume smaller then Jupiter’s orbit

112. The orbits of stars around the center of the Milky Way Galaxy suggest that the mass at the center of the Galaxy is about 4 million solar masses. What is believed to be the diameter of this supermassive object?

A) about half a parsec

B) about 8 ly

C) smaller than Earth’s orbit

D) smaller than the diameter of the Sun

113. If the galactic center of the Milky Way is now thought to contain a supermassive black hole, why is the Sun NOT falling into it under the black hole’s extreme gravity?

A) The Sun’s mass is so small that even the extreme mass concentration at the galactic center will not exert a significant force on it.

B) The inward force exerted on the Sun from the black hole is offset by the force exerted outward by the hidden “dark” matter beyond the Sun’s orbit.

C) The mutual gravitational forces of local stars in the Orion spiral arm are sufficient to overcome the strong inward force and keep the Sun moving in its orbit.

D) The Sun has sufficient velocity that it can orbit the galactic center in a circle.

114. The star S2 orbits Sagittarius A* (the galactic center of the Milky Way) with a period of 15.2 years and a semimajor axis of 950 au. What does Kepler’s third law suggest as the total amount of mass inside the orbital radius of S2?

A) 1.3  10⁶ solar masses

B) 2.6  10⁶ solar masses

C) 3.7  10⁶ solar masses

D) 9.4  10⁶ solar masses

115. If the black hole in the center of the Galaxy is 4  10⁶ solar masses, what would be its radius (that is, the radius of its event horizon)?

A) 1.2  10⁷ m (about twice the radius of Earth)

B) 1.2  10⁹ m (about twice the radius of the Sun)

C) 1.2  10¹⁰ m (about one-fifth of Mercury’s orbital radius)

D) 1.2  10¹² m (almost twice Jupiter’s orbital radius)

116. If the black hole in the center of the Milky Way Galaxy were 4  10⁶ solar masses, what would be the orbital period of Earth around this object in an orbit of radius 1 au (assuming that Newtonian physics and Kepler’s third law apply)?

A) less than 1 second

B) 3 minutes

C) 4 hours

D) 5 weeks

117. Which of these properties is NOT characteristic of the center of the Milky Way Galaxy?

A) synchrotron (nonthermal) radiation produced by electrons spiraling in a powerful magnetic field

B) huge filaments of gas similar in structure to solar prominences

C) rapidly fluctuating emissions from a gigantic millisecond pulsar

D) halo of lobes of hot, ionized X-ray-emitting gas

Section: 16-6

118. The vast majority of the stars in the galactic halo are isolated field stars. What is the origin of these stars?

A) These very old stars are left over from the original infall of the material that formed the Galaxy.

B) These are stars left over from the formation of the globular clusters.

C) These are stars that were moving too fast and were expelled from globular clusters.

D) These are the remnants of small galaxies that have been absorbed by the Milky Way Galaxy.

119. Where are many of the older, metal-poor stars found in the Milky Way Galaxy?

A) throughout the whole Galaxy

B) at the galactic center

C) in the disk and spiral arms

D) in globular clusters and in the galactic halo

120. Where in space should an astronomer look for a globular cluster?

A) in the Milky Way disk, moving in a circular orbit around the galactic center

B) in elliptical galaxies, because they are composed of old stars and do not exist in young systems like spiral galaxies

C) in the Milky Way galactic halo, orbiting the galactic center in a long elliptical orbit around the galactic center

D) in the asteroid belt

121. What is a galactic halo?

A) system of arcs and other gas clouds surrounding a galactic nucleus

B) system of satellite galaxies surrounding the Milky Way Galaxy

C) spherical distribution of stars and globular clusters centered on a nuclear bulge

D) large disk of stars and molecular clouds extending outward from a nuclear bulge

122. Does the halo of the Milky Way Galaxy include other galaxies?

A) No. It is not possible for one galaxy to orbit another galaxy.

B) No. The halo is limited to field stars and globular clusters.

C) Yes. The halo region includes the Andromeda Galaxy.

D) Yes. The halo region includes several small galaxies.

123. Which of these do astronomers NOT find in orbit around the galactic center?

A) isolated field stars

B) globular clusters

C) small dwarf galaxies

D) large spiral galaxies

124. Where do black holes with masses more than 100 times the mass of the Sun occur in the Milky Way Galaxy?

A) nowhere

B) only in the galactic center

C) in the galactic center and within some globular clusters

D) in the galactic center, within some globular clusters, and in several locations between the spiral arms

125. The halo of the Milky Way Galaxy consists of a number of objects that orbit the galactic center and periodically pass through the galactic plane. Which of these objects are NOT among the galactic halo objects?

A) supermassive black holes

B) galaxies

C) globular clusters

D) field stars

126. The galactic halo contains outer and inner shells of stars. How does the outer shell compare to the inner shell?

A) The two shells are virtually identical except for their distance from the galactic nucleus.

B) The stars of the outer shell are much younger than those of the shell layer.

C) The stars of the outer shell are relatively metal-rich.

D) The stars of the outer shell orbit in the opposite sense from those of the inner shell.

127. The galactic halo contains outer and inner shells of stars. How does the outer shell compare to the inner shell?

A) The two shells are virtually identical except for their distance from the galactic nucleus.

B) The outer shell contains all of the globular clusters.

C) The inner shell contains all of the globular clusters

D) The stars of the outer shell are relatively metal-rich.

128. A small galaxy orbits a larger one. After many orbits, the stars in the small galaxy have dispersed throughout the halo of the larger. This is an example of

A) conservation of angular momentum.

B) galactic cannibalism.

C) globular cluster formation.

D) supermassive black hole formation.

Section: 16-7

129. How do astronomers obtain an estimate of the amount of mass that is inside the Sun’s orbital path in the Milky Way Galaxy?

A) by counting stars, assuming an average stellar mass, and calculating the total mass

B) by observing the bending of light from distant galaxies as the light passes near the center of the Milky Way

C) by observing the movement of the Galaxy toward neighboring galaxies because of mutual gravitational attraction

D) by applying Kepler’s law to the motion of the Sun and other nearby stars

130. What is the motion of stars in the bulge across the inner part of the Milky Way Galaxy?

A) The stars move in roughly circular orbits.

B) The stars oscillate back and forth across the bulge, retracing the same path again and again.

C) The stars are part of a density wave that moves around the galactic center.

D) The stars basically remain stationary within the bulge.

131. The stars in the Milky Way Galaxy

A) obey Hubble’s law of recession.

B) are all receding from the galactic center.

C) move generally around the galactic center.

D) number between 8 and 10 million.

132. The speed of the Sun in its orbit around the Milky Way Galaxy is deduced by reference to and observations of

A) the orbital motions of stars near the Sun.

B) globular clusters.

C) Cepheid variables, which provide a distance standard.

D) the galactic center, about which the Sun is orbiting.

133. The MOST important reason globular clusters are useful for finding the speed of the Sun in its orbit around the Milky Way is that

A) globular clusters are bright and easily seen at large distances.

B) globular clusters on average rotate at the same speed as the Sun around the center of the Galaxy.

C) globular clusters are distributed uniformly around the Galaxy.

D) the average velocity of all the globular clusters must be zero if the globular clusters are to maintain their orbits around the galactic center.

134. What is a “hypervelocity” star?

A) It is one of the stars in rapid orbital motion close to the supermassive black hole in the galactic center.

B) A star moving faster than the escape speed from the galaxy.

C) This is the name given to a few stars that have been detected moving around the galactic center in the opposite direction from the general galactic rotation. Although their speeds are not unusual, their speeds relative to normal stars are very large.

D) These are stars that move around the galactic center in the middle of the galactic plane, but travel much faster than neighboring stars—seemingly in defiance of Kepler’s third law.

135. In order to measure the speed of the Sun with respect to the center of the Galaxy, it was necessary to establish a rest frame. What was used to establish this frame?

A) the average of several nearby galaxies

B) the globular clusters in the halo because they remain at rest while the Galaxy rotates beneath

C) the globular clusters in the halo because their velocities must average to zero

D) the Great Attractor because its enormous mass acts like a stationary point

136. In order to measure the speed of the Sun, one must have a rest frame to use as a reference. What rest frame is used for this measurement?

A) the average velocity of nearby galaxies

B) the black hole at the galactic center

C) the average velocity of the globular clusters

D) cosmic background radiation

137. Which of these statements correctly describes the rotation of the Milky Way Galaxy?

A) The disk rotates like a solid object (objects at all distances take the same time to complete an orbit), and the globular clusters have random orbits with no net rotation of the halo about the center of the Galaxy.

B) Objects in the disk have random orbits with no net rotation of the disk about the center of the Galaxy, and the globular clusters rotate differentially (objects farther from the center take longer to complete an orbit than objects closer to the center).

C) The disk rotates differentially (objects farther from the center take longer to complete an orbit than objects closer to the center), and the globular clusters rotate differentially (objects farther from the center take longer to complete an orbit than objects closer to the center).

D) The disk rotates differentially (objects farther from the center take longer to complete an orbit than do objects closer to the center), and the globular clusters have random orbits with no net rotation of the halo about the center of the Galaxy.

138. For which one of these objects has the phenomenon of differential rotation NOT been observed?

A) Mars

B) Jupiter

C) Sun

D) Milky Way Galaxy

139. The time for the Sun to orbit the galactic center once in its motion in the Milky Way is

A) 28,000 years.

B) 230 million years.

C) about 500,000 years.

D) 2.3 million years.

140. Which parameters of star motion in the Milky Way are represented by its rotation curve?

A) orbital speed as a function of star distance from the galactic center

B) orbital period of the stars as a function of their distance from the galactic center

C) orbital speed of the stars as a function of their individual masses

D) star position above or below the galactic plane as a function of distance from the galactic center

141. If the Sun were traveling around the galactic center along with companion stars (all in circular orbits) as depicted in Figure 16-19 from the text, from which of these stars would one measure the largest Doppler shift of their light?

A) stars at the same orbital distance as the Sun

B) stars in directions 45° from the Sun’s direction of motion

C) All the companion stars are moving along with the Sun and have no relative velocity with respect to it.

D) stars directly between the Sun and the galactic center

142. A radio astronomer points a radio telescope through the Milky Way’s disk in a direction directly away from the center of the Galaxy. If the astronomer measures the Doppler shift of the 21-cm radio line of hydrogen from interstellar clouds at several different distances from the Sun along this line of sight, what should be the result? Refer to Figure 16-8 from the text.

A) The Doppler shift should have a constant value, regardless of distance, as a result of the Sun’s speed of motion around the center of the Galaxy.

B) The Doppler shift should be almost zero for all clouds, regardless of distance.

C) The Doppler shift should rise to a maximum at some distance, then decrease again.

D) The Doppler shift should be larger the farther away the cloud is.

143. Which parameter is plotted as a function of distance from the galactic center in a rotation curve of a galaxy?

A) mass of matter inside the distance from the galactic center

B) thickness of the galactic disk

C) mass of cool hydrogen gas

D) speed of stars orbiting the galactic center

Section: 16-8

144. What physical process has allowed astronomers to rule out brown dwarfs as the source of dark matter in the Milky Way Galaxy?

A) The gravitational fields of brown dwarfs can bend the light from background stars.

B) Brown dwarfs emit copious amounts of infrared radiation, which can penetrate the interstellar medium.

C) Molecules in the atmospheres of brown dwarfs emit radio waves that can be detected from Earth.

D) Brown dwarfs occasionally eclipse (block the light from) more distant stars.

145. The present estimate for the total mass of the Milky Way Galaxy in units of solar mass is about _____.

A) 2.3  10⁸

B) 10¹²

C) 1.1  10¹¹

D) 10⁶⁶

146. What has microlensing told astronomers about the role of brown dwarfs in the Milky Way Galaxy?

A) Brown dwarfs cannot be a major component of the dark matter in the Galaxy.

B) Brown dwarfs are a major component of the matter that is falling into the supermassive black hole in the galactic center.

C) Brown dwarfs are one of the most important components of the Galaxy’s dark matter.

D) Microlensing has stated almost nothing yet about brown dwarfs—microlensing is too difficult to observe.

147. Much of the mass of the Milky Way Galaxy appears to be in the form of “dark matter” of unknown composition. At present, dark matter can be detected only because

A) it emits synchrotron radiation at radio wavelengths.

B) its gravitational pull affects orbital motions in the Galaxy.

C) it bends light from distant quasars.

D) it blocks out the light from distant stars in the plane of the Galaxy.

148. What do astronomers know about the geometry of the dark matter distribution in the vicinity of the Milky Way Galaxy?

A) The dark matter appears to be a lens-shaped disk like the Galaxy’s disk of visible stars, only larger.

B) The dark matter appears to be a spherical halo.

C) Dark matter, including that near the Milky Way Galaxy, appears to be part of a uniform density distribution that fills all space.

D) The geometry of the dark matter distribution, like its constituents, is totally unknown at the present time.

149. The dark matter halo is estimated to extend out about 12 times farther from the galactic center than the solar system’s position. How does this compare with the distance to the nearest large galaxy, Andromeda (M31), which is 2.2 million light-years away?

A) The dark matter halo reaches only about 1 percent of the way to M31.

B) The dark matter halo reaches about one-seventh of the way to M31.

C) The dark matter halo just falls short of M31. If the Andromeda Galaxy has its own dark matter halo, the two must overlap.

D) Earth’s dark matter halo engulfs M31.

150. The presence of a very large amount of unseen matter (“dark matter”) in the halo of the Milky Way Galaxy is deduced from

A) the rotation curve of the Galaxy, in which orbital speeds of stars appear to obey Kepler’s law.

B) the rotation curve of the Galaxy, in which orbital speeds of stars in the outer regions of the Galaxy are significantly higher than is predicted by Kepler’s law in which the value for the observed mass in the Galaxy is used.

C) the unexpected absence of luminous matter (stars, and so on) beyond a certain distance from the galactic center.

D) the high amount of interstellar absorption in certain directions.

151. A rotation curve is a plot of the rotational velocity of a galaxy as a function of the distance away from the galactic center. Predictions for rotation curves for spiral galaxies suggest that the curve should decline beyond the galactic bulge. Actual measurements, however, show that the rotation curve is flat beyond the bulge. What does this deviation from prediction state about spiral galaxies, including the Milky Way?

A) Spiral galaxies are not rotating as rapidly as previously believed.

B) The spiral arms move together as a rigid object.

C) There is a great deal of unseen mass surrounding the visible galaxy.

D) The centers of spiral galaxies contain supermassive black holes.

152. A rotation curve is a plot of the rotational velocity of a galaxy as a function of the distance away from the galactic center. What does the rotation curve of the Milky Way Galaxy look like?

A) The curve is a straight line, slanted upward, suggesting that the velocity increases linearly with radius.

B) The curve rises steeply, levels off, and then experiences the “Keplerian falloff” expected from Kepler’s third law.

C) The curve rises steeply and then oscillates up and down showing the effect of the spiral arms.

D) The curve rises steeply, and then levels off or rises gradually until well beyond the edge of the visible galaxy.

153. What fraction of the mass of the Milky Way Galaxy is in a form that astronomers have been able to see?

A) about 50%

B) 100%—whoever heard of matter that can’t be seen?

C) about 10%

D) about 90%

154. What fraction of the mass of the Milky Way Galaxy appears to be in the form of “dark matter,” which astronomers cannot see but can detect through its gravitational influence?

A) about 50%

B) about 10%

C) 0%—whoever heard of matter that can’t be seen?

D) about 90%

155. What is microlensing?

A) beaming of radiation by accretion disks

B) use of small telescopes to enhance contrast by eliminating scattered light

C) focusing of starlight by the gravitational fields of “small” objects like planets or brown dwarfs

D) focusing of starlight by planetary atmospheres

156. What is microlensing?

A) minute shift in the apparent position of a star as a massive object passes in front of it

B) temporary brightening of a star as a massive object passes in front of it

C) temporary disappearance of a star as a massive object passes in front of it

D) gradual reduction in brightness of a star as a massive object passes in front of it

157. In a recent survey of stars in the sky, it was found that occasionally a star that is normally of constant brightness will slowly brighten and then fade back to its original brightness again over the space of several weeks. What is believed to be the cause of this change in brightness?

A) massive coronal ejection on a star with abnormally strong magnetic fields

B) gradual motion of the star through a break or hole between interstellar dust clouds

C) microlensing by compact objects such as brown dwarfs passing in front of the star

D) “lighthouse beam” of a slowly rotating neutron star companion

Section: 16-9

158. Of the entries in this list of questions about the Milky Way Galaxy, which ONE has been answered at the present time?

A) Did the central black hole form first or did the Galaxy form first?

B) Does the Galaxy have a central bar or not?

C) What is dark matter?

D) What is creating the positrons near the center of the Milky Way?