Chapter 18: Quasars and Other Active Galaxies

Section: Introduction

1. The energy output per second of a typical quasar is equal to that emitted by the Sun in

A) 1 year.

B) its whole lifetime.

C) 200 years.

D) 1 million years.

2. The energy output per second of a typical quasar is equal to that emitted by the Sun in 200 years. How much energy is this? The Sun’s luminosity is 3.8  10²⁶ watts.

A) 6.8  10²⁸ Joules

B) 1.2  10³⁴ Joules

C) 2.4  10³⁶ Joules

D) 5.5  10³⁹ Joules

3. The energy output per second of a typical quasar is equal to that emitted by the Sun in 200 years. So, if the luminosity of the Sun is L, what is the luminosity of a typical quasar?

A) 3.2  10⁷ L

B) 6.3  10⁹ L

C) 1.9  10¹¹ L

D) 5.3  10¹² L

4. Astronomy with a radio telescope was initiated by

A) the British Broadcasting Corporation in England.

B) Marconi in Europe.

C) the National Radio Astronomical Observatories of the United States, with the support of the National Science Foundation and the American Astronomical Society.

D) an amateur astronomer, Grote Reber, after Jansky had detected radio energy from the Galaxy.

5. One of the first three astronomical objects detected at radio wavelengths by Grote Reber in the late 1930s was

A) Jupiter.

B) the Sun.

C) the galactic center.

D) the Moon.

6. The first three strong radio sources, discovered by Reber in the 1930s and 1940s, included all of these EXCEPT

A) galactic nucleus.

B) supernova remnant.

C) globular cluster.

D) distant galaxy.

7. The major surprise about Cygnus A, one of the first three sources of strong radio emission detected by Grote Reber in the late 1930s, when examined with large optical telescopes, was that it

A) had no corresponding optical counterpart at all.

B) was at the center of the Milky Way Galaxy.

C) was a very distant object that was relatively faint at optical wavelengths yet extremely luminous at radio wavelengths.

D) was a very bright supernova remnant.

8. The discovery of the peculiar galaxy Cygnus A was a surprise to astronomers because

A) the redshift of its radio wavelength signal was as high as anything measured up to that time.

B) it was first discovered at X-ray wavelengths and detected at optical and radio wavelengths only some time later.

C) it was so bright at optical wavelengths that no one expected it to be a galaxy.

D) it was very faint at visible wavelengths but extremely bright at radio wavelengths.

9. Redshifts revealed the radio source Cygnus A to be moving at 17,000 km/s. At this speed it could not have been a part of the Milky Way Galaxy. At this speed how long would it have taken to cross the whole of the visible Galaxy?

A) a few years

B) a few million years

C) a few hundred million years

D) 10 billion years, almost the age of the Galaxy

10. When, by whom, and by what means was the first quasar detected?

A) in 1944 by Reber with a small radio telescope

B) in 1951 by Baade and Minkowski with the 200-inch Hale telescope on Mount Palomar

C) in 1992 by Zeeman with the Hubble Space Telescope

D) in 1955 by a British team using the Cambridge radio telescope

Section: 18-1

11. Cygnus A, a strong radio source, is also designated 3C 405. What does the “3C” stand for?

A) “3C” stands for the Third Cambridge Catalog, an important list of radio sources.

B) This radio source is a Type 3C radio source as opposed to a Type 1A or a Type 2B.

C) Cygnus A was discovered through a joint program run by Cambridge, Connecticut, and Calgary universities.

D) A chopper is a device needed to cut off and delineate a signal from a source, and three such choppers are required to isolate an intelligible signal from Cygnus A.

12. Most quasi-stellar objects (QSOs) produce their strongest emissions in which region of the electromagnetic spectrum?

A) X- ray

B) visible

C) infrared

D) radio

13. What is the MOST outstanding feature of a quasar compared with other objects in deep space?

A) small size

B) great distance from Earth

C) short lifetime

D) prodigious output of energy

14. Quasars were originally thought to be stars in the Milky Way Galaxy. Which of these properties of quasars was LEAST unusual in comparison to normal stars?

A) They were dim in the visible part of the spectrum.

B) They were very energetic in the radio part of the spectrum.

C) Their spectra were dominated by emission lines rather than absorption lines.

D) Their spectra showed patterns that were, at first, unrecognizable.

15. Quasars typically appear to be

A) extremely massive objects in the Milky Way Galaxy, their spectra showing very high gravitational redshift.

B) moving away from Earth at very high speeds, up to about 90% of the speed of light.

C) moving across Earth’s line of sight at very high speeds, as seen in time-lapse photographs.

D) moving toward Earth at high speeds, as high as 90% of the speed of light.

16. The specific characteristics that identify MOST quasars are

A) that they look like elliptical galaxies but with high spectral redshifts.

B) luminous, starlike appearance, and very high spectral blueshift, indicating that they are approaching the Sun very fast, and rapid intensity fluctuations, indicating small intrinsic size.

C) luminous, starlike appearance with very high redshifts and hence very large distances, indicating very energetic sources.

D) spiral galaxy appearance and very high spectral blueshift, indicating that they are coming toward the Sun at high speed.

17. The puzzle about quasar spectra, which was finally solved by Maarten Schmidt, was

A) that the characteristic pattern of hydrogen spectral lines was seen but at much greater blueshifts than is usual from hydrogen in stellar sources.

B) that the brightnesses of hydrogen lines in the familiar spectral sequence were seen to fluctuate wildly over times of seconds from such a bright and therefore presumably large object.

C) that the characteristic pattern of hydrogen spectral lines was seen but at much greater redshifts than is usual from hydrogen in stellar sources.

D) that a familiar spectral line sequence of hydrogen lines was detected from these starlike objects, but they had intensity ratios between lines radically different from those seen in spectra from nearby stars.

18. Astronomers initially had difficulty identifying the emission lines in quasar spectra at optical wavelengths because

A) the emission lines were smeared out by the extremely high speed of the quasars, making them hard to measure.

B) the emission lines were from ionized atoms that had not been seen before.

C) the emission lines were found to be created from elements that do not exist on Earth.

D) no one expected to see the pattern of spectral lines characteristic of the ultraviolet hydrogen spectra in the visible region.

19. The emission lines in quasar spectra were difficult to identify initially because

A) no one expected violet and ultraviolet spectral lines to be shifted so far toward the red.

B) the observed emission lines were so broad because of internal motions in the quasar that they were difficult to identify.

C) they appeared to be created by elements that did not exist on Earth.

D) they were very faint and could not be measured accurately.

20. The observed characteristics of a quasar are a

A) diffuse circular image, no redshift of the spectrum, and often a very bright radio source.

B) starlike image, with a highly variable Doppler spectrum that shifts from red to blue, and often a very bright radio source.

C) starlike image, a highly redshifted spectrum, and sometimes an intense radio emitter.

D) starlike image, an extremely blueshifted spectrum, and often an intense radio emitter.

21. Which observation of quasars convinced astronomers that quasars were very distant objects?

A) extremely red spectrum, reddened by extreme interstellar absorption of the blue part of the spectrum, meaning that the source must be a very long way away

B) extreme redshift of visible Balmer and UV Lyman hydrogen emission lines, indicating high recessional velocities and hence, by the Hubble law, very large distances

C) extreme faintness at visible wavelengths; the inverse square law for visible light showed that they must be very distant

D) appearance as pointlike star images under the highest magnification, meaning that they must be very far away

22. Faint starlike objects producing intense radio energy that eventually were called quasars were found to have an unusual feature in their optical spectra. This feature was

A) an extreme redshift of emission lines that indicated high recessional velocities and hence great distances, requiring extremely high energy output to be detected.

B) a periodic variation of the Doppler shift from red to blue, indicating a light source oscillating back and forth over a few weeks.

C) two sets of spectral lines that indicated simultaneous motion of sources toward and away from the Sun, possibly from a rapidly expanding shell of material around the radio source.

D) an extreme blueshift, meaning that these stars in the Milky Way Galaxy were coming toward Earth at very high velocities.

23. An intense radio source is found to coincide with a starlike object whose spectrum contains a pattern of intense emission lines in the visible range that matches that of the Lyman UV hydrogen spectral lines but is very redshifted. What is this object?

A) exploding shell of a supernova

B) black hole

C) quasar

D) pulsar

24. A starlike object seen on deep sky photographs coincides with an intense radio source and has a spectrum in which the characteristic Lyman pattern of hydrogen spectral lines has been shifted from the ultraviolet to the visible spectral range. What is this object?

A) quasar

B) pulsar

C) supernova explosion

D) black hole

25. Quasars appear to be

A) very distant and intrinsically bright objects moving in random directions at high speeds.

B) very distant, intrinsically faint objects with very high blueshifts in their spectra.

C) relatively close with very high redshifts.

D) very distant, intrinsically bright objects with very high redshifts.

26. All quasars appear to be

A) moving toward Earth at very high speeds.

B) moving away from Earth at very high speeds.

C) very distant, intrinsically faint objects.

D) relatively close, very bright objects.

27. The nearest quasar is

A) in the Milky Way Galaxy.

B) in the Local Group of galaxies.

C) about 600 million light-years from the Milky Way.

D) barely visible at the remote edge of the observable universe, 13 billion light-years away.

28. A quasar is observed to have a redshift of about 0.06, with a recessional velocity about 6% of the speed of light. Approximately what distance, in light-years, does the Hubble law give for this object? The Hubble constant is about 70 kilometers per second per megaparsec.

A) 100 million

B) 800 million

C) 3.2 billion

D) 12.1 billion

29. The closest quasar is Cygnus A, 635 million light-years away. What does the Hubble law give for its redshift?

A) 0.0326

B) 0.046

C) 0.091

D) 0.15

30. The redshift of the quasar Cygnus A is about 0.046. In the spectrum of this quasar what is the wavelength of the H_α line in the hydrogen spectrum (in nanometers)? The rest wavelength of the H_α line is 656.3 nanometers.

A) 30.2

B) 626.1

C) 656.3

D) 686.5

31. Quasars

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 astronomers can measure.

D) peaked in number about 2 billion years after the Big Bang.

32. The distance to the bright quasar 3C 273 is estimated to be

A) 2 billion ly.

B) 20,000 ly.

C) just beyond the Milky Way.

D) 3 million ly.

33. The highest recession velocities that have recently been detected for quasars are

A) 3000 km/s, or about 1% the speed of light.

B) more than 99% the speed of light.

C) about 10% the speed of light.

D) almost half the speed of light.

34. Quasars have now been detected at redshifts so large that, if translated to “recession velocities,” indicate motion away from the Milky Way Galaxy of

A) about 90% the speed of light.

B) speeds as large as 3  10³ km/s, or 1% the speed of light.

C) about 1/10 the speed of light.

D) speeds greater than the speed of light, indicating that the interpretation of redshifts in terms of velocity is faulty.

35. The extreme redshifts of quasar spectra are caused by

A) very high recession speeds of the sources away from the Milky Way Galaxy.

B) absorption of all but the red parts of the quasar spectrum by intergalactic matter.

C) Zeeman effects from the very intense magnetic fields in the vicinity of the source.

D) high gravitational fields at the surfaces of the quasars (gravitational redshift).

36. Today, the number density of quasars in the universe is quite small. For how long in the past has this been true?

A) 7 billion years

B) 9 billion years

C) 11 billion years

D) 13 billion years

37. Which of these statements MOST accurately characterizes the abundance of quasars over the universe’s history?

A) Quasars were most common very early in the universe’s history, about 100 million years after the Big Bang, and their abundance has been declining ever since.

B) Quasars have been increasing in abundance over time, so that they are most common today.

C) Quasars peaked in their abundance about 11–12 billion years ago.

D) Quasars go through cycles in which their abundance increases dramatically then decreases. At the present day, they are at a low point in the cycle.

38. Quasars emit significant amounts of radiation from the Lyman-alpha transition. When the spectrum is observed on Earth, it is found that the Lyman-alpha line is accompanied by many absorption lines, called the Lyman-alpha forest. What is the origin of these lines? See Figure 18-5 in the text.

A) The absorption lines are caused by the rotation of the quasar. Different parts of the quasar thus give rise to Lyman-alpha lines with different Doppler shifts.

B) The absorption lines are the result of gravitational lensing by objects between the quasar and Earth.

C) The emitted Lyman-alpha radiation is absorbed by many gas clouds between the quasar and Earth. The lines are receding at various velocities and thus are absorbed at different Doppler-shifted wavelengths.

D) Because the quasar’s jets are aimed at various directions, the jet plasma has Doppler shifts that are different from those of the quasar itself. The result is a variety of Lyman-alpha wavelengths in the spectrum received on Earth.

Section: 18-2

39. Quasars are

A) sources of great energy, very large in actual size, and shaped like galaxies.

B) sources of intense radio energy only, not visible at other wavelengths, and relatively large but very distant.

C) prolific sources of energy, starlike in appearance, and intrinsically small.

D) starlike sources of great energy located in the Milky Way Galaxy and intrinsically very small.

40. What are the typical characteristics of a quasar in terms of energy generation?

A) output of 100 galaxies from a volume with diameter of 1 light-day

B) output of 10⁶ galaxies from a volume with diameter of 1 light-day

C) output of 100 galaxies from a volume with diameter of 1 light-year

D) output of single galaxy from a volume with diameter of 1 light-year

41. “It is starlike in appearance, showing very high redshift and an energy output of at least 100 galaxies from a small region about 1 light-day across.” This statement describes which of these astronomical objects?

A) red supergiant star

B) center of the Milky Way Galaxy

C) quasar

D) supernova explosion in a neighboring galaxy

42. The energy output of a bright quasar is equivalent to

A) that of the Milky Way Galaxy.

B) 10⁶ spiral galaxies.

C) 1000 Suns.

D) 100 Milky Way galaxies.

43. Compared with the total output of energy from the Milky Way Galaxy, what is the equivalent output of a typical quasar?

A) only about 1/10 of the Galaxy’s output but from a small volume of space

B) 100 times brighter

C) about a million times brighter

D) about the same energy output

44. The fact that quasars can be detected from distances from which even the biggest and most luminous galaxies cannot be seen means that

A) they must be in directions where gravitational focusing by the masses of nearer galaxies makes them visible from Earth.

B) they must be far more luminous than the brightest galaxies.

C) they must be in directions where intergalactic absorption by dark matter is minimum, allowing observers to see them.

D) their spectra have not been as redshifted by their motion as those of galaxies and hence they can still be seen.

45. A quasar is now thought to be a

A) long-lived supernova explosion.

B) very luminous object at a very large distance from the Sun.

C) nearby star, ejected with great violence and velocity from the center of a galaxy.

D) distant but very luminous and active star in the Milky Way Galaxy.

46. The fluctuation time for the brightnesses of quasars can be as short as a

A) few seconds.

B) month.

C) day.

D) year.

47. Variations in the luminosity of quasars over the span of a few hours are an indication

A) of the rapid rotation of the sources.

B) that these objects are actually binary systems in which one sees mutual eclipses.

C) of the relatively small size of the emitting regions.

D) of obscuring gas and dust clouds moving in front of them from Earth’s point of view.

48. How can astronomers determine the size of an emission region in a very distant and unresolvable source?

A) by measuring the object’s mass and using a reasonable value for the average density for matter to calculate its volume and hence its diameter

B) by using radio interferometry because this technique can resolve far greater detail than optical imaging

C) by measuring brightness variability because an object cannot vary more rapidly than the time taken for light to cross the source

D) by measuring the redshift of its spectrum because the redshift will depend on the source size

49. What is the relationship between the size of an object and the rate at which its radiant output can vary?

A) Small objects can vary more rapidly.

B) Large objects can vary more rapidly.

C) The relationship depends on the wavelength of the radiant output. Large objects can vary more rapidly at long wavelengths, and smaller objects can vary more rapidly at shorter wavelengths.

D) There is no relationship between size and variation rate.

50. What observations convince astronomers that the energy source of a quasar is physically very small?

A) rapid fluctuations in brightness since variations over 1 day mean that the source must be less than 1 light-day across

B) instant disappearance of the quasar when occulted by the Moon’s edge as the Moon moves in front of a quasar, indicating a very small source size

C) sharpness of the emission lines in their optical spectra since motions within a large source would smear out the line shapes

D) extremely small size of the image of a quasar, even from Hubble Space Telescope images and radio interferometry measurements

51. What observational fact convinces astronomers that the source of energy in a typical quasar is physically very small?

A) rapid variation in the energy output of the source

B) extreme distance of all quasars

C) starlike appearance of quasars in Earth’s sky

D) narrowness of the emission lines in the spectra of quasars

52. Which observations of the radiation from quasars indicate that quasars are physically very small objects compared with galaxies?

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

53. An astronomer is observing the radiation from a distant active galaxy and notices that the amplitude of the signal varies in strength regularly over a certain period. The maximum possible size for the source of this radiation can now be calculated from the

A) period and the amplitude of the signal.

B) period but not the amplitude of the signal.

C) amplitude but not the period of the signal.

D) frequency of the radiation composing the signal.

54. Suppose the X-ray signal from a quasar fluctuates with a period of 3 hours. What is the maximum size of the source?

A) 150,000 km, about the diameter of Jupiter

B) 10.8 au, about the diameter of Jupiter’s orbit

C) 4.5 ly, about the distance between the Sun and the nearest star

D) 10,800 ly, a tenth the size of the Milky Way Galaxy

55. Why is it that astronomers will NOT see fluctuations in light output in times shorter than about 1 day when they observe an extragalactic source whose diameter is about 1 light-day?

A) Absorption of light by intergalactic matter will smooth out rapid fluctuations within the beam.

B) The light from different parts of the source will be Doppler-shifted by different amounts, allowing observers to see only an average shift.

C) It is inconceivable that a source of this size could vary on such short time scales.

D) Arrival times will be different from different parts of the source, which will smooth out short-term fluctuations.

56. The surprising observational fact about quasars is that they appear to

A) produce the energy output of greater than 100 galaxies in a volume similar to that of the solar system.

B) be the largest known structures in the universe, although they produce only modest amounts of energy.

C) be associated with ancient supernova explosions.

D) be moving rapidly toward Earth while emitting large amounts of energy.

57. The luminosity of the Milky Way Galaxy is about 10 billion times the luminosity of the Sun. During peak periods of brightness, the quasar 3C 279 has a luminosity about ten thousand times the luminosity of the Galaxy. Suppose the flux (energy per second per emitting area) from the Sun remains the same, but it grows in radius. How big would it have to be to produce the same luminosity as 3C 273 at its peak?

A) about the size of Pluto’s orbit

B) about 100 times the diameter of Pluto’s orbit

C) about a light-year across

D) about a tenth the diameter of the Galaxy

Section: 18-3

58. An active galaxy is measured to be 100 Mpc away. How many million years ago did the light observers see now leave this galaxy?

A) 30.7

B) 100

C) 326

D) 652

59. Which of these is NOT a characteristic of active galaxies?

A) jets

B) strong emission lines in the spectrum

C) Doppler shifts indicating a large rate of rotation

D) bright starlike nuclei

60. Which of these would NOT necessarily be categorized as “active galaxies”?

A) radio galaxies

B) double-radio sources

C) BL Lacertae objects

D) galaxies with supermassive black holes

61. Seyfert galaxies are

A) spiral galaxies with bright, starlike nuclei and strong emission lines.

B) irregular galaxies with no shape or structure.

C) very small elliptical galaxies.

D) the largest galaxies in the universe.

62. Seyfert galaxies are a distinct class of galaxies because they

A) are very close to the Milky Way and appear to be gravitationally bound to it.

B) have very bright, very hot starlike central cores with variable energy output.

C) are completely devoid of structure, appearing to be amorphous spheres of gas and dust.

D) are in the constellation of Seyfert in Earth’s southern hemisphere sky.

63. Which of these is a characteristic of a Seyfert galaxy?

A) A Seyfert galaxy has no spiral arms.

B) The nucleus of a Seyfert galaxy is unusually bright compared with the spiral arms.

C) The nucleus of a Seyfert galaxy is unusually faint compared with the spiral arms.

D) The spectrum of a Seyfert galaxy shows particularly narrow absorption lines.

64. Which one of these properties do Seyfert galaxies and the Milky Way Galaxy share?

A) bright starlike nucleus

B) spiral galaxy classification

C) significantly varying energy output over a few weeks

D) spectrum containing strong emission lines from elements such as highly ionized iron

65. Which one of these objects typically has the greatest luminosity?

A) giant elliptical galaxy

B) Seyfert galaxy

C) radio galaxy

D) quasar

66. Seyfert galaxies are

A) elliptical galaxies with extremely bright nuclei.

B) supergiant elliptical galaxies that are periodically disturbed by supernova explosions within them.

C) spiral galaxies with extremely active cores.

D) active galaxies that shine mainly by radiation from two relatively widely spaced radio lobes.

67. A spiral galaxy with a bright, starlike nucleus showing strong emission lines is called a

A) quasar.

B) gravitational lens.

C) BL Lacertae object.

D) Seyfert galaxy.

68. Observationally, the biggest difference between quasars and other active galaxies such as Seyfert and radio galaxies appears to be that

A) quasars appear to be located inside elliptical galaxies, whereas Seyfert and radio galaxies are spirals.

B) Seyfert and radio galaxies do not have the bright, starlike nuclei of quasars.

C) the brightness of Seyfert and radio galaxies does not vary with time.

D) Seyfert and radio galaxies are less powerful energy emitters than quasars.

69. BL Lacertae objects appear to be

A) giant irregular galaxies with neither spiral arms nor the smooth shape of elliptical galaxies.

B) elliptical galaxies with bright, starlike nuclei.

C) spiral galaxies with bright, starlike nuclei.

D) active galaxies, most of whose energy is emitted by two widely spaced radio lobes.

70. A BL Lacertae object is a(n)

A) emission nebula containing a young T Tauri star.

B) rapidly spinning neutron star.

C) eclipsing binary star with a black hole as one component.

D) active galactic nucleus.

71. The spectrum of a BL Lacertae object shows

A) strong, highly redshifted emission lines.

B) doubled emission lines split by the Doppler shift from oppositely directed jets of material.

C) absorption lines of highly ionized atoms.

D) A fairly smooth continuum with exceptionally weak spectral lines.

72. What is a “peculiar” galaxy?

A) a galaxy with a supermassive black hole at the center

B) an older name for an active galaxy

C) a galaxy that looks like it is exploding

D) an active galaxy with large radiation output at many different wavelengths across the spectrum

73. Which of these designations does NOT refer to the shape of a galaxy?

A) peculiar

B) barred spiral

C) elliptical

D) spiral

74. The typical timescale for variability of a blazar (a parameter related to true physical size of these objects) is

A) about 1 week.

B) less than 1 day.

C) a few seconds.

D) about 1 year.

75. Which of these objects is NOT classified as an active galaxy?

A) Seyfert galaxy

B) BL Lacertae object

C) quasar

D) barred spiral

76. An electron moving in a magnetic field in space is forced to move in a spiral pattern. As it does so, it emits

A) characteristic X-rays as the electron is slowed down by this motion.

B) visible light, mostly blue in color because the electron is forced to move in a circular path.

C) synchrotron radiation, mostly radio waves, since the electrons are accelerating.

D) nothing since such electrons are moving with a constant speed and are not therefore accelerating.

77. A moving electron in a magnetic field in space follows a spiral pattern, emitting what type of radiation as it does so?

A) synchrotron radiation

B) no radiation at all, since it is moving smoothly without acceleration

C) Cherenkov radiation

D) Lyman radiation

78. Synchrotron radiation is produced whenever

A) electrons jump from level to level in an atom.

B) electrons spiral around a magnetic field.

C) atoms in a molecule vibrate back and forth.

D) electrons move in a transparent medium at a speed faster than the speed of light in the medium.

79. Which of these phenomena will produce synchrotron radiation?

A) spiraling high-speed electrons in a magnetic field

B) heating of matter by compression as it spirals into a black hole

C) radioactive decay of an atomic nucleus

D) slowing down of charged particles as they enter a dense medium such as the atmosphere of a star or of Earth

80. What mechanism appears to produce the double-radio sources seen in intergalactic space?

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) 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

81. The mechanism that leads to the double-radio source phenomenon appears to be

A) two active galaxies in orbit around a common center, like a binary star system.

B) two oppositely directed jets of energetic particles from an active galactic object colliding with other intergalactic material.

C) a radio-bright galaxy crossed by a dark, absorbing, edge-on disk, splitting the radio emissions into two as seen from Earth.

D) gravitational lensing of a distant radio source by a nearby galaxy or black hole.

82. The mechanism that appears to generate two extensive regions of radio emission near active galaxies is

A) two very hot gas clouds, suspended by magnetic fields above the rotation axis of a galaxy, emitting 21-cm radio waves.

B) a radio source split by a dark absorbing disk across its center as seen from Earth.

C) the double image of a single source behind the galaxy produced by gravitational lensing by the galaxy.

D) two oppositely directed jets of energetic particles that collide with intergalactic gas.

83. The radio emission from the jets in a double-radio source is

A) recombination radiation from electrons combining with protons to form neutral hydrogen gas as the gas cools within the jets.

B) 21-cm emission from neutral hydrogen atoms.

C) thermal emission from very hot matter ejected from the accretion disk around the central black hole.

D) synchrotron radiation from relativistic electrons spiraling in magnetic fields.

84. Many bright radio sources at very large distances from the Sun appear to emit energy from two relatively widely spaced sources. What mechanism is thought to produce these double-radio emission regions?

A) Two oppositely directed beams of light and radio energy illuminate the intergalactic dust and gas.

B) Two oppositely directed plasma jets from a central source produce radio energy as the plasma slows down.

C) These sources are two galaxies orbiting each other like binary stars.

D) The sources are in fact a single source and its reflection in a dense intergalactic gas and dust “mirror,” or plasma sheet.

85. What is now considered to be the mechanism for the production of the double lobes of radio emission that appear on either side of many galaxies?

A) radio emission from the two halves of galactic halos containing many globular clusters that generate radio noise

B) supernova explosion of a massive star and ejection outward in two opposite directions of very hot gas that emits thermal energy

C) two oppositely directed jets of relativistic particles spiraling around magnetic fields

D) material streaming in toward a black hole, perpendicular to and on opposite sides of the accretion disk

86. Centaurs A is a giant radio galaxy. Its radio emission is primarily from the

A) galactic nucleus.

B) lobes outside the galaxy.

C) jet leading from the nucleus to the lobes.

D) entire galaxy, including the jet and the lobes.

87. See Figure 18-10 in the text. NGC 1265, an active galaxy in the Perseus cluster, is described as a head-tail source because

A) it resembles a horse’s tail and is located near the Horsehead nebula in Orion.

B) the jets are emitted from the radio lobes and stream toward the nucleus, the reverse of what happens in a normal radio galaxy.

C) this galaxy is peculiar and is exploding and sending out many jets that interact with the surrounding intergalactic material.

D) the two jets interact with the rapidly moving intergalactic medium, which sends them streaming out behind the galaxy in a pair of arcs.

Section: 18-4

88. Evidence obtained over the last few years indicates that a quasar is MOST likely

A) the remnant core of an exploding star or supernova.

B) the focused image of a distant galaxy by the gravitational lens effect of a closer galaxy.

C) evidence of very intense star-building activity in certain distant dust and gas clouds.

D) powered by material near the central supermassive black hole of a very distant, very active galaxy.

89. Massive black holes have been discovered in the cores of what kinds of galaxies?

A) giant elliptical only

B) spirals only

C) giant ellipticals and spirals only

D) dwarf ellipticals, giant ellipticals, and spirals

90. Where do astronomers find supermassive black holes?

A) only in the centers of giant elliptical galaxies

B) only in the centers of active galaxies

C) in the centers of both active and normal galaxies, but only those at relatively high redshift values, indicating that they existed in the distant past

D) in the centers of both active and normal galaxies, both nearby and far away

91. What recent evidence seems to indicate that several nearby galaxies may contain supermassive black holes at their centers?

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.

92. What observational evidence seems to indicate the presence of a supermassive black hole at the center of the Milky Way’s neighboring galaxy, M31, in Andromeda?

A) slow but measurable motion of the Milky Way Galaxy toward M31 under the intense gravitational force of its black hole

B) very intense X-ray emission from a very small central core, indicating a very hot source

C) spectroscopic measurements of extremely high and symmetric Doppler shifts from stars orbiting rapidly around a massive object near the galactic center

D) small but dark region on detailed photographs of M31, showing an area from which light cannot escape

93. Which of these statements has NOT provided observational evidence of the existence of supermassive black holes in the centers of galaxies?

A) The centers of these galaxies are completely dark even though it is clear that the background (the other side of the galaxy) should be bright with stars.

B) Large amounts of energetic X-rays are being emitted from the galactic centers.

C) Stars show very high speeds near the galactic centers.

D) The number density of stars near the galactic center is extraordinarily high.

94. What appears to be the central energy-generating system or “engine” that is producing prodigious amounts of energy in the centers of galaxies, active galaxies, and quasars?

A) steady series of supernova explosions, the late evolutionary stages of massive stars

B) rapidly rotating core of matter, where friction between it and the surrounding matter causes tremendous heat and energy output

C) supermassive black hole, where matter is compressed on falling into the hole and heated to extremely high temperatures

D) There is no central “engine” in these sources. Their high gravity has focused radiation from many sources beyond them by gravitational lensing and they thus appear to be very bright.

95. The strongest direct evidence for supermassive black holes at the centers of normal galaxies comes from

A) gravitational lensing.

B) the Doppler shifts of stars.

C) gamma-ray emissions.

D) the motions of galaxies during mergers.

96. The energy-generation mechanism for active galaxies is believed to be

A) large numbers of supernovae erupting simultaneously.

B) cosmic radioactive decay.

C) black holes colliding.

D) accretion disks around supermassive black holes.

97. Astronomers believe a supermassive black hole of three billion solar masses exists at the center of the giant elliptical galaxy M87. Which one of these is NOT a piece of evidence used to deduce this?

A) This galaxy is a powerful source of X-rays.

B) The density of stars near the center of the galaxy is extraordinarily high.

C) Multiple images of the galaxy have been formed by gravitational lensing.

D) The galaxy has a bright, starlike nucleus.

Section: 18-5

98. The jets of material moving away from a supermassive black hole are generated by high pressures and temperatures at the base of the accretion disk. What causes these high temperatures?

A) radioactivity

B) friction

C) hydrogen fusion

D) helium fusion

99. Jets of gas coming from an AGN are associated with

A) any supermassive black hole.

B) Schwarzschild black hole only.

C) slowly rotating Kerr black holes only.

D) rapidly rotating black holes only.

100. What do astronomers expect to be the relationship between the number of active galaxies they detect and the number of active galaxies that actually exist?

A) The energy output from active galaxies is so large that it is easy to detect them. Thus, astronomers believe that they have detected all that exist.

B) The nuclei of many active galaxies are hidden by dust in the accretion disk. Thus, astronomers expect the actual number to be much greater than the observed number.

C) Astronomers cannot detect active galaxies unless the jets are pointed more or less toward Earth. The jets point toward Earth in only a few cases, so they expect the actual number to be greater than the observed number.

D) Active galaxies are created in galaxy collisions. But galaxy collisions occurred only in the early universe and are no longer occurring. The active galaxies all died off while their light was traveling to Earth.

101. Observations indicate that BL Lacertae objects are

A) quasars colliding with smaller galaxies and initiating vigorous star formation.

B) black holes in binary star systems, where matter pulled from the companion stars forms a hot accretion disk around the black holes.

C) distant spiral galaxies with high rates of supernova explosions.

D) radio galaxies whose jets and radio lobes point almost directly at Earth.

102. The central “engine” of an active galaxy appears to be

A) stars falling into a supermassive black hole, then their remnants being thrown out in all directions.

B) supernova explosions in an extremely dense star cluster at the center of the galaxy.

C) the violent merger of two galaxies in which the collision throws out jets of matter along the rotation axis of the larger galaxy.

D) a supermassive black hole at the center of an accretion disk and material being projected out perpendicular to the disk in both directions.

103. In which direction(s) are the jets of matter ejected from a supermassive black hole to produce the many effects surrounding quasars and active galaxies?

A) two opposite directions perpendicular to the accretion disk of the black hole

B) single direction above the accretion disk, the precession of which sweeps the jet along the surface of a cone

C) two opposite directions across the accretion disk and rotating with it

D) They are tangential to the accretion disk in two opposite directions, 180° apart; the matter is spun off by the rapid disk rotation.

104. The intensity of radiation coming from an active galaxy

A) remains remarkably constant during its long life.

B) decreases slightly at a constant rate as energy is radiated away.

C) ebbs and flows as the mass of the accretion disk around the black hole ebbs and flows.

D) ebbs and flows as the mass of the black hole ebbs and flows.

105. When the jet of an active galaxy is aimed directly at Earth, astronomers might classify the object as a

A) radio galaxy.

B) BL Lacertae object.

C) pulsar.

D) Seyfert galaxy.

106. If the central engine of a double-lobed radio source is a black hole swallowing matter from an accretion disk, where do the jets of matter come from that are observed traveling outward from the galaxy?

A) The jets are accelerated near the rotating black hole and ejected outward in the black hole’s equatorial plane.

B) The jets are composed of material that has been accelerated from two hemispheres toward the black hole with such speed that it escapes again on two opposite sides of the black hole.

C) The jets arise in the weak galactic magnetic field, not in the region near the black hole.

D) The jets are squirted out by high pressure in the accretion disk before the matter reaches the black hole.

107. Double-radio sources, quasars, and BL Lacertae objects are now considered to be the same basic type of object—an accretion disk producing two oppositely directed jets—but they appear to observers to be very different because

A) the relativistic particles ejected in the double jets are different in each case: electrons in double-radio sources, protons in quasars, and quarks in BL Lacertae objects.

B) they are at different distances from Earth, so one sees more detail and different properties in those that are closer to Earth.

C) they are of different ages.

D) astronomers view them at different angles to the line of the double jets. Face-on to the accretion disk the source will look like a BL Lacertae object, edge-on as double-radio sources, and between these positions a quasar.

108. In the unified model of active galaxies, the main difference among quasars, BL Lacertae objects, and radio galaxies appears to be that

A) astronomers see the accretion disk around the central black hole from a different angle in each case—face-on for BL Lacertae, edge-on for radio galaxies, and in between for quasars.

B) the rate at which matter is falling into the central black hole is different in each case—largest in quasars, less in BL Lacertae objects, and least in radio galaxies.

C) the mass of the central black hole is different in each case—largest in quasars, less in BL Lacertae, and least in radio galaxies.

D) the galaxy type with which they are associated is different in each case—spiral for BL Lacertae, elliptical for quasars, and irregular for radio galaxies.

109. Quasars, double-radio sources, and BL Lacertae objects may well be the same kind of object. The different appearance of them being simply that

A) their position in the universe is different: Quasars are in the Milky Way Galaxy, double-radio sources are associated with other galaxies, and BL Lacertae objects are in the vast voids of space between galaxies.

B) the orientation of Earth’s line of sight to the axis of their ejected jets of matter is different.

C) their ages are different: BL Lacertae objects evolving through the double-radio source phase end as quasars.

D) their sizes are different: Quasars are star-sized, double-radio sources are larger, and BL Lacertae objects are galaxy-sized.

110. How has the number of quasars varied over the history of the universe?

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, 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.

111. Does the presence of an AGN in a galaxy affect star formation rates in the galaxy?

A) No. Star formation rates in galaxies with AGN are similar to those in galaxies of the same type and size without AGN.

B) Yes. The presence of an AGN and its jets sends out shock waves that cause star formation to occur at an elevated rate throughout the disk of the galaxy.

C) Yes. It has a negative effect. The presence of the AGN heats the gas and dust throughout the galaxy, thus making it less likely to collapse to form stars.

D) This question has different answers for different host galaxies. An AGN in a spiral galaxy causes increased star formation rates, but an AGN in an elliptical galaxy has no effect on the (very low) star formation rate.

Section: 18-6

112. What is an Einstein ring?

A) arc of relativistic particles ejected from a galactic nucleus

B) example of the image formed by a gravitational lens

C) ring of material in some spiral galaxies created by a collision with a smaller galaxy

D) accretion disk around a black hole

113. An astronomer is examining quasars and happens upon two quasars in nearly the same part of the sky that have identical spectra. They cannot see any massive object in the region between the two quasars. What is a reasonable explanation for this discovery?

A) The discovery is not really a surprise because all quasars are formed by the same processes and all have virtually identical spectra.

B) The two quasars are part of a binary pair of quasars that were formed together, are at the same stage in their lives, and have the same spectra.

C) The two quasars cannot be gravitational lensing images because there is no visible lensing object in the space between them.

D) The two quasars are most likely gravitational lensing images, and the lensing object is mostly dark matter with little visible emission.

114. Gravitational lensing is caused by the bending of light as it passes a massive object. Why does gravitational lensing so often involve quasars?

A) because they are among the most massive objects known

B) because they are all so far away

C) because their radiation is narrowly confined to particular regions of the spectrum

D) because astronomers can observe most quasars up off the galactic plane where they are not obscured by gas and dust

115. For gravitational lensing of a distant quasar to occur, the galaxy producing the lensing must

A) be rotating rapidly to produce the requisite curvature of space to bend the light beam.

B) be almost perfectly placed on a line between Earth and the quasar.

C) contain a supermassive black hole at its center.

D) be a dwarf galaxy; otherwise the quasar’s light cannot pass through it and reach Earth.

116. In observations of a double quasar image produced by gravitational lensing, fluctuations on the intensity of one image appear to be delayed compared with equivalent variations on the other image. Explain.

A) Rotation of the lensing galaxy has delayed the beam of light on one side compared with the other because of the Doppler effect.

B) One beam has had to pass through material with a greater refractive index than for the other beam within the lensing galaxy.

C) The gravitational field of the lensing object slows down the light on one beam compared with the other.

D) The path light takes through the gravitational lens to produce one image is longer than that for the other image.

Section: 18-7

117. What is the origin of fast radio bursts?

A) gamma ray bursts

B) active galactic nuclei

C) mergers of black holes

D) Astronomers do not yet understand the origin of these phenomena.

118. What is the evidence that fast radio bursts are extragalactic in origin?

A) They occur in all directions, not just toward the disk of the Milky Way.

B) The burst periods are too small to be nearby objects.

C) The bursts are all redshifted.

D) The bursts are all very faint.