ANSWERS TO COMPUTATIONAL, GOT IT?, AND SELECT MARGIN QUESTIONS

ANS-1

CHAPTER 1

MARGIN QUESTION 1-3: Explain why Figure 1-14 must have been taken facing west. Hint: Examine Figure 1-17. ANSWER: In the northern hemisphere, stars move southward (to the right) as they rise and northward (to the right) as they set. If this figure were taken in the morning, the stars would be moving in the wrong direction.

MARGIN QUESTION 1-10: Why are some of the paths in Figure 1-30 wider than others? ANSWER: The angle that the shadow strikes Earth determines how wide the eclipse path is. This angle varies with latitude, time from noon, and distance of the Moon from Earth. For example, near the equator, near noon, and with the Moon relatively far from Earth, the shadow is thinnest, while near the poles, especially far from noon, and when the Moon is closest to Earth, the shadow is particularly wide.

43. 1 more sidereal month than synodic month

53. R⊙ ≈ 6.53 × 108 m

Toolbox 1-1: ~0.53°. This is the same angle as the Moon makes in the sky; ~0.27°; ~1.1°

GOT IT? 73: Describe the Orion constellation. ANSWER: The region of the celestial sphere, with connected boundaries running at right angles to each other, containing the Orion asterism and everything else in that region.

GOT IT? 74: In what season in the southern hemisphere is Earth closest to the Sun? In what season in the northern hemisphere is Earth closest to the Sun? If the changing distance from Earth to the Sun caused the seasons, what should be true about the answers to the first two parts of this question? ANSWER: Summer. Winter. They should both be the same, since the whole Earth would then be affected equally by the changing amount of heat with distance from the Sun.

GOT IT? 75: What observational evidence do we have that the Moon does not make its own light? ANSWER: If the Moon made its own light, then we should see all parts of it glowing equally, or at least all of it glowing all the time.

GOT IT? 76: What is the difference between the “far” side of the Moon and the “dark” side of the Moon? ANSWER: The far side of the Moon is the side always facing away from Earth, while the dark side is the side of the Moon facing away from the Sun at any time.

GOT IT? 77: Through how many constellations does the Sun pass each year? What are these constellations called? ANSWER: 13, Zodiac constellations.

GOT IT? 78: Which of the following statements is correct for someone standing at one of Earth’s poles?

  1. The Sun rises and sets every day.
  2. The Sun is directly overhead on the summer solstice in that hemisphere.
  3. The Sun is in the sky continuously for 6 months.
  4. The Moon is never visible.
  5. The stars rise vertically and set vertically.

ANSWER: c

CHAPTER 2

MARGIN QUESTION 2-3: Why do I say “of the same type” in the last paragraph on this page? ANSWER: There are two types of conjunctions for the inferior planets. The interval from a superior conjunction to the following inferior one, or vice versa, is not a synodic period, whereas the period from one superior conjunction to the next superior conjunction (or one inferior conjunction to the next) is a synodic period.

MARGIN QUESTION 2-6: We saw in Chapter 1 that the Moon’s orbit around Earth is not circular. Where in its orbit is the Moon moving fastest, and where is it moving slowest? ANSWER: By Kepler’s second law, the Moon is moving fastest where it is closest to Earth (perigee) and slowest where it is farthest away (apogee).

MARGIN QUESTION 2-7: Sitting in a moving car, how can you experimentally verify that your body has inertia? ANSWER: To show that your body has inertia, drive a car and then rapidly put on the brakes. You will feel yourself forced forward against the seatbelt. Without inertia, you would slow down at the same rate as the car does without having to be restrained by the seatbelt.

17.

20. 5.2 square AU in 2013; 26 square AU in 5 years

21. a = 100 AU; maximum distance is almost 200 AU

22. 2.8 years

28. At 2 AU, 1 year = 2.8 present years

29. At ½ AU, 1 year = 0.35 present year

30. 0.01 × present gravitational attraction

31. same acceleration; same length of the year

Toolbox 2-1: ~2.48 × 1013 mi, ~3.99 × 1013 km

Toolbox 2-2: Energy increases 9-fold; energy decreases to ¼; 2000 J; 0 J; change its moment of inertia, I, and its angular velocity ω.

ANS-2

Toolbox 2-3: Your weight in pounds; same; 9.8 m/s2 or 32 ft/s2; 9 × 1021 N; It is one-quarter the present force.

GOT IT? 38: A comet coming inward from the Kuiper Belt, outside the orbit of Neptune, experiences a gravitational force from the Sun. Does the presence of the planets affect the comet’s orbit? Explain your reasoning. ANSWER: Yes. All objects exert gravitational attraction on all other objects at all times.

GOT IT? 39: How would the weight of an astronaut on the Moon compare to her weight on Earth? ANSWER: She would be lighter on the Moon. She would weigh 1/6 as much as she does on Earth.

GOT IT? 40: How would the mass of an astronaut on the Moon compare to his mass on Earth? ANSWER: His mass would be the same as on Earth.

GOT IT? 41: An astronomer observes a new comet and calculates that it will exit the solar system and not return. Which of the following best describes the path of the comet?

  1. a nearly straight line
  2. a circle
  3. an ellipse
  4. a hyperbola
  5. some other shape

ANSWER: d, hyperbolic

CHAPTER 3

MARGIN QUESTION 3-2: The speed of sound is about 0.34 km/s (0.21 mi/s). How can this and the information in this section be used to determine a person’s distance from a lightning strike? ANSWER: To determine a person’s distance to a lightning strike, measure the time interval between the lightning and subsequent thunder. Since you see the lightning virtually instantaneously after it strikes, while the thunder travels at the much slower “speed of sound,” the distance to the strike is just the speed of sound times the time between the lightning and thunder clap.

MARGIN QUESTION 3-6: Why do the human eye and brain clear the images that they receive many times per second? ANSWER: The brain clears images it receives in order to detect motion. Without clearing what we see very often, the brain would get saturated with light and would be unable to detect changes, i.e., motion.

MARGIN QUESTION 3-9: Where in a typical house would infrared detectors indicate most activity? ANSWER: At home, infrared detectors are useful for detecting things in the kitchen, in the furnace room, in fireplaces, and wherever else there are heat sources.

4. 9 times more

17.

18. Palomar gathers 106 times more light than the human eye.

20. (a) 222×; (b) 100×; (c) 36×

Toolbox 3-1: 406 nm; violet; 203 nm; assuming green at 500 nm, E = 4.0 × 10−19 J

GOT IT? 33: Why do stars twinkle? ANSWER: As seen from space, they don’t. As seen from Earth, they twinkle because the light from them passes through the Earth’s atmosphere, which continually changes density and thereby changes the direction of the starlight passing through it. These changes, typically every ½ second, are what we see as twinkling.

GOT IT? 34: Why do all research telescopes use primary mirrors rather than objective lenses? ANSWER: Lenses do not refract all colors equally, thereby creating chromatic aberration, while mirrors do not have this problem. Lenses also cannot be prevented from sagging (and hence distorting images) as their telescopes change direction, while lenses can be prevented from sagging. It is also very hard to grind a lens to the right shape to focus light, but easy to make mirrors of the right shape. Lenses often have air bubbles, which distort light, but mirrors don’t have this problem since light never enters them. Lenses do not allow all colors of light to pass through them equally.

GOT IT? 35: For the purpose of observing very faint objects, which of the following features of a telescope is most important? Explain your answer.

  1. its maximum magnification
  2. its ability to resolve colors
  3. the size of its objective lens or primary mirror
  4. the type of mount it has (if necessary, please see Appendix H for information on mounts)
  5. its weight

ANSWER: c, the larger the diameter of the primary or objective, the more photons the telescope can collect, hence the brighter dim objects appear.

GOT IT? 36: Of the following types of electromagnetic radiation, which is most dangerous to life?

  1. radio waves
  2. X rays
  3. ultraviolet radiation
  4. infrared radiation
  5. visible light

ANSWER: b, of all listed, X rays have the shortest wavelength and hence each x-ray photon carries the most energy.

CHAPTER 4

MARGIN QUESTION 4-1: Referring to Figure 3-6, if necessary, where on the horizontal axis are radio waves relative to visible light in Figure 4-2? ANSWER: Far to the right.

MARGIN QUESTION 4-3: From what we have just discussed, what do you think causes fireworks to have their distinctive colors? ANSWER: Transitions of electrons in the elements and molecules used in the fireworks emit specific wavelengths, giving fireworks their distinctive colors.

ANS-3

MARGIN QUESTION 4-5: Which Balmer line in Figure 4-11 is Hα? ANSWER: Hα on Figure 4-11 is the transition from n = 3 to n = 2. This transition’s energy is least (the resulting photon has the longest wavelength) of all Balmer transitions.

MARGIN QUESTION 4-6: Does a police siren approaching you sound higher or lower in pitch than the siren at rest relative to you? ANSWER: An approaching siren has a higher pitch than does the same siren at rest relative to you.

15. 7½ times

16. 238 nm

17. 6520 K

18. coming toward us; 13 km/s

19. moving away from us; 21 km/s

20. about 90,000 km/s

24. λ = 500 nm (same as now), 4× brighter than now

28. Rigel, no; Deneb, no; Arcturus, yes; Vega, no; Betelgeuse, no

Toolbox 4-1: ~5270 K; ~9700 nm; infrared

Toolbox 4-2: ⅛ kg; ~0.35 kg; ~17½

Toolbox 4-3: 3 × 104 km/s or 0.1c; ~3.3 × 10−4; −0.1

GOT IT? 29: What color does the Sun emit most intensely? ANSWER: Blue-green, as you can see on a blackbody diagram.

GOT IT? 30: A star of which of the following colors is coolest?

  1. blue
  2. red
  3. orange
  4. yellow
  5. violet

ANSWER: b, red.

GOT IT? 31: If a yellow star cools off, what color will it become next? ANSWER: Orange.

GOT IT? 32: Do we see all the colors from each star? Why or why not? ANSWER: No, because some wavelengths from each star are scattered on their way to us, creating dark absorption lines in the spectra of all stars (including the Sun).

CHAPTER 5

MARGIN QUESTION 5-1: Which elements on Earth may have been unchanged since the universe began? ANSWER: Much of the hydrogen and helium, and some of the lithium on Earth formed shortly after the universe began. All other elements here were definitely formed from these building blocks since then.

MARGIN QUESTION 5-5: How might some of the meteoroids in the solar system come from asteroids? ANSWER: When asteroids strike each other or when a comet strikes an asteroid, the rock and metal debris splashed into space from the asteroid become meteoroids.

MARGIN QUESTION 5-6: Why is Earth’s albedo continually changing? ANSWER: The surface and atmospheric features of Earth, including cloud cover, ocean surface activity, snow, ice, and vegetation cover, change, thereby changing the amount of light scattered back into space (i.e., the albedo).

GOT IT? 25: Which elements on Earth were created during the Big Bang? ANSWER: Hydrogen (H), helium (He), and lithium (Li).

GOT IT? 26: How long has the universe existed and how long has the solar system existed? ANSWER: 13.8 billion years; 4.6 billion years.

GOT IT? 27: Name an object on Earth that is most similar in shape to a typical moon in our solar system. ANSWER: Potato.

GOT IT? 28: Of the four giant planets, which gets the most radiation from the Sun? Explain why. (Tables 5-1 and 5-2 might help.) ANSWER: Jupiter, because it is largest and closest to the Sun.

CHAPTER 6

MARGIN QUESTION 6-1: List three things that cause Earth’s albedo to change. ANSWER: Volcanic ash ejected by volcano eruptions; changing cloud coverage; changing amounts of air pollution; and changing amount of smoke from forest fires, among others.

MARGIN QUESTION 6-2: An increase in the amount of vegetation on Earth will have what effect on the carbon dioxide level in the atmosphere? ANSWER: Increased vegetation decreases the carbon dioxide level in the atmosphere by converting some of the CO2 (along with water) into oxygen and glucose.

MARGIN QUESTION 6-4: What happens to the Van Allen belts when Earth’s magnetic field is flipping? ANSWER: When the Van Allen belts are flipping, Earth’s magnetic field and the resulting Van Allen belts vanish, allowing more high energy particles to enter Earth’s atmosphere than occurs today.

MARGIN QUESTION 6-8: Why haven’t the ocean tides on Earth put our planet into synchronous rotation with respect to the Moon? ANSWER: Earth’s angular momentum is too great for it to have been slowed down enough to be in synchronous rotation with the Moon today.

GOT IT? 54: Can the Earth’s ozone layer, which has been depleted over decades, be naturally replenished? If so, how? ANSWER: Yes, by ultraviolet radiation from the Sun splitting normal O2 in the atmosphere into oxygen atoms, each of which combines with different O2 molecules to create a new pair of ozone, O3, molecules.

GOT IT? 55: Since infrared radiation (heat) from the Sun does not enter a greenhouse, how do greenhouses get warm? ANSWER: Visible light enters the greenhouse, is absorbed, and heats the greenhouse’s surfaces, which then radiate heat into the greenhouse, where it is trapped.

GOT IT? 56: How can we tell by looking at the Moon that it rotates? ANSWER: We always see the same side of the Moon. Since the Moon and Earth are orbiting their barycenter, if the Moon was not rotating, we would see different sides of it throughout the cycle of phases.

ANS-4

GOT IT? 57: How were the craters on the Moon formed? ANSWER: From impacts.

GOT IT? 58: When was the last year that anyone landed on the Moon? ANSWER: 1972, Apollo 17.

CHAPTER 7

MARGIN QUESTION 7-2: If Mercury were struck by a large planetesimal, why would this collision not produce a moon, as happened when Earth was struck early in its history? ANSWER: Two possible reasons the large impact on Mercury didn’t create a moon there are that the impacting body hit head on, putting debris in all directions, but not in orbit, or that the impacting body was composed of low-density material that didn’t have enough mass to cause an ejection.

MARGIN QUESTION 7-3: Why isn’t Mercury in synchronous rotation with respect to the Sun? ANSWER: Mercury is not in synchronous orbit around the Sun because the planet’s orbit is too elliptical to keep the same face toward the Sun.

MARGIN QUESTION 7-6: Why does little cratering occur on Venus today, even compared to the present low rate of cratering on our Moon? ANSWER: Venus’s thick atmosphere vaporizes virtually every object heading toward the planet’s surface, preventing cratering from occurring there today.

MARGIN QUESTION 7-10: Why do you think it is easier to stand up in a dust devil on Mars than in the same equivalent-speed event here on Earth? ANSWER: The air on Mars is less dense than the air on Earth, so the air pressure on Mars is lower and winds of equivalent speed have less impact there than on Earth.

MARGIN QUESTION 7-11: Would our Moon have to be closer or farther away to orbit in the same direction but rise in the west? ANSWER: By Kepler’s third law, our Moon would have to be closer to Earth for it to be moving so fast that, like Phobos, it would rise in the west.

GOT IT? 47: Plants on Earth convert carbon dioxide to oxygen and other substances. Give three reasons why that process will never occur on carbon dioxide-rich Venus. ANSWER: Too hot for organic molecules to form there and to interact to form plants; the surface is frequently being replaced by large-scale volcanic activity, which would wipe out all life there; there isn’t any liquid water there, which life would need to form and evolve.

GOT IT? 48: Which statement about life on Mars is correct?

  1. Rovers from Earth have observed life on the surface of Mars.
  2. Rovers from Earth have found remnants of simple life on Mars.
  3. Rovers from Earth have found remnants of advanced civilizations on Mars.
  4. Rovers from Earth have found life underground.
  5. Rovers from Earth have not found evidence of life there.

ANSWER: e

GOT IT? 49: What color does the surface of Venus have, as seen from that world, and why?

ANSWER: Yellow, because sunlight is filtered through sulfuric acid in the clouds, which primarily allows yellow light through.

GOT IT? 50: Which terrestrial planet has the coldest surface, and why? ANSWER: Nighttime on Mercury, closest planet to the Sun, because its nights are so long and there is virtually no atmosphere to hold in the heat deposited on the daytime side when that side moves into night.

CHAPTER 8

MARGIN QUESTION 8-3: What creates most of the heat inside Io that causes it to have volcanoes and geysers? ANSWER: Friction created by tidal distortion (rubbing of rock on rock) in Io as it orbits Jupiter creates the heat necessary to form volcanoes and geysers there.

MARGIN QUESTION 8-8: What two effects cause Saturn’s belt and zone system? ANSWER: Convection of Saturn’s outer layers and the planet’s rotation cause that planet to have a belt and zone system.

MARGIN QUESTION 8-11: Besides Mercury, what other world we have studied has scarps? ANSWER: The Moon, and the moons Iapetus and Rhea.

MARGIN QUESTION 8-13: Why do Uranus’s rings remain in orbit? ANSWER: Uranus’s rings are held in orbit by two shepherd moons, Cordelia and Ophelia.

MARGIN QUESTION 8-15: Is our Moon inside or outside Earth’s Roche limit? ANSWER: Our Moon is outside Earth’s Roche limit. Otherwise, the Moon would have been pulled apart and made into a ring system around Earth.

GOT IT? 45: How would descending into Jupiter differ from descending from space onto the Earth? ANSWER: Because of the pressure, temperature, and density changes as one goes into Jupiter, there are no phase transitions, meaning that there is no boundary between the gaseous atmosphere and what we would normally think of as a liquid. There would be no splashdown like one experiences when landing on the oceans on Earth.

GOT IT? 46: Why does the moon Io have active volcanoes while our Moon does not? ANSWER: Io is being heated by tidal effects from Jupiter and the other Galilean moons. This heat melts some of Io’s interior, allowing volcanic activity to occur there. Our Moon does not receive enough tidal heating from its slightly elliptical orbit around the Earth to create enough molten rock to come out through volcanoes.

ANS-5

GOT IT? 47: Is Cassini’s division in Saturn’s rings empty? Justify your answer. ANSWER: No, we can see the dust particles in it from forward-scattered light when we look at Saturn from its nighttime side.

GOT IT? 48: Jupiter’s Great Red Spot is most accurately described by which of the following?

  1. an impact site
  2. a dormant volcano
  3. an active volcano
  4. a hurricane
  5. a tornado

ANSWER: d

CHAPTER 9

MARGIN QUESTION 9-1: What objects are classified today as planets? What objects are classified as dwarf planets? ANSWER: Planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Dwarf planets are Pluto, Ceres, Eris, Haumea, and Makemake.

MARGIN QUESTION 9-3: Why do astronomers doubt that the asteroid belt was once made up of a single planet? ANSWER: The total mass of all the objects in the asteroid belt combined is much less than that of any planet.

MARGIN QUESTION 9-7: What evidence do we have that Comet Shoemaker-Levy 9 was not one solid chunk of rock surrounded by ice? ANSWER: Shoemaker-Levy 9 broke into numerous pieces under the relatively weak tidal influence of Jupiter. A solid comet would not have been so easily pulled apart.

MARGIN QUESTION 9-9: Why are stony-iron meteorites so rare compared to stony or iron meteorites? ANSWER: Stony-iron meteorites come from the boundaries between the rocky and metal parts of asteroids. These are very thin layers; hence, the meteorites from them are rare.

28. 5.0 × 1014 tons

GOT IT? 39: What are the differences between meteors, meteorites, and meteoroids? ANSWER: Meteoroids are small pieces of rock and metal debris orbiting the Sun. Meteors are meteoroids passing through our atmosphere, which heats them up and creates their tails. Meteorites are pieces of meteors that land intact on Earth.

GOT IT? 40: Where are asteroids found in the solar system? ANSWER: Asteroids are found in many places, including in the asteroid belt, in highly elliptical orbits around the Sun, in the same orbits as the planets (as Trojans), and in the Kuiper belt and Oort cloud.

GOT IT? 41: Why is Pluto no longer considered a planet? ANSWER: Unlike the “planets,” it does not have enough mass to gravitationally clear its orbit of other debris.

GOT IT? 42: Are planets the only solar system bodies with moons? If not, what other types of solar system bodies have them? ANSWER: No, moons have been discovered orbiting dwarf planets and asteroids.

GOT IT? 43: What is the typical separation between asteroids in the asteroid belt? ANSWER: About a million kilometers.

GOT IT? 44: Of the following objects, which contains the largest bodies?

  1. asteroids
  2. meteoroids
  3. moons
  4. planetary rings
  5. comets

ANSWER: a, asteroids.

CHAPTER 10

MARGIN QUESTION 10-3: Which of the Sun’s three atmospheric layers is coolest? Which is densest? ANSWER: A portion of the Sun’s chromosphere is the coolest atmospheric layer, while its photosphere is the densest part.

MARGIN QUESTION 10-8: Why does the Sun not collapse under the influence of its own enormous gravitational attraction? ANSWER: The Sun doesn’t collapse because some of the energy it generates in its core pushes outward, counteracting the inward force of gravity.

MARGIN QUESTION 10-9: Why did the growing Sun become hotter as measured from Earth if its surface temperature has remained roughly constant? ANSWER: As the Sun grew, its surface area increased. This allowed more photons to leave it each second and hence to strike the Earth, thereby heating it further.

MARGIN QUESTION 10-10: Why did the earlier neutrino detectors not detect the predicted number of neutrinos from the Sun? ANSWER: The earlier neutrino detectors were only sensitive to one type of neutrino. Since some of the Sun’s neutrinos transformed from one type to another before reaching Earth, the early detectors did not detect all of them.

5. next maximum in 2024, next minimum in 2020

16. 1400 kg/m3

17. 4.8%

18. 500 nm = visible light; 58 nm = ultraviolet; 1.9 nm = X ray

32. ≈ 25d, equator

Toolbox 10-1: 6 × 1017 helium atoms; 2.25 × 1014 J

GOT IT? 31: Does the Sun shine by burning gas, like methane? Justify your answer. ANSWER: No. If it did so, it would have only enough fuel to shine for about 5000 years.

GOT IT? 32: What causes sunspots? ANSWER: Sunspots are regions of low-density gas caused by magnetic fields piercing the Sun’s photosphere.

GOT IT? 33: Does the Sun rotate? Justify your answer. ANSWER: Yes, as seen by the motion of its sunspots.

GOT IT? 34: Does the Sun have a solid or liquid surface, like the Earth? ANSWER: No, the Sun is gas all the way to its center.

ANS-6

GOT IT? 35: Does the Sun have an atmosphere? Explain your reasoning. ANSWER: Yes. We can see the solar atmosphere during solar eclipses.

CHAPTER 11

MARGIN QUESTION 11-1: For which object—the Moon, Mars, or the star Sirius—is the parallax angle smallest as seen from Earth? ANSWER: The parallax angle for Sirius is smaller than that of Mars and our Moon because Sirius is farthest away of the three.

MARGIN QUESTION 11-8: A main-sequence star of which spectral type—F5, A8, or K0—is largest? ANSWER: Hotter main-sequence stars are larger than cooler main-sequence stars. Therefore, the A8 main-sequence stars are larger than F5 or K0 main-sequence stars.

MARGIN QUESTION 11-10: For what types of stars does the mass-luminosity relationship not apply? ANSWER: The mass-luminosity relationship does not apply for any type of star other than those on the main sequence.

MARGIN QUESTION 11-11: How do the spectral lines of a spectroscopic binary change as observed from Earth? ANSWER: The spectral lines of the two stars in a spectroscopic binary simultaneously Doppler shift in opposite directions, as seen from Earth unless the two stars orbit perpendicular to our line of sight. In the latter case, the spectral lines do not change at all.

15. 25 times brighter

29. 4.3 pc

31.

32. 1585 times brighter

35.

36. ≈ 10,000−15,000 K

Toolbox 11-1: ~4.85 × 10−6 pc; 3.26 × 10−2″; ~2.6 pc or ~8.47 ly; ~237 pc; ~4.22 × 10−3″

Toolbox 11-2: 40 times; 631 times; 5.5

Toolbox 11-3: ~11.4 pc, ~37.1 ly; +1.74 (Fomalhaut); −1.44 (Sirius A)

Toolbox 11-4: We can ignore M2 (planet’s mass) as tiny compared to the Sun’s mass, M1. Since the Sun’s mass in these units is 1, the equation becomes 1 = a3/P2, or P2 = a3; ~1.56 M⊙; 73 years

GOT IT? 47: Of all the spectral classes of stars (O, B, A, F, G, K, M), which is least common? ANSWER: O, the highest mass stars.

GOT IT? 48: Comparing them side by side, which of the following most accurately describes the Sun’s luminosity relative to other stars?

  1. It is among the most luminous of stars.
  2. It is of average luminosity.
  3. It is less luminous than the average luminosity of stars.
  4. It is among the least luminous of all stars.
  5. All stars are equally luminous.

ANSWER: b

GOT IT? 49: What colors are the hottest and coolest stars? ANSWER: Hottest: violet peak in blackbody spectrum, blue in appearance as our eyes are less sensitive to violet than to blue. Coolest: red.

GOT IT? 50: If a star is a red giant, how does its surface temperature compare to that of the Sun? ANSWER: Red giants are cooler stars than the Sun.

CHAPTER 12

MARGIN QUESTION 12-5: How are T Tauri stars different from main-sequence stars, like the Sun? ANSWER: Unlike solar-mass main-sequence stars, T Tauri stars are emitting large amounts of gas and dust, and they are changing brightness significantly.

MARGIN QUESTION 12-6: Which star arrives on the main sequence first, one that is 0.5 M⊙ or one that is 2 M⊙? ANSWER: Higher-mass stars arrive on the main sequence more quickly than lower-mass stars, so a 2-M⊙ star would arrive there more rapidly than a 0.5-M⊙ star.

MARGIN QUESTION 12-10: Will the Sun undergo a helium flash? Why or why not? ANSWER: The Sun will undergo a helium flash because its core will remain degenerate as it expands into the giant phase.

MARGIN QUESTION 12-11: How many helium atoms does it take to make one oxygen atom? ANSWER: It takes four helium atoms to make one oxygen atom.

MARGIN QUESTION 12-12: How do astronomers observe that Cepheids are changing size? ANSWER: By observing their cyclic changes in brightness and Doppler shift, which indicate that Cepheids are changing size.

26. 2000

27. 200 times longer

GOT IT? 40: Which “last” longer, high-mass or low-mass stars? Explain your reasoning. ANSWER: Low-mass stars last longer because even though they have less mass than higher-mass stars, they fuse much more slowly than more massive ones. Hence, higher-mass stars use up all their fuel faster than lower-mass ones.

GOT IT? 41: Is Jupiter a “failed” star? Why or why not? ANSWER: No, Jupiter would have to be 75 times more massive than it is to fuse hydrogen in its core like main-sequence stars do. Jupiter is not even close to being a star.

GOT IT? 42: Which star, if either, is at the center of a binary star system? Explain your reasoning. ANSWER: Neither, because they both orbit their common center of mass, which is between them.

GOT IT? 43: Did the Sun form shortly after the universe came into existence? Explain your answer. ANSWER: No, the Sun formed from the debris ejected by earlier generations of stars. The Sun formed 9.2 billion years after the universe came into existence.

GOT IT? 44: Are stars still forming today? Explain your answer. ANSWER: Yes, as we see in such places as the Orion molecular cloud.

ANS-7

CHAPTER 13

MARGIN QUESTION 13-10: If the lighthouse model of pulsars is correct, do we see all the nearby pulsars? Why or why not? ANSWER: No. We do not see the nearby pulsars whose beams do not sweep in our direction.

MARGIN QUESTION 13-12: Why do glitches change the rotation rates of pulsars? ANSWER: Glitches change the rotation rates of a pulsar by redistributing its mass. To conserve its angular momentum, the pulsar’s rotation rate must also change.

28. about 7470 years ago

GOT IT? 41: How will the Sun end its “life cycle”? Will there be anything left of it after fusion in it ceases? ANSWER: The Sun will shed its outer layers as a planetary nebula, a relatively wimpy explosion. After this occurs, its core will remain as a white dwarf.

GOT IT? 42: Where was the iron in your blood formed? ANSWER: The iron was formed in the outer layers of massive stars as they exploded as supernovae.

GOT IT? 43: What are cosmic rays? ANSWER: Cosmic rays are high-speed particles, mostly protons and electrons, but also nuclei of other elements.

GOT IT? 44: When a star leaves the main sequence, where is the energy being generated that pushes its outer layers outward? ANSWER: Stars expand because of fusion occurring in shells just outside their cores.

CHAPTER 14

MARGIN QUESTION 14-1: How much mass, m, would have to be destroyed to create an amount of energy, E? ANSWER: By Einstein’s equation E = mc2, to create energy, E, requires mass E/c2.

MARGIN QUESTION 14-6: How would a 1-M⊙ black hole 1 AU from Earth affect our planet? ANSWER: A black hole with the same mass as the Sun at the same distance from Earth as the Sun would have the same gravitational attraction on Earth as the Sun. The black hole would have no other effects on Earth.

MARGIN QUESTION 14-11: Why does at least one particle in the Hawking process always fall into the black hole? ANSWER: At least one particle must enter the black hole in the Hawking process to conserve the linear momentum of the pair of particles that the process creates.

11. 8.9 km, 89 km

15. 10 m

Toolbox 14-1: ~1.0 × 1012 kg; 9 × 109 km; ~2130 M⊙

GOT IT? 21: Are black holes completely black (that is, emitting or scattering no light) and are they holes (in the sense of being completely empty)? Explain. ANSWER: No, black holes are not completely black because they emit Hawking radiation from just outside their event horizons.

GOT IT? 22: How does the mass of a young black hole compare to the masses of the neutron star from which it just formed? ANSWER: The mass of a young black hole and the neutron star from which it just formed are identical. Nor are black holes empty, as they contain the mass that has entered them.

GOT IT? 23: Are black holes giant vacuum cleaners in space that are eventually going to suck up everything in the universe? Explain your reasoning. ANSWER: No, black holes can only attract distant objects with the same force as any other object containing the same amount of mass as the black hole.

GOT IT? 24: Which of the following most accurately describes the fate of a black hole?

  1. It will remain unchanged as long as the universe exists.
  2. It will eventually suck in everything else in the universe.
  3. It will evaporate.
  4. It will eventually start shining due to fusion in its core.
  5. It will eventually start pulsating by periodically growing larger and then smaller.

ANSWER: c, due to Hawking radiation.

CHAPTER 15

MARGIN QUESTION 15-1: Referring to Figure 15-4, how would the brightness of a Cepheid variable with peak luminosity of 1000 L⊙ change if it were observed every 5 days? ANSWER: A Cepheid variable with peak luminosity of 1000 L⊙ has a period of 5 days, so if it were observed every 5 days, its luminosity would appear unchanged from one observation to the next.

MARGIN QUESTION 15-3: The presence of supernova remnants at the center of our Galaxy implies what other activity is occurring in that region? ANSWER: Since stars that become supernovae, and the remnants they leave, have short lives in astronomical terms, supernova remnants at the center of our Galaxy imply that new star formation must be occurring there.

13. 20 times

18. about once every 3750 years

Toolbox 15-1: 1 × 105 pc; ~6.92; M = 9.83, m − M = 5

GOT IT? 28: Does the Sun move in the Milky Way? If so, describe its motion. ANSWER: Yes, the Sun moves in a snake-like path above and below the plane of the Galaxy, while making a nearly circular orbit around the center of the Galaxy.

GOT IT? 29: About what fraction (or percent) of the matter in the Milky Way can astronomers see with current technology? ANSWER: 10%, the rest is either “dark matter” or is located behind too many interstellar gas clouds for us to see through with present technology.

ANS-8

GOT IT? 30: Where in the Galaxy is the solar system located?

  1. in the nucleus
  2. in the halo
  3. in a spiral arm
  4. between two spiral arms
  5. in the central bulge

ANSWER: d

GOT IT? 31: Is the Milky Way in close contact with other galaxies? Explain. ANSWER: Yes, astronomers have observed the Milky Way cannibalizing other galaxies.

CHAPTER 16

MARGIN QUESTION 16-1: Comparing it to the galaxies in Figures 16-1 or 16-3, what is the spiral classification of M74, shown in Figure 16-4b? ANSWER: M74 is an Sc galaxy.

MARGIN QUESTION 16-5: The Large Magellanic Cloud is visible to the naked eye in the southern hemisphere. From Figure 16-13a, what terrestrial things do you think it could be mistaken for? ANSWER: The Large Magellanic Cloud is sometimes mistaken for part of the Milky Way or for a cloud in the night sky.

MARGIN QUESTION 16-10: If the dark matter in Figure 16-29d suddenly vanished, what would we see of the clusters of galaxies behind it? ANSWER: If the dark matter between us and a cluster of galaxies suddenly vanished, the shapes we would see of many of those galaxies would change (since the dark matter lenses the light from them).

MARGIN QUESTION 16-11: If the Hubble constant were 3 times its present value, how much slower or faster would superclusters be moving apart? ANSWER: If the Hubble constant were 3 times its present value, superclusters would be moving apart 3 times faster.

MARGIN QUESTION 16-12: In what way are telescopes time machines? ANSWER: The light entering telescopes travels at a finite speed, so what we see through them are events and objects as they were some time in the past.

21. 306 Mpc

22. 7200 km/s

Toolbox 16-1: v = 1.74 × 105 km/s; d = 3.0 Mpc; slower

GOT IT? 36: What shape(s) do galaxies have? ANSWER: Galaxies are spiral, spiral with a bar in the middle (barred spirals), oval and spherical (ellipticals), and irregularly shaped (irregulars).

GOT IT? 37: With most galaxies in the universe moving away from us equally in all directions, are we at the center of the universe? Explain. ANSWER: No, because as the universe expands, every place in it experiences the same recession of galaxies from it as we do.

GOT IT? 38: Are galaxies uniformly distributed throughout the universe? Explain your answer. ANSWER: No, galaxies are distributed on the boundaries surrounding great voids containing very few galaxies and little intergalactic gas. Visualize the galaxies as located where the fiber is in a sponge.

GOT IT? 39: Is the Milky Way Galaxy moving through the universe? Justify your answer. ANSWER: Yes, the Milky Way is orbiting with other galaxies in the Local Group. Our Galaxy will collide with the Andromeda Galaxy. The two galaxies are now moving toward each other.

CHAPTER 17

MARGIN QUESTION 17-1: How are the spectra of quasars different from the spectra of stars? ANSWER: Quasar spectra have emission lines, whereas stellar spectra have absorption lines.

MARGIN QUESTION 17-3: Is the Milky Way a peculiar galaxy? Why or why not? ANSWER: No. There is no emission from the Milky Way that gives the impression that it is exploding.

MARGIN QUESTION 17-5: What other astronomical objects have been observed after their light passed through a gravitational lens? ANSWER: Besides quasars, astronomers have observed galaxies that have been gravitationally lensed.

GOT IT? 22: What is a quasar? ANSWER: A quasar is a supermassive black hole in the center of a galaxy. The black hole is surrounded by gas and dust that is spiraling inward. Some of this gas and dust is heated so much that it squirts out in two jets, which are the quasar.

GOT IT? 23: What does a quasar look like through an optical telescope? ANSWER: As seen through an optical telescope, a quasar looks like a star.

CHAPTER 18

MARGIN QUESTION 18-2: If the universe were twice as old as it is now, how would the Hubble constant compare to the value it has today? ANSWER: Since the inverse of the Hubble constant is its age, if the universe were twice as old, the Hubble constant would be half as large as it is today.

MARGIN QUESTION 18-11: If dark matter did not exist, would any gravitational lensing occur in the universe? ANSWER: Even without dark matter, gravitational lensing would occur due to the gravitational effects of superclusters of galaxies, clusters of galaxies, individual galaxies, and other mass distributions.

Toolbox 18-1: 27.6 × 109 years; 6.9 × 109 years

GOT IT? 21: Which of the following best describes the overall motion in the universe?

  1. The universe is expanding faster and faster.
  2. The universe is expanding but more slowly all the time.
  3. The universe is expanding at a constant rate.
  4. The universe is collapsing at a constant rate.
  5. The universe is collapsing faster and faster.

ANSWER: a

GOT IT? 22: What is the weakest force in nature? ANSWER: Gravity (the strong force is the strongest).

GOT IT? 23: What do astronomers believe to be the fate of the universe? ANSWER: The universe will continue to expand forever and eventually all the stars will use up all their fuel.

GOT IT? 24: Has the universe existed forever? If not, when did it come into existence? ANSWER: No, the universe formed about 13.8 billion years ago.

CHAPTER 19

MARGIN QUESTION 19-2: Synchronous rotation occurred in what other situation that we explored earlier in this book? ANSWER: Examples of synchronous rotation that we have considered are the orbits of most of the moons in the solar system around their respective planets, including our Moon orbiting Earth, and the synchronous rotation of Pluto with respect to its moon Charon.

MARGIN QUESTION 19-6: What negative consequence could arise from an alien civilization discovering a Pioneer spacecraft? ANSWER: If an alien civilization discovered a Pioneer spacecraft, it could trace the spacecraft back to Earth. Visits from this alien civilization might introduce diseases for which we have no defense, or they might lead to interplanetary war if we couldn’t get along with our new neighbors.

GOT IT? 15: Have astronomers found concrete evidence for the existence of extraterrestrial life? If so, please describe these discoveries. ANSWER: No incontrovertible evidence for extraterrestrial life has been found.

GOT IT? 16: Have scientists found credible evidence that extraterrestrials have visited Earth? Explain your answer. ANSWER: No credible (that is, physical) evidence for visits to Earth by extraterrestrials has been found.

GOT IT? 17: Has life been created in the laboratory? If so, when and where? ANSWER: Scientists have not created life in the laboratory.

APPENDIX A

3.141 × 109, 3.1831 × 10−9;

27182820000, 0.0000000000367879

APPENDIX B

580 K, 55 km, temperature decreases with height; +8, +10, September 1, September 6; 8L⊙, 6000 K

APPENDIX D

5527°C, 9980°F; −270°C, −454°F