# 21st Century Astronomy The Solar System Fifth Edition By Kay -Palen -Test Bank

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###### 21st Century Astronomy The Solar System Fifth Edition By Kay -Palen -Test Bank

1. The speed of light was first determined by which scientist?
1. Galileo
2. Newton
3. Kepler
4. Rømer
5. Einstein

ANS: D         DIF: Easy              REF: Section 5.1 MSC: Remembering OBJ: Summarize the electromagnetic properties of light.

1. The speed of light in a vacuum is
1. 300,000 m/s.
2. 300,000 mph.
3. 300,000 km/s.
4. 300,000,000 mph.

ANS: C         DIF: Easy              REF: Section 5.1 MSC: Remembering OBJ: Summarize the electromagnetic properties of light.

1. What is the difference between visible light and X-rays?
1. Speed; X-rays go faster than visible light.
2. Speed; X-rays go slower than visible light.
3. Wavelength; X-rays have a shorter wavelength than visible light.
4. Wavelength; X-rays have a longer wavelength than visible light.
5. X-rays are made up of particles, whereas visible light is made up of waves.

ANS: C         DIF: Easy              REF: Section 5.1 MSC: Understanding OBJ: Summarize the electromagnetic properties of light.

1. How does the speed of light traveling through a medium (such as air or glass) compare to the speed of light in a vacuum?
1. It is the same as the speed of light in a vacuum.
2. It is always less than the speed of light in a vacuum.
3. It is always greater than the speed of light in a vacuum.
4. Sometimes it is greater than the speed of light in a vacuum and sometimes it is less, depending on the medium.
5. Light can’t travel through a medium; it only can go through a vacuum.

ANS: B         DIF: Easy              REF: Section 5.1 MSC: Remembering OBJ: Summarize the electromagnetic properties of light.

1. A light-year is a unit that is used to measure

ANS: E         DIF: Easy              REF: Section 5.1 MSC: Remembering OBJ: Summarize the electromagnetic properties of light.

1. Which formula denotes how the speed of light is related to its wavelength and frequency?
1. c = λf
2. c = λ/f
3. c = f
4. c = 1/λf
5. There is no relationship between wavelength and frequency.

ANS: A         DIF: Easy              REF: Section 5.1 MSC: Remembering OBJ: Relate color, wavelength, and energy of photons.

1. The color of visible light is determined by its
1. distance from you.

ANS: B         DIF: Easy              REF: Section 5.1 MSC: Remembering OBJ: Relate color, wavelength, and energy of photons.

1. How do the wavelength and frequency of red light compare to the wavelength and frequency of blue light?
1. Red light has a longer wavelength and higher frequency than blue light.
2. Red light has a longer wavelength and lower frequency than blue light.
3. Red light has a shorter wavelength and higher frequency than blue light.
4. Red light has a shorter wavelength and lower frequency than blue light.

ANS: B         DIF: Easy              REF: Section 5.1 MSC: Remembering OBJ: Relate color, wavelength, and energy of photons.

1. What wavelengths of light can the human eye see?
1. 8 µm to 7.5 µm
2. 8 nm to 7.5 nm
3. 380 cm to 750 cm
4. 380 nm to 750 nm
5. 8 m to 7.5 m

ANS: D         DIF: Easy              REF: Section 5.1 MSC: Remembering OBJ: List the names and wavelength ranges of the electromagnetic spectrum.

1. What does amplitude reveal about light?
1. wavelength
2. frequency
3. speed
4. brightness

ANS: D         DIF: Easy              REF: Section 5.1 MSC: Remembering OBJ: Define the bold-faced vocabulary terms within the chapter.

1. The unit Hertz is a measure of what quantity?
1. wavelength
2. frequency
3. speed
4. brightness

ANS: B         DIF: Easy              REF: Section 5.1 MSC: Remembering OBJ: Define the bold-faced vocabulary terms within the chapter.

1. When talking about a wave, what does the term “medium” refer to?
1. the size of an object
2. the substance through which the wave travels
3. the brightness level
4. the vacuum

ANS: B         DIF: Easy              REF: Section 5.1 MSC: Remembering OBJ: Define the bold-faced vocabulary terms within the chapter.

1. A nanometer is a measure of which quantity?
1. wavelength
2. frequency
3. speed
4. brightness

ANS: A         DIF: Easy              REF: Section 5.1 MSC: Remembering OBJ: Define the bold-faced vocabulary terms within the chapter.

1. Which of the following photons carries the smallest amount of energy?
1. a blue photon of the visible spectrum, whose wavelength is 450 nm
2. an infrared photon, whose wavelength is 105 m
3. a red photon in the visible spectrum, whose wavelength is 700 nm
4. a microwave photon, whose wavelength is 102 m
5. an ultraviolet photon, whose wavelength is 300 nm

ANS: D         DIF: Medium        REF: Section 5.1 MSC: Applying OBJ: Relate color, wavelength, and energy of photons.

1. Einstein showed that the _________ could be explained if photons carried quantized amounts of energy.
1. warping of space and time
2. Heisenberg uncertainty principle
3. photoelectric effect
4. theory of special relativity
5. Bohr model of the atom

ANS: C         DIF: Medium        REF: Section 5.1 MSC: Remembering OBJ: Explain how and when light acts like a wave, and when it acts like a particle.

1. Light has aspects of
1. only a wave.
2. only a particle.
3. both a particle and a wave.
4. neither a particle nor a wave.

ANS: C         DIF: Medium        REF: Section 5.1 MSC: Remembering OBJ: Explain how and when light acts like a wave, and when it acts like a particle.

1. Saying that something is quantized means that it
1. is a wave.
2. is a particle.
3. travels at the speed of light.
4. can only have discrete quantities.
5. is smaller than an atom.

ANS: D         DIF: Medium        REF: Section 5.1 MSC: Remembering OBJ: Explain how and when light acts like a wave, and when it acts like a particle.

1. A red photon has a wavelength of 650 nm. An ultraviolet photon has a wavelength of 250 nm. The energy of an ultraviolet photon is _________ the energy of a red photon.
1. 6 times larger than
2. 8 times larger than
3. 6 times smaller than
4. 8 times smaller than
5. the same as

ANS: A         DIF: Medium        REF: Section 5.1 MSC: Applying OBJ: Relate color, wavelength, and energy of photons.

1. Light with a wavelength of 600 nm has a frequency of
1. 2 × 105 Hz
2. 5 × 107 Hz
3. 2 × 1010 Hz
4. 5 × 1012 Hz
5. 5 × 1014 Hz

ANS: E         DIF: Medium        REF: Section 5.1 MSC: Applying OBJ: Relate color, wavelength, and energy of photons.

1. Which of the following lists different types of electromagnetic radiation in order from the shortest wavelength to the longest wavelength?
1. radio waves, infrared, visible, ultraviolet, X-rays
2. gamma rays, ultraviolet, visible, infrared, radio waves
3. gamma rays, X-rays, infrared, visible, ultraviolet
4. X-rays, infrared, visible, ultraviolet, radio waves
5. radio waves, ultraviolet, visible, infrared, gamma rays

ANS: B         DIF: Medium        REF: Section 5.1 MSC: Remembering OBJ: List the names and wavelength ranges of the electromagnetic spectrum.

1. As wavelength increases, the energy of a photon _________ and its frequency _________.
1. increases; decreases
2. increases; increases
3. decreases; decreases
4. decreases; increases

ANS: C         DIF: Medium        REF: Section 5.1 MSC: Remembering OBJ: Relate color, wavelength, and energy of photons.

1. If the frequency of a beam of light were to increase, its period would _________ and its wavelength would _________.
1. decrease; increase
2. increase; decrease
3. increase; increase
4. decrease; decrease
5. stay the same; stay the same

ANS: D         DIF: Medium        REF: Section 5.1 MSC: Applying OBJ: Relate color, wavelength, and energy of photons.

1. The fact that the speed of light is constant as it travels through a vacuum means that
1. photons with longer wavelengths have lower frequencies.
2. radio wave photons have shorter wavelengths than gamma ray photons.
3. X-rays can be transmitted through the atmosphere around the world.
4. ultraviolet photons have less energy than visible photons.

ANS: A         DIF: Medium        REF: Section 5.1 MSC: Understanding OBJ: Summarize the electromagnetic properties of light.

1. If the wavelength of a beam of light were to double, how would that affect its frequency?
1. The frequency would be four times higher.
2. The frequency would be two times higher.
3. The frequency would be two times lower.
4. The frequency would be four times lower.
5. There is no relationship between wavelength and frequency.

ANS: C         DIF: Medium        REF: Section 5.1 MSC: Applying OBJ: Relate color, wavelength, and energy of photons.

1. If the Sun instantaneously stopped giving off light, what would happen on the day-side of Earth?
1. It would immediately get dark.
2. It would get dark 8.3 minutes later.
3. It would get dark 27 minutes later.
4. It would get dark 1 hour later.
5. It would get dark 24 hours later.

ANS: B         DIF: Medium        REF: Section 5.1 MSC: Applying OBJ: Summarize the electromagnetic properties of light.

1. When an electron moves from a higher energy level in an atom to a lower energy level,
1. the atom is ionized.
2. a continuous spectrum is emitted.
3. a photon is emitted.
4. a photon is absorbed.
5. the electron loses mass.

ANS: C         DIF: Easy              REF: Section 5.2 MSC: Applying OBJ: Relate spectral features to changes in energy state of an atom.

1. If you observe an isolated hot cloud of gas, you will see
1. an absorption spectrum.
2. a continuous spectrum.
3. an emission spectrum.
4. a rainbow spectrum.
5. a dark spectrum.

ANS: C         DIF: Easy              REF: Section 5.2 MSC: Applying OBJ: Illustrate the processes of atomic absorption and emission of light.

1. Which of these objects would emit an absorption spectrum?
1. an incandescent lightbulb
2. a fluorescent lightbulb
3. an isolated hot gas cloud
4. a hot, solid object
5. a thin, cool gas cloud that lies in front of a hotter blackbody

ANS: E         DIF: Easy              REF: Section 5.2 MSC: Applying OBJ: Illustrate the processes of atomic absorption and emission of light.

1. If you observe a star, you will see
1. an absorption spectrum.
2. a continuous spectrum.
3. an emission spectrum.
4. a rainbow spectrum.
5. a dark spectrum.

ANS: A         DIF: Easy              REF: Section 5.2 MSC: Applying OBJ: Illustrate the processes of atomic absorption and emission of light.

1. In the energy level diagram shown in the figure below, the electron is excited to the E4 energy level. If the electron transitions to an energy level giving off a photon, which level would produce a photon with the largest energy?
1. E1
2. E2
3. E3
4. E4
5. E5

ANS: A         DIF: Easy              REF: Section 5.2 MSC: Remembering OBJ: Relate spectral features to changes in energy state of an atom.

1. In the energy level diagram shown in the figure below, the electron is excited to the E4 energy level. If the electron transitions to an energy level giving off a photon, which level would produce a photon with the largest frequency?
1. E1
2. E2
3. E3
4. E4
5. E5

ANS: A         DIF: Easy              REF: Section 5.2 MSC: Remembering OBJ: Relate spectral features to changes in energy state of an atom.

1. In the energy level diagram shown in the figure below, the electron is excited to the E4 energy level. If the electron transitions to an energy level giving off a photon, which level would produce a photon with the largest wavelength?
1. E1
2. E2
3. E3
4. E4
5. E5

ANS: C         DIF: Easy              REF: Section 5.2 MSC: Remembering OBJ: Relate spectral features to changes in energy state of an atom.

1. In the energy level diagram shown in the figure below, the electron is excited to the E2 energy level. If the atom absorbs a photon with the exact frequency to move the electron to another energy level, which energy level would correspond to the largest frequency difference?
1. E1
2. E2
3. E3
4. E4
5. E5

ANS: E         DIF: Easy              REF: Section 5.2 MSC: Understanding OBJ: Relate spectral features to changes in energy state of an atom.

1. In the energy level diagram shown in the figure below, the electron is excited to the E2 energy level. If the atom absorbs a photon with the exact wavelength to move the electron to another energy level, which energy level would correspond to the largest wavelength difference?
1. E1
2. E2
3. E3
4. E4
5. E5

ANS: E         DIF: Easy              REF: Section 5.2 MSC: Understanding OBJ: Relate spectral features to changes in energy state of an atom.

1. Astronomers measure the amount of various elements in other stars and most commonly compare them to which of the following when studying the composition of a star?
1. solar abundance
2. big bang abundance
3. terrestrial abundance
4. water

ANS: A         DIF: Easy              REF: Section 5.2 MSC: Remembering OBJ: Define the bold-faced vocabulary terms within the chapter.

1. In the figure below, you see a stellar spectrum. The dip in the data near 650 nm corresponds most closely with which of the following?
1. sodium emission
2. sodium absorption
3. hydrogen emission
4. hydrogen absorption
5. iron absorption

ANS: D         DIF: Medium        REF: Section 5.2 MSC: Understanding OBJ: Relate spectral features to changes in energy state of an atom.

1. Why is a neutral iron atom a different element than a neutral carbon atom?
1. A carbon atom has fewer neutrons in its nucleus than an iron atom.
2. An iron atom has more protons in its nucleus than a carbon atom.
3. An iron atom has more electrons than a carbon atom.
4. A carbon atom is bigger than an iron atom.

ANS: B         DIF: Medium        REF: Section 5.2. MSC: Understanding OBJ: Relate spectral features to changes in energy state of an atom

1. In the quantum mechanical view of the atom, electrons are often depicted as
1. a cloud that is centered on the nucleus.
2. a particle orbiting the nucleus.
3. free to orbit at any distance from the nucleus.
4. a particle inside the nucleus.

ANS: A         DIF: Medium        REF: Section 5.2 MSC: Understanding OBJ: Illustrate the processes of atomic absorption and emission of light.

1. The n = 5 electronic energy level in a hydrogen atom is 1.5 × 1019 J higher than the n = 3 level. If an electron moves from the n = 5 level to the n = 3 level, then a photon of wavelength
1. 3 nm, which is in the ultraviolet region, is emitted.
2. 3 nm, which is in the ultraviolet region, is absorbed.
3. 1,300 nm, which is in the infrared region, is absorbed.
4. 1,300 nm, which is in the infrared region, is emitted.
5. No light will be absorbed or emitted.

ANS: D         DIF: Difficult       REF: Section 5.2 MSC: Applying OBJ: Relate spectral features to changes in energy state of an atom.

1. The Doppler shift can be used to determine the _________ of an object.
1. energy
2. temperature
4. color
5. three-dimensional velocity

ANS: C         DIF: Easy              REF: Section 5.3 MSC: Remembering OBJ: Explain why radial motion produces a Doppler shift.

1. A spaceship is traveling toward Earth while giving off a constant radio signal with a wavelength of 1 meter (m). What will the signal look like to people on Earth?
1. a signal with a wavelength less than 1 m
2. a signal with a wavelength more than 1 m
3. a signal moving faster than the speed of light
4. a signal moving slower than the speed of light
5. a signal with a wavelength of 1 m, moving the normal speed of light

ANS: A         DIF: Easy              REF: Section 5.3 MSC: Applying OBJ: Explain why radial motion produces a Doppler shift.

1. Which of these stars would have the biggest redshift?
1. a star moving at low speed toward you
2. a star moving at high speed toward you
3. a star moving at low speed away from you
4. a star moving at high speed away from you
5. a star that is not moving away from you or toward you

ANS: D         DIF: Easy              REF: Section 5.3 MSC: Remembering OBJ: Explain why radial motion produces a Doppler shift.

1. A spaceship is traveling from planet B on the left, toward planet A on the right. The spaceship is traveling at a speed of 15,000 km/s to the left while it sends out a signal with a wavelength of 4 m. If astronomers living on planets A and B measure the radio waves coming from the spaceship, what wavelengths will they measure?
1. Planet A measures 6 m, and planet B measures 2 m.
2. Planet A measures 2 m, and planet B measures 6 m.
3. Planet A measures 4.2 m, and planet B measures 3.8 m.
4. Planet A measures 3.8 m, and planet B measures 4.2 m.
5. Both Planet A and planet B measure 4 m.

ANS: D         DIF: Difficult       REF: Section 5.3 MSC: Applying OBJ: Explain why radial motion produces a Doppler shift.

1. What does it mean to say that an object is in thermal equilibrium?
1. It isn’t absorbing any energy.
2. It isn’t radiating any energy.
3. It is radiating more energy than it is absorbing.
4. It is absorbing more energy than it is radiating.
5. It is absorbing the same amount of energy that it is radiating.

ANS: E         DIF: Easy              REF: Section 5.4 MSC: Remembering OBJ: Summarize what it means for a system to be in equilibrium.

1. The Kelvin temperature scale is used in astronomy because
1. at 0 K an object has absolutely zero energy.
2. water freezes at 0 K.
3. water boils at 100 K.
4. hydrogen freezes at 0 K.
5. the highest temperature possible is 1000 K.

ANS: A         DIF: Easy              REF: Section 5.4 MSC: Remembering OBJ: Relate temperature to the rate of thermal motions.

1. You observe the spectrum of two stars. Star A has an emission line from a known element at 600 nm. Star B has emission lines from the same atom, but the emission line is occurring at 650 nm. One possible explanation for this observation is: that star A is
1. cooler than star B.
2. farther away from us than star B.
3. moving toward us faster than star B.
4. made of different elements than star B.
5. larger than star B.

ANS: C         DIF: Easy              REF: Section 5.3 MSC: Applying OBJ: Explain why radial motion produces a Doppler shift.

1. In the figure below, which blackbody spectrum corresponds to the object with the highest temperature?
1. A
2. B
3. C
4. D
5. E

ANS: A         DIF: Easy              REF: Section 5.4 MSC: Understanding OBJ: Characterize how blackbody spectra describe the luminosity, temperature, and color of an object.

1. In the figure below, which blackbody spectrum corresponds to the object that would appear the most red to the human eye?
1. A
2. B
3. C
4. D
5. E

ANS: C         DIF: Easy              REF: Section 5.4 MSC: Understanding OBJ: Characterize how blackbody spectra describe the luminosity, temperature, and color of an object.

1. In the figure below, which blackbody spectrum corresponds to the object that would appear white to the human eye?
1. A
2. B
3. C
4. D
5. E

ANS: A         DIF: Easy              REF: Section 5.4 MSC: Understanding OBJ: Characterize how blackbody spectra describe the luminosity, temperature, and color of an object.

1. As a blackbody’s temperature increases, it also becomes _________ and _________.
1. more luminous; redder
2. more luminous; bluer
3. less luminous; redder
4. less luminous; bluer
5. more luminous; stays the same color

ANS: B         DIF: Medium        REF: Section 5.4 MSC: Remembering OBJ: Characterize how blackbody spectra describe the luminosity, temperature, and color of an object.

1. Compare two blackbody objects, one at 200 K and one at 400 K. How much larger is the flux from the 400 K object compared to the flux from the 200 K object?
1. 2 times larger
2. 4 times larger
3. 8 times larger
4. 16 times larger
5. They have the same flux.

ANS: D         DIF: Medium        REF: Section 5.4 MSC: Applying OBJ: Characterize how blackbody spectra describe the luminosity, temperature, and color of an object.

1. At what temperature does water freeze?
1. 0 K
2. 32 K
3. 100 K
4. 273 K
5. 373 K

ANS: D         DIF: Medium        REF: Section 5.4 MSC: Remembering OBJ: Relate temperature to the rate of thermal motions.

1. You observe a red star and a blue star and are able to determine that they are the same size. Which star has a higher surface temperature, and which star is more luminous?
1. The red star has a higher surface temperature and more luminous.
2. The red star has a higher surface temperature, and the blue star is more luminous.
3. The blue star has a higher surface temperature and more luminous.
4. The blue star has a higher surface temperature, and the red star is more luminous.
5. They have the same luminosities and temperatures.

ANS: C         DIF: Medium        REF: Section 5.4 MSC: Remembering OBJ: Illustrate the relationship between flux and luminosity.

1. At what peak wavelength does your body radiate the most given that your temperature is approximately that of Earth, which is 300 K?
1. 105 m
2. 103 m
3. 102 m
4. 10 m
5. 1,000 m

ANS: A         DIF: Medium        REF: Section 5.4 MSC: Applying OBJ: Characterize how blackbody spectra describe the luminosity, temperature, and color of an object.

1. Why do some stars in the sky appear blue, whereas other stars appear red?
1. The red stars have higher surface temperatures than the blue stars.
2. The blue stars have higher surface temperatures than the red stars.
3. The blue stars are closer to us than the red stars.
4. The red stars are closer to us than the blue stars.
5. The blue stars are moving toward us, while red stars are moving away from us.

ANS: B         DIF: Medium        REF: Section 5.4 MSC: Applying OBJ: Characterize how blackbody spectra describe the luminosity, temperature, and color of an object.

1. Consider an incandescent lightbulb. If you wanted to turn a 10-W lightbulb into a 100-W lightbulb, how would you change the temperature of the filament inside the bulb?
1. Raise its temperature by a factor of 3.2.
2. Raise its temperature by a factor of 1.8.
3. Raise its temperature by a factor of 10.
4. Lower its temperature by a factor of 2.6.
5. Lower its temperature by a factor of 5.4.

ANS: B         DIF: Difficult       REF: Section 5.4 MSC: Applying OBJ: Characterize how blackbody spectra describe the luminosity, temperature, and color of an object.

1. Star A and star B appear equally bright in the sky. Star A is twice as far away from Earth as star B. How do the luminosities of stars A and B compare?
1. Star A is twice as luminous as star B.
2. Star B is twice as luminous as star A.
3. Star A is four times as luminous as star B.
4. Star B is four times as luminous as star A.
5. Stars A and B have the same luminosity.

ANS: C         DIF: Medium        REF: Section 5.5 MSC: Applying OBJ: Use the inverse square law to relate luminosity, brightness, and distance.

1. Star C and star D have the same luminosity. Star C is twice as far away from Earth as star D. How do the brightnesses of stars C and D compare?
1. Star C appears four times as bright as star D.
2. Star C appears twice as bright as star D.
3. Star D appears twice as bright as star C.
4. Star D appears four times as bright as star C.
5. Stars C and D appear equally bright.

ANS: D         DIF: Medium        REF: Section 5.5 MSC: Applying OBJ: Use the inverse square law to relate luminosity, brightness, and distance.

1. The average red giant in the night sky is about 1,000 times more luminous than the average main-sequence star. If both kinds of stars have about the same brightness, how much farther away are the red giants compared to the main-sequence stars?
1. 32 times farther
2. 1,000 times farther
3. 65 times farther
4. 6 times farther
5. The red giants and main-sequence stars have approximately the same distances.

ANS: A         DIF: Difficult       REF: Section 5.5 MSC: Applying OBJ: Use the inverse square law to relate luminosity, brightness, and distance.

1. You are driving on the freeway when a police officer records a shift of −7 nm when he or she your speed with a radar gun that operates at a wavelength of 0.1 m. How fast were you going?
1. 43 mph
2. 83 mph
3. 21 mph
4. 65 mph
5. 47 mph

ANS: E         DIF: Difficult       REF: Working It Out 5.2 MSC: Applying OBJ: Use the Doppler equation to relate radial velocity with shifts in the wavelengths of spectral lines.

1. You record the spectrum of a star and find that a calcium absorption line has an observed wavelength of 394.0 nm. This calcium absorption line has a rest wavelength is 393.3 nm. What is the radial velocity of this star?
1. 5,000 km/s
2. 500 km/s
3. 50 km/s
4. 5 km/s
5. 5 km/s

ANS: B         DIF: Medium        REF: Working It Out 5.2 MSC: Applying OBJ: Use the Doppler equation to relate radial velocity with shifts in the wavelengths of spectral lines.

1. If you find that the hydrogen alpha line in a star’s spectrum occurs at a wavelength of 656.45 nm, what is the star’s radial velocity? Note that the rest wavelength of this line is 656.30 nm.
1. 150 km/s away from you
2. 150 km/s toward you
3. 350 km/s toward you
4. 70 km/s away from you
5. 70 km/s toward you

ANS: D         DIF: Difficult       REF: Working It Out 5.2 MSC: Applying OBJ: Use the Doppler equation to relate radial velocity with shifts in the wavelengths of spectral lines.

1. If Jupiter has a temperature of 165 K, at what wavelength does its spectrum peak? Use the electromagnetic spectrum in the figure below to answer this question.
1. 18 nm—orange visible wavelengths
2. 1,800 mm—microwave wavelengths
3. 1,800 nm—infrared wavelengths
4. 18,000 nm—ultraviolet wavelengths
5. 18,000 nm—infrared wavelengths

ANS: E         DIF: Medium        REF: Working It Out 5.3 MSC: Understanding OBJ: Use Wien’s law to relate the temperature and peak wavelength of blackbody emission.

1. If the typical temperature of a red giant is 3000 K, at what wavelength is its radiation the brightest? Use the electromagnetic spectrum in the figure below to help you answer this question.
1. 1 µm—infrared wavelengths
2. 1 µm—red visible wavelengths
3. 20 µm—infrared wavelengths
4. 20 µm—red visible wavelengths
5. 700 µm—red visible wavelengths

ANS: A         DIF: Medium        REF: Working It Out 5.3        MSC: Understanding OBJ: Use Wien’s law to relate the temperature and peak wavelength of blackbody emission.

1. If a star has a peak wavelength of 290 nm, what is its surface temperature?
1. 1000 K
2. 2000 K
3. 5000 K
4. 10,000 K
5. 100,000 K

ANS: D         DIF: Difficult       REF: Working It Out 5.3 MSC: Applying OBJ: Use Wien’s law to relate the temperature and peak wavelength of blackbody emission.

1. A black car left in the sunlight becomes hotter than a white car left in the sunlight under the same conditions because
1. the white car absorbs more sunlight than the black car.
2. the white car reflects more sunlight than the black car.
3. the black car absorbs only blue photons and reflects red photons, whereas the white car absorbs only red photons and reflects blue photons.
4. the atoms in the black car are smaller than the atoms in the white car.

ANS: B         DIF: Easy              REF: Working It Out 5.4 MSC: Applying OBJ: Calculate a planet’s temperature based on its parent star and albedo.

1. Which of the following factors does not directly influence the temperature of a planet?
1. the luminosity of the Sun
2. the distance of the planet from the Sun
3. the albedo of the planet
4. the size of the planet
5. the atmosphere of the planet

ANS: D         DIF: Easy              REF: Working It Out 5.4 MSC: Remembering OBJ: Calculate a planet’s temperature based on its parent star and albedo.

1. An asteroid with an albedo of 0.1 and a comet with an albedo of 0.6 are orbiting at roughly the same distance from the Sun. How do their temperatures compare?
1. They both have the same temperature.
2. The comet is hotter than the asteroid.
3. The asteroid is hotter than the comet.
4. You must know their sizes to compare their temperatures.
5. You must know their compositions to compare their temperatures.

ANS: C         DIF: Medium        REF: Working It Out 5.4 MSC: Understanding OBJ: Calculate a planet’s temperature based on its parent star and albedo.

1. Which of these planets would be expected to have the highest average temperature?
1. a light-colored planet close to the Sun
2. a dark-colored planet close to the Sun
3. a light-colored planet far from the Sun
4. a dark-colored planet far from the Sun
5. There is not enough information to know which would be hotter.

ANS: B         DIF: Medium        REF: Working It Out 5.4 MSC: Applying OBJ: Calculate a planet’s temperature based on its parent star and albedo.

1. If Saturn has a semimajor axis of 10 astronomical units (AU) and an albedo of 0.7. If Saturn were to emit the same amount of energy as it absorbs from the Sun, what is Saturn’s expected temperature?
1. 130 K
2. 15 K
3. 35 K
4. 170 K
5. 65 K

ANS: E         DIF: Difficult       REF: Working It Out 5.4 MSC: Applying OBJ: Calculate a planet’s temperature based on its parent star and albedo. SHORT ANSWER

1. Compare and contrast the wavelengths, frequencies, speeds, and energies of red and blue photons.

ANS: Red and blue photons both travel at the speed of light, which is 3 × 108 m/s. Red photons have longer wavelengths, lower frequencies, and lower energy levels than blue photons. DIF: Easy  REF: Section 5.1 MSC: Remembering OBJ: Relate color, wavelength, and energy of photons.

1. How is the energy of a photon related to its, frequency, wavelength, and speed?

ANS: Energy and frequency are directly related by a constant. (E=hf) Energy and wavelength are inversely related. (E=hc/(lambda)) Energy is independent of speed, whereas light always travels at the same speed, c. DIF: Easy  REF: Section 5.1  MSC: Understanding OBJ: Relate color, wavelength, and energy of photons.

1. What is the intensity of light, and how does it depend on wavelength?

ANS: Intensity is the total amount of energy a beam of light carries and is independent of the wavelength or frequency of the light. DIF: Medium  REF: Section 5.1   MSC: Understanding OBJ: Summarize the electromagnetic properties of light.

1. What is an electromagnetic wave?

ANS: Unlike a wave on water, which requires water, light is a self-sustaining electromagnetic wave. The varying electric field causes a varying magnetic field, and a varying magnetic field causes a varying electric field. DIF: Medium  REF: Section 5.1 MSC: Understanding OBJ: Summarize the electromagnetic properties of light.

1. The first five energy levels of hydrogen are E1 = 0 eV, E2 =2 eV, E3 = 12.1 eV, E4 = 12.7 eV, and E5 = 13.1 eV. If the electron is in the n = 4 level, what energies can a single emitted photon have?

ANS: To emit a photon, the electron needs to drop to a lower level, and the energy of the photon will be equal to the difference between the two energy levels. So, an atom in this state could give of a photon with 0.6 eV, 2.5 eV, or 12.7 eV of energy. DIF: Easy  REF: Section 5.2  MSC: Applying OBJ: Relate spectral features to changes in energy state of an atom.

1. Explain what the term “ground state” means.

ANS: The ground state is the lowest energy state that an atom can have. DIF: Easy  REF: Section 5.2 MSC: Understanding OBJ: Relate spectral features to changes in energy state of an atom.

1. Explain how continuous, emission, and absorption spectra are produced.

ANS: Continuous spectra are produced by viewing a hot glowing object, a blackbody. Emission spectra are observed when you see a hot gas directly. Absorption spectra are observed when a gas is between you and a source of blackbody radiation and the gas has relatively low density and has a lower temperature than the source.   DIF: Easy  REF: Section 5.2   MSC: Understanding OBJ: Illustrate the processes of atomic absorption and emission of light.

1. How are atoms excited, and why do they become de-excited?

ANS: An atom becomes excited when an electron absorbs just the right amount of energy to allow it to jump up to a higher energy level. Although it is possible to cause induced emission of a photon and have the electron fall back down to a lower level, most of the time this simply happens randomly and spontaneously. DIF: Easy  REF: Section 5.2  MSC: Understanding OBJ: Illustrate the processes of atomic absorption and emission of light.

1. Explain how an emission line is formed, and why it is unique to a given element.

ANS: Emission lines are formed when an electron transitions from a higher energy level to a lower energy level. These transitions are unique to each element because each element has its own set of energy levels. Each transition releases a photon of a specific energy, wavelength, and frequency. DIF: Medium  REF: Section 5.2   MSC: Understanding OBJ: Relate spectral features to changes in energy state of an atom.

1. The difference in energy between the n = 2 and n = 1 electronic energy levels in the hydrogen atom is 1.6 × 1018 If an electron moves from the n = 1 level to the n = 2 level, will a photon be emitted or absorbed? What will its energy be, and what type of electromagnetic radiation is it? Use the electromagnetic spectrum shown in the figure below to answer this question.

ANS: The n = 2 energy level is higher than the n = 1 energy level, so a photon with energy equal to 1.6 × 1018 J must be absorbed to make this transition. Its wavelength is equal to λ = hc/E = (6.6 × 1034 J s × 3 × 108 m/s)/1.6 ×1018 J = 1.2 × 107 m = 120 nm, which is in the ultraviolet region. DIF: Difficult  REF: Section 5.2  MSC: Applying OBJ: Relate spectral features to changes in energy state of an atom.

1. Describe, in your own words, why electrons cannot orbit the nucleus like the planets orbit the Sun.

ANS: In this model, the electron is constantly undergoing acceleration and therefore would constantly be giving off electromagnetic radiation. This would cause the electron to quickly spiral into the nucleus. DIF: Difficult  REF: Section 5.2   MSC: Understanding OBJ: Illustrate the processes of atomic absorption and emission of light.

1. Why do we see black lines in an absorption spectrum if the absorbed photons are (almost) instantaneously reemitted by the atoms in the cloud?

ANS: Originally all the light was traveling in the same direction, but absorbed photons, when reemitted, can be emitted in any direction. The number of photons emitted in the same direction as they were originally traveling is just a small fraction of the total number of photons. DIF: Difficult  REF: Section 5.2 MSC: Remembering OBJ: Illustrate the processes of atomic absorption and emission of light.

1. For a star that lies in the plane of Earth’s orbit around the Sun, how does the observed wavelength of the hydrogen absorption line at 656.28 nm in its spectrum change in wavelength (if at all) with the time of year?

ANS: As the Earth moves toward the star, the absorption line wavelength will get shorter. As the Earth moves away from the star, the absorption line wavelength will get longer. DIF: Easy  REF: Section 5.3  MSC: Applying OBJ: Explain why radial motion produces a Doppler shift.

1. A spaceship approaches Earth at 0.9 times the speed of light and shines a powerful searchlight onto Earth. How fast will the photons from this searchlight be moving when they hit Earth?

ANS: At the speed of light, 3 × 105 km/s. DIF: Easy  REF: Section 5.3  MSC: Applying OBJ: Explain why radial motion produces a Doppler shift.

1. If you are standing in a fixed location, you may notice that the pitch of a passing train’s whistle changes. What produces this effect?

ANS: Because sound is a wave, it can also experience a Doppler shift. As the train approaches, the whistle’s pitch is raised (blue shifted). The pitch drops after the train passes (red shifted). When the train is closest to you, you hear the unshifted pitch (or the rest frequency) of the whistle. DIF: Medium  REF: Section 5.3 MSC: Understanding OBJ: Explain why radial motion produces a Doppler shift.

1. Suppose you observe a star emitting a certain emission line of helium at 584.8 nm. The rest wavelength of this line is 587.6 nm. How fast is the star moving? Is it moving toward you or away from you?

ANS: The Doppler formula states that the radial velocity of an object is directly proportional to the shift of the spectral line it emits. The exact formula is  Plugging in values gives us νr = (584.8 nm − 587.6 nm) × 3 × 105 km/s/587.6 nm = −1,430 km/s. The negative sign indicates that the emission line has been blue shifted, so the star is moving toward us. DIF: Medium  REF: Section 5.3  MSC: Applying OBJ: Explain why radial motion produces a Doppler shift.

1. Imagine a satellite is orbiting a planet. This satellite gives off radio waves with a constant wavelength of 1 m. An observer on Earth then measures the signal from the satellite when it is directly between Earth and the planet. How does the wavelength received compare to the wavelength that the satellite gave off?

ANS: The received signal will be exactly 1 m because the satellite is not moving toward or away from Earth. DIF: Difficult  REF: Section 5.3   MSC: Applying OBJ: Explain why radial motion produces a Doppler shift.

1. Explain what is meant when someone says “thermal motions.”

ANS: Temperature can be related to the average kinetic energy of molecules and, therefore, to the average velocity of the molecules at a given temperature. These motions are random and are often referred to as “thermal motions.” DIF: Easy  REF: Section 5.4 MSC: Remembering OBJ: Define the bold-faced vocabulary terms within the chapter.

1. Sketch two blackbody curves, one for a hot blue object and the second for a cooler red object. Be sure to label your axes.

ANS: The hot blue curve should have a higher intensity and a shorter wavelength at the spectral peak than the red curve. DIF: Easy  REF: Section 5.4 MSC: Remembering OBJ: Characterize how blackbody spectra describe the luminosity, temperature, and color of an object.

1. How does temperature relate to the speed of gas particles?

ANS: Higher temperature gas particles have higher velocities. DIF: Easy  REF: Section 5.4 MSC: Remembering OBJ: Relate temperature to the rate of thermal motions.

1. Name four physical properties of an object that we can determine by analyzing the radiation that it emits, and briefly describe how these properties are determined. Cite the names of any laws that apply.

ANS: We can learn the following: (1) measure the spectrum, determine the wavelength where the most photons are emitted, and use Wien’s law to derive the temperature of the object; (2) measure the spectrum and determine the radial velocity of the object using the Doppler shift; (3) measure the spectrum and determine the chemical composition of the object from the absorption or emission lines it emits; and (4) measure the distance of an object by comparing its luminosity and brightness. DIF: Difficult  REF: Section 5.4   MSC: Remembering OBJ: Characterize how blackbody spectra describe the luminosity, temperature, and color of an object.

1. Imagine you observed three different stars: a red star, a blue star, and a yellow star. You are able to determine that each of these stars has the same radius. Answer each question below and explain how you know.

A: Which star has the highest surface temperature? B: Which star is the most luminous? C: Which star is the brightest? ANS: A: The blue star has the highest surface temperature because Wien’s law says hotter objects radiate at shorter or bluer wavelengths. B: The blue one is also the most luminous. The Stefan-Boltzmann law says that the hotter an object is, the larger the flux will be; thus, the hottest star is also the most luminous because they all have the same radius. C: You cannot tell from the information given. The brightness of a star depends on both luminosity and distance. Because you don’t know the distances to these stars, you can’t know which one is the brightest. DIF: Difficult  REF: Section 5.4  MSC: Applying OBJ: Characterize how blackbody spectra describe the luminosity, temperature, and color of an object.

1. If you were driving down a deserted country road and you saw a light in the distance, what would you need to measure or know about it in order to calculate how far away it was?

ANS: You would need to know the light’s luminosity and measure its brightness. DIF: Easy  REF: Section 5.5   MSC: Understanding OBJ: Use the inverse square law to relate luminosity, brightness, and distance.

1. Imagine you see a street lamp that is 100 m away from you and is 10,000 times more luminous than a firefly. How close would you have to be to the firefly to make it look as bright as the street lamp?

ANS: For objects with equal brightness, L µ d2, and the firefly has to be the square root of 10,000 or 100 times closer than the street lamp to have the same brightness. Thus, the firefly must be 1 m away. DIF: Difficult  REF: Section 5.5  MSC: Applying OBJ: Use the inverse square law to relate luminosity, brightness, and distance.

1. How much would you have to change the temperature of an object if you wanted to increase its flux by a factor of 100?

ANS: Because flux is proportional to T4, you would have to raise the object’s temperature by a factor of 1001/4 = 3.16. DIF: Medium  REF: Working It Out 5.3   MSC: Applying OBJ: Use the Stefan-Boltzmann law to relate temperature, flux, and luminosity of a blackbody.

1. If you want a blackbody’s peak wavelength to be cut in half, by how much do you need to increase its temperature?

ANS: Wien’s law states that λpeak = (2,900,000 nm K)/T. Because we want to cut the peak wavelength in half, we need λpeak,newpeak,old = 1/2. λpeak,newpeak,old = [(2,900,000 nm K)/Tnew]/[(2,900,000 nm K)/Told] = Told/Tnew = 1/2. So, the temperature must double to cut the peak wavelength in half. DIF: Difficult  REF: Working It Out 5.3   MSC: Applying OBJ: Use Wien’s law to relate the temperature and peak wavelength of blackbody emission.

1. What two factors control a planet’s surface temperature if it has no atmosphere, and no internal source of heat?

ANS: A planet’s distance from the Sun and its albedo determine its temperature. DIF: Easy  REF: Working It Out 5.4   MSC: Applying OBJ: Calculate a planet’s temperature based on its parent star and albedo.

1. How can the average temperature of Earth stay approximately constant even though Earth is always getting energy from the Sun?

ANS: Earth is also giving off energy into space in the form of blackbody radiation. DIF: Medium  REF: Working It Out 5.4   MSC: Applying OBJ: Calculate a planet’s temperature based on its parent star and albedo.

1. Astronomers have now found a large number of exoplanets, which are planets that orbit around stars other than the Sun. Imagine astronomers found a planet identical to Earth orbiting a star that had the same radius as the Sun, but with a temperature that is twice the temperature of the Sun. How far would this new planet need to be away from its star to have the same average temperature as Earth?

ANS: In order to have the same average temperature as Earth, the planet must receive the same energy from its star that Earth receives from the Sun. A star with a temperature two times that of the Sun would have 24 = 16 times the flux of the Sun. Because the two stars have the same radius, this new star’s luminosity would also be 16 times that of the Sun. Brightness is proportional to 1/distance2, so if this new planet was times further from its star than Earth is from the Sun, it would have the same brightness as the Sun does when viewed from Earth. Thus, this planet would need to be 4 AU from its star. DIF: Difficult  REF: Working It Out 5.4   MSC: Applying OBJ: Calculate a planet’s temperature based on its parent star and albedo.

1. What would you expect the temperature of a comet to be if its distance was 100 AU from the Sun? Assume that it is very icy and reflective so that its albedo is equal to 0.6. Does it matter what the radius of the comet is?

1. What is a protostar?
1. a planet like Jupiter
2. a hot star
3. a large ball of gas not yet hot enough at its core to be a star
4. a large ball of gas too hot at its core to be a star
5. a star with too much angular momentum

ANS: C         DIF: Easy              REF: Section 7.1 MSC: Remembering OBJ: Define the bold-faced vocabulary terms within the chapter.

1. What is a meteorite?
1. a streak of light in the sky
2. a rock that fell to Earth from space
3. a fireball
4. a volcanic rock
5. an iron-rich rock

ANS: B         DIF: Easy              REF: Section 7.1 MSC: Remembering OBJ: Define the bold-faced vocabulary terms within the chapter.

1. What have astronomers and geologists studied to arrive at the same conclusions about Earth’s origins?
1. volcanism in the solar system
2. comets
3. meteorites
4. the Moon
5. the oceans

ANS: C         DIF: Easy              REF: Section 7.1 MSC: Remembering OBJ: Describe how astronomers and geologists arrived at the same conclusions about Earth’s origins from different pieces of evidence.

1. The icy planetesimals that remain in the solar system today are called
1. comet nuclei.

ANS: D         DIF: Easy              REF: Section 7.1 MSC: Remembering OBJ: Define the bold-faced vocabulary terms within the chapter.

1. Which of the following is not a characteristic of the early Solar System, based on current observations?
1. The early solar nebula must have been flattened.
2. The material from which the planets formed was swirling about the Sun in the same average rotational direction.
3. The first objects to form started out small and grew in size over time.
4. The initial composition of the solar nebula varied between its inner and outer regions.
5. Temperatures decreased with increasing distance from the Sun.

ANS: D         DIF: Medium        REF: Section 7.1 MSC: Remembering OBJ: Illustrate the nebular hypothesis for solar system formation.

1. The smallest grains of dust stick together in an accretion disk by which force?
1. gravitational force
2. electrostatic force
3. magnetic force
4. quantum mechanical force
5. strong force

ANS: B         DIF: Medium        REF: Section 7.2 MSC: Remembering OBJ: Describe the formation sequence of planetesimals in an accretion disk.

1. In order for two clumps of dust to stick together in an accretion disk, they must collide at roughly
1. 100 m/s.
2. 10 m/s.
3. 1 m/s.
4. 5 m/s.
5. 1 m/s or less.

ANS: E         DIF: Medium        REF: Section 7.2 MSC: Remembering OBJ: Describe the formation sequence of planetesimals in an accretion disk.

1. What is a planetesimal?
1. bodies of ice and rock 100 meters or more in diameter
2. bodies of ice and rock 10 meters or less in diameter
3. bodies of ice and rock about 1 meter in diameter
4. another name for dwarf planets
5. planets that haven’t cleared their orbits

ANS: A         DIF: Easy              REF: Section 7.2 MSC: Remembering OBJ: Define the bold-faced vocabulary terms within the chapter.

1. According to the conservation of angular momentum, if an ice-skater who is spinning with her arms out wide slowly pulls them close to her body, this will cause her to . . .
1. spin faster.
2. spin slower.
3. maintain a constant rate of spin.
4. fall down.

ANS: A         DIF: Easy              REF: Section 7.2 MSC: Understanding OBJ: Explain conservation of angular momentum.

1. Approximately how much mass was there in the protoplanetary disk out of which the planets formed, compared to the mass of the Sun?
1. 50 percent
2. 25 percent
3. 10 percent
4. 5 percent
5. < 1 percent

ANS: E         DIF: Medium        REF: Section 7.2 MSC: Remembering OBJ: Illustrate how accretion disks transfer angular momentum so that stars and planets can collapse.

1. In the figure shown below, the direction of the disk’s rotation is indicated. What is the direction of the protostellar Sun’s rotation?
1. impossible to tell
2. in the opposite direction as the disk’s rotation
3. in the same direction as the disk’s rotation
4. perpendicular to the disk’s rotation

ANS: C         DIF: Medium        REF: Section 7.2 MSC: Understanding OBJ: Explain conservation of angular momentum.

1. Consider the figure shown below. At which point in time does the collapsing cloud have the greatest angular momentum?
1. 1
2. 2
3. 3
4. 1 and 2, because the protostar has not yet formed
5. The cloud has the same angular momentum at each point in time.

ANS: E         DIF: Medium        REF: Section 7.2 MSC: Understanding OBJ: Illustrate how accretion disks transfer angular momentum so that stars and planets can collapse.

1. The fact that Jupiter’s radius is contracting at a rate of 1 mm per year results in
1. Jupiter’s rotation rate slowing down with time.
2. Jupiter’s shape being noticeably oblate.
3. Jupiter moving slightly farther from the Sun with time.
5. Jupiter having a strong magnetic field.

ANS: D         DIF: Difficult       REF: Section 7.2 MSC: Applying OBJ: Explain conservation of angular momentum.

1. If a collapsing interstellar cloud formed only a protostar without an accretion disk around it, what would happen?
1. The forming protostar would be significantly less massive than it would have been otherwise.
2. The forming protostar would be rotating too fast to hold itself together.
3. Only giant planets would form around the protostar.
4. Only terrestrial planets would form around the protostar.
5. More planets would form around the protostar.

ANS: B         DIF: Difficult       REF: Section 7.2 MSC: Remembering OBJ: Illustrate how accretion disks transfer angular momentum so that stars and planets can collapse.

1. Conservation of angular momentum slows a cloud’s collapse
1. equally in all directions.
2. only when the cloud is not rotating initially.
3. mostly along directions perpendicular to the cloud’s axis of rotation.
4. mostly at the poles that lie along the cloud’s axis of rotation.
5. to a complete stop.

ANS: C         DIF: Difficult       REF: Section 7.2 MSC: Understanding OBJ: Illustrate how accretion disks transfer angular momentum so that stars and planets can collapse.

1. What is a primary atmosphere?
1. the atmospheres that all planets have today
2. the gas captured during the planet’s formation
3. the gas captured after the planet’s formation
4. the oxygen and nitrogen in Earth’s atmosphere
5. the gas closest to the planet’s surface

ANS: B         DIF: Easy              REF: Section 7.3 MSC: Remembering OBJ: Define the bold-faced vocabulary terms within the chapter.

1. What is a secondary atmosphere?
1. the atmosphere that escapes
2. the gas captured during the planet’s formation
3. the gas farthest from the surface
4. the atmosphere that remains after the planet has formed
5. the gas closest to the planet surface

ANS: D         DIF: Easy              REF: Section 7.3 MSC: Remembering OBJ: Define the bold-faced vocabulary terms within the chapter.

1. Consider four spheres of equal mass and size. Which has the most potential energy?
1. a sphere on the top shelf of a bookshelf
2. a sphere rolling on the floor at the base of the bookshelf
3. a sphere sitting at rest on the floor at the base of the bookshelf
4. a sphere on the middle shelf of a bookshelf
5. a sphere that fell from the top shelf to the floor

ANS: A         DIF: Easy              REF: Section 7.3 MSC: Applying OBJ: Explain conservation of energy.

1. The atmosphere of which of these Solar System bodies is primary, as opposed to secondary, in origin?
1. Venus
2. Earth
3. Saturn’s moon Titan
4. Saturn
5. Mars

ANS: D         DIF: Easy              REF: Section 7.3 MSC: Remembering OBJ: Compare and contrast primary and secondary atmospheres.

1. The primary atmospheres of the planets are made mostly of
1. carbon and oxygen.
2. hydrogen and helium.
3. oxygen and nitrogen.
4. iron and nickel.
5. nitrogen and argon.

ANS: B         DIF: Easy              REF: Section 7.3 MSC: Remembering OBJ: Compare and contrast primary and secondary atmospheres.

1. When you push your palms together and rub them back and forth, you are demonstrating one way of converting _________ energy into _________ energy.
1. potential; thermal
2. kinetic; potential
3. thermal; kinetic
4. kinetic; thermal
5. potential; total

ANS: D         DIF: Easy              REF: Section 7.3 MSC: Applying OBJ: Explain conservation of energy.

1. The solid form of a volatile material is generally referred to as a(n)
1. refractory material.

ANS: C         DIF: Easy              REF: Section 7.3 MSC: Remembering OBJ: Distinguish between refractory and volatile materials.

1. Based on the figure shown below, which planet(s) is(are) most likely to have the largest fraction of its(their) mass made of highly volatile materials such as methane and ammonia?
1. Venus, Earth, and Mars
2. Earth
3. Saturn
4. Jupiter
5. Uranus

ANS: E         DIF: Medium        REF: Section 7.3 MSC: Applying OBJ: Relate the temperature of an accretion disk to the presence of different types of materials (e.g., refractory, volatile, organic, ice) within the disk.

1. What happens to the kinetic energy of gas as it falls toward and eventually hits the accretion disk surrounding a protostar?
1. It is immediately converted into photons, giving off a flash of light on impact.
2. It is converted into thermal energy, heating the disk.
3. It is converted into potential energy as the gas plows through the disk and comes out the other side.
4. It becomes the kinetic energy of the orbit of the gas in the accretion disk around the protostar.
5. It disappears into interstellar space.

ANS: B         DIF: Medium        REF: Section 7.3 MSC: Applying OBJ: Use conservation of energy to argue why material falling on an accretion disk heats the disk up.

1. What sets the temperature of the pocket of gas in a protoplanetary disk?
1. its distance from the forming star
2. how much kinetic energy was converted to heat
3. how much radiation from the forming star shines on the gas
4. a combination of A, B, and C

ANS: D         DIF: Medium        REF: Section 7.3 MSC: Applying OBJ: Use conservation of energy to argue why material falling on an accretion disk heats the disk up.

1. Whether or not a planet is composed mostly of rock or gas is set by
1. its mass.
2. its temperature.
3. its distance from the star when it formed.
4. a combination of A, B, and C

ANS: D         DIF: Difficult       REF: Section 7.3 MSC: Applying OBJ: Relate the temperature of an accretion disk to the presence of different types of materials (e.g., refractory, volatile, organic, ice) within the disk.

1. Which of the following is a terrestrial planet?
1. Mercury
2. Jupiter
3. Venus
4. both A and B
5. both A and C

ANS: E         DIF: Easy              REF: Section 7.4 MSC: Remembering OBJ: Define the bold-faced vocabulary terms within the chapter.

1. Which of the following is a giant planet?
1. Mercury
2. Jupiter
3. Venus
4. both A and B
5. both A and C

ANS: B         DIF: Easy              REF: Section 7.4 MSC: Remembering OBJ: Define the bold-faced vocabulary terms within the chapter.

1. Which is the best description of a moon?
1. any small icy body in the solar system
2. any small rocky body in the solar system
3. any natural satellite of a planet or asteroid
4. a captured asteroid
5. a captured comet

ANS: C         DIF: Easy              REF: Section 7.4 MSC: Remembering OBJ: Define the bold-faced vocabulary terms within the chapter.

1. What is the most important factor in determining whether or not a planet will be rocky like terrestrial planets or gaseous like giant planets?
1. the time at which the planet forms
3. the planet’s distance from the Sun
4. whether the planet has moons
5. the planet’s internal temperature

ANS: C         DIF: Easy              REF: Section 7.4 MSC: Applying OBJ: Show how temperature differences in our accretion disk led to the formation of terrestrial and giant planets.

1. Why do the outer giant planets have massive gaseous atmospheres of hydrogen and helium whereas the inner planets do not?
1. These gases were more abundant in the outer regions of the accretion disk where the outer planets formed.
2. The outer planets grew massive quickly enough to gravitationally hold on to these gases before the solar wind dispersed the accretion disk.
3. The inner planets are made of rock.
4. Frequent early collisions by comets with the inner planets caused most of their original atmospheres to dissipate.

ANS: B         DIF: Easy              REF: Section 7.4 MSC: Understanding OBJ: Compare and contrast terrestrial and giant planets.

1. Comets and asteroids are
1. other names for moons of the planets.
2. primarily located within 1 astronomical unit (AU) of the Sun.
3. all more massive than Earth’s Moon.
4. material left over from the formation of the planets.
5. other names for meteors.

ANS: D         DIF: Easy              REF: Section 7.4 MSC: Remembering OBJ: Describe how planetesimals become planets.

1. The Moon probably formed
1. out of a collision between Earth and a Mars-sized object.
2. when Earth’s gravity captured a planetesimal.
3. when the accretion disk around Earth fragmented.
4. when planetesimals collided to form a more massive object.
5. when a piece of Earth broke off and entered orbit.

ANS: A         DIF: Easy              REF: Section 7.4 MSC: Applying OBJ: Describe how planetesimals become planets.

1. What prevented the Moon from maintaining any atmosphere with which it originally formed?
1. It repeatedly collided with planetesimals.
2. It is too close to the Sun.
3. The solar wind blew it away.
4. It is not massive enough.
5. It is tidally locked to Earth.

ANS: D         DIF: Medium        REF: Section 7.4 MSC: Applying OBJ: Describe how planetesimals become planets.

1. Which of the following is not considered evidence of cataclysmic impacts in the history of our Solar System?
1. Uranus is “tipped over” so that it rotates on its side.
2. Valles Marineris on Mars is a huge scar, many times deeper than the Grand Canyon, which spans one-fourth the circumference of the planet.
3. Mercury has a crust that has buckled on the opposite side of an impact crater.
4. Mimas has a crater whose diameter is roughly one-third of the Moon’s size.
5. Mercury, Earth’s Moon, and many other small bodies are covered with many impact craters.

ANS: B         DIF: Medium        REF: Section 7.4 MSC: Remembering OBJ: Describe how planetesimals become planets.

1. The difference in composition between the giant planets and the terrestrial planets is most likely caused by the fact that
1. the giant planets are much larger.
2. only the terrestrial planets have iron cores.
3. the terrestrial planets are closer to the Sun.
4. the giant planets are made mostly of carbon.
5. only small differences in chemical composition existed in the solar nebula.

ANS: C         DIF: Medium        REF: Section 7.4 MSC: Applying OBJ: Show how temperature differences in our accretion disk led to the formation of terrestrial and giant planets.

1. Two competing models of the formation of giant gaseous planets suggest they form either from gas accreting onto a rocky core or from
1. fragmentation of the accretion disk that surrounds the protostar.
2. the merger of two large planetesimals.
3. planets stolen from another nearby protostar.
4. materials condensing out of the solar wind.
5. an eruption of material from the protostar.

ANS: A         DIF: Medium        REF: Section 7.4 MSC: Remembering OBJ: Describe how planetesimals become planets.

1. Was it ever possible (or is it currently possible) for Jupiter to become a star?
1. Yes, it is in the process of becoming a star in the near future.
2. Yes, but it cooled off before it could become a star.
3. No, it would have to be at least 13 times more massive.
4. No, its composition is too different from stars for it to become one.
5. No, it used to be massive enough, but the solar wind has blown off too much of its mass.

ANS: C         DIF: Medium        REF: Section 7.4 MSC: Applying OBJ: Describe how planetesimals become planets.

1. How much material in an accretion disk goes into forming the planets, moons, and smaller objects?
1. most of it
2. roughly half of it
3. none; these objects were not formed in the accretion disk
4. a small amount of it

ANS: D         DIF: Medium        REF: Section 7.4 MSC: Remembering OBJ: Describe how planetesimals become planets.

1. Why do the terrestrial planets have a much higher fraction of their mass in heavy chemical elements (as opposed to lighter chemical elements) than the giant planets?
1. Terrestrial planets are low in mass and high in temperature, thus their lighter chemical elements eventually escaped to the outer reaches of the Solar System.
2. The heavier elements in the forming solar nebula sank to the center of the Solar System, thus the inner terrestrial planets formed mostly from heavy chemical elements.
3. The giant planets were more massive than terrestrial planets, and the giant planets preferentially pulled the lighter elements from the inner to the outer Solar System.
4. Terrestrial planets formed much earlier than giant planets before the hydrogen and helium had a chance to cool and condense onto them.
5. Terrestrial planets are colder and thus more massive chemical elements condensed on them than on the giant planets.

ANS: A         DIF: Difficult       REF: Section 7.4 MSC: Applying OBJ: Compare and contrast terrestrial and giant planets.

1. Which property of an extrasolar planet cannot be determined using the Doppler effect?
1. orbital period
2. orbital distance
3. orbital speed
4. mass

ANS: E         DIF: Easy              REF: Section 7.5 MSC: Remembering OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. What is the habitable zone?
1. the distance from a star where liquid water can exist
2. the location on the sky where planets can be found
3. the distance from a star where liquid can exist
4. the distance from a star where planets have oxygen in the atmosphere
5. 1 AU from any star

ANS: A         DIF: Easy              REF: Section 7.5 MSC: Remembering OBJ: Define the bold-faced vocabulary terms within the chapter.

1. Which method can be used to determine the radius of an extrasolar planet?
1. Doppler shift
2. transit
3. microlensing
4. direct imaging
5. none of the above

ANS: B         DIF: Easy              REF: Section 7.5 MSC: Remembering OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. Most planets currently found around other stars are
1. rocky in composition like terrestrial planets.
2. 2 to 10 MEarth, which is smaller than Neptune.
3. 2 to 10 MJupiter.
4. located at distances much larger than Jupiter’s distance from the Sun.
5. similar in mass to Earth.

ANS: B         DIF: Easy              REF: Section 7.5 MSC: Remembering OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. Which is not a scientific goal of NASA’s Kepler mission?
1. finding Earth-sized planets
2. finding rocky planets
3. finding Earth-sized planets that could have liquid water
4. finding intelligent life on other planets
5. All the above are goals of the Kepler mission

ANS: D         DIF: Easy              REF: Section 7.5 MSC: Applying OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. Consider a star that is more massive and hotter than the Sun. For such a star, the habitable zone would
1. be located inside 1 AU.
2. be located outside 1AU.
3. not exist at any radii.

ANS: B         DIF: Easy              REF: Section 7.5 MSC: Applying OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. The Kepler mission is designed to search for extrasolar planets using the _________ method.
1. Doppler shift
2. transit
3. microlensing
4. direct imaging

ANS: B         DIF: Easy              REF: Section 7.5 MSC: Remembering OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. Earth-sized planets have been found using the _________ method(s).
1. Doppler shift
2. transit and Doppler shift
3. microlensing
4. direct imaging
5. transit

ANS: B         DIF: Medium        REF: Section 7.5 MSC: Remembering OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. Astronomers believe that the “hot Jupiters” found orbiting other stars must have migrated inward over time
1. by slowly accreting large amounts of gas and increasing their gravitational pull.
2. by losing their gas because of evaporation.
3. by losing orbital angular momentum.
4. after colliding with another planet.
5. after a close encounter between their star and another star.

ANS: C         DIF: Medium        REF: Section 7.5 MSC: Applying OBJ: Describe how planetary migration accounts for hot Jupiters being located very close to their host stars.

1. The borderline between the most massive planet and the least massive brown dwarf occurs at
1. 4 Jupiter masses.
2. 13 Jupiter masses.
3. 120 Jupiter masses.
4. 80 Jupiter masses.
5. 45 Jupiter masses.

ANS: B         DIF: Medium        REF: Section 7.5 MSC: Remembering OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. Have astronomers detected any Earth-sized planets around normal stars yet?
1. Yes, the Kepler spacecraft is just starting to find them.
2. Yes, although the ones detected lie much closer to their stars than we do to ours.
3. Yes, although the ones detected lie much farther from their stars than we do from ours.
4. No, we do not have the technology to detect such low-mass planets yet.
5. No; although we have the technology to detect low-mass planets, we haven’t found any others yet.

ANS: A         DIF: Medium        REF: Section 7.5 MSC: Applying OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. Why have astronomers using the radial velocity method found more Jupiter-sized planets at a distance of 1 AU around other stars than Earth-sized planets?
1. A Jupiter-sized planet occults a larger area than an Earth-sized planet.
2. A Jupiter-sized planet exerts a larger gravitational force on the star than an Earth-sized planet, and the Doppler shift of the star is larger.
3. A Jupiter-sized planet shines brighter than an Earth-sized planet.
4. Earth-sized planets are much rarer than Jupiter-sized planets.
5. Actually, the planets found at these distances all have been Earth-sized.

ANS: B         DIF: Medium        REF: Section 7.5 MSC: Understanding OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. When astronomers began searching for extrasolar planets, they were surprised to discover Jupiter-sized planets much closer than 1 AU from their parent stars. Why is this surprising?
1. These planets must have formed at larger radii where temperatures were cooler and then migrated inward.
2. Jupiter-sized, rocky planets were thought to be uncommon in other solar systems.
3. These planets must be the remnants of failed stars.
4. Earth-like planets must be rarer than Jupiter-sized planets in other solar systems.
5. Jupiter-sized planets so close to the star are different than in our Solar System.

ANS: A         DIF: Medium        REF: Section 7.5 MSC: Applying OBJ: Describe how planetary migration accounts for hot Jupiters being located very close to their host stars.

1. Which of the following is false?
1. Hundreds of extrasolar planets have been discovered to date from radial velocity surveys.
2. The most common types of extrasolar planets found to date have masses 10 times the mass of Jupiter and lie within 5 AU from their parent star.
3. Some planetary systems have been found that contain multiple planets.
4. A star can brighten significantly because of gravitational lensing when a planet that orbits it passes directly in front of the star.
5. The Kepler mission has begun to find terrestrial planets similar in size to Earth.

ANS: B         DIF: Medium        REF: Section 7.5 MSC: Applying OBJ: Describe how planetary migration accounts for hot Jupiters being located very close to their host stars.

1. Astronomers have used radial velocity monitoring to discover
1. extrasolar planetary systems that are similar to our own Solar System.
2. Earth-sized planets around other stars.
3. Earth-sized planets at distances of 10 AU from their parent stars.
4. extrasolar planetary systems that contain more than one planet.
5. all of the above

ANS: D         DIF: Medium        REF: Section 7.5 MSC: Applying OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. An observer located outside our Solar System, who monitors the velocity of our Sun over time, will find that the Sun’s velocity varies by ± 12 m/s over a period of 12 years, due to
1. Jupiter’s gravitational pull.
2. Earth’s gravitational pull.
3. variations in its brightness.
4. convection on the Sun’s surface.
5. the sunspot cycle.

ANS: A         DIF: Medium        REF: Section 7.5 MSC: Understanding OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. Detecting a planet around another star using the transit method is difficult because the
1. planet must pass directly in front of the star.
2. planet must have a rocky composition.
3. star must be very dim.
4. star must be moving with respect to us.
5. planet’s orbital period is usually longer than 1 month.

ANS: A         DIF: Medium        REF: Section 7.5 MSC: Applying OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. In the figure below, which of the dips in the brightness of the star is(are) caused by the transit of the planet with the largest orbital period?
1. A
2. B
3. C
4. A and B
5. B and C

ANS: C         DIF: Medium        REF: Section 7.5 MSC: Applying OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. Figure 7.4 shows data from the transit study of a star in which three different planets repeatedly transit in front of the star (A, B, and C). Which dip is(are) caused by the transit of the planet with the smallest radius?
1. A
2. B
3. C
4. A, B, and C
5. impossible to tell from these data

ANS: A         DIF: Medium        REF: Section 7.5 MSC: Applying OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. Using the Doppler effect data shown in the figure below, determine the approximate orbital period of the extrasolar planet.
1. 1 year
2. 3 years
3. 6 years
4. 8 years
5. 12 years

ANS: C         DIF: Medium        REF: Section 7.5 MSC: Applying OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. Using the Doppler effect data for a particular star shown in Figure 7.5 and assuming the star is about the same mass as our Sun, determine the approximate orbital distance of its exoplanet.
1. 1 AU
2. 4 AU
3. 18 AU
4. 36 AU
5. 3 AU

ANS: E         DIF: Difficult       REF: Section 7.5 MSC: Applying OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. From the data shown in Figure 7.5, which property of an extrasolar planet cannot be determined?
1. orbital period
2. orbital distance
4. mass
5. All of the above properties can be determined.

ANS: C         DIF: Difficult       REF: Section 7.5 MSC: Applying OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. What is the best method to detect Earth-sized exoplanets with the telescopes and instrumentation that exist today?
1. Doppler shift
2. Transit
3. Microlensing
4. Direct imaging

ANS: B         DIF: Difficult       REF: Section 7.5 MSC: Remembering OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. Which of the following is false?
1. The masses of exoplanets can be determined using the radial velocity technique.
2. Most of the exoplanets detected to date have masses that are between 2 and 10 MEarth.
3. Some exoplanets have been found in the habitable zone around their stars.
4. Using the transit technique, the Kepler satellite has detected rocky planets.
5. No images of exoplanets have been obtained because they are too far away.

ANS: E         DIF: Difficult       REF: Section 7.5 MSC: Applying OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. In the figure shown below, what can be directly measured from the information given?
1. the mass of the planet
2. percentage reduction in light
3. size of the planet
4. orbital radius of the planet
5. distance of the star

ANS: B         DIF: Difficult       REF: Section 7.5 MSC: Understanding OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. What is the ratio of the orbital angular momentum of Earth compared to its spin angular momentum? Note that Earth has a radius of 6 × 106 m, and 1 AU is 1.5 × 1011
1. 1
2. 70
3. 640
4. 25,000
5. 3 × 106

ANS: E         DIF: Difficult       REF: Working It Out 7.1 MSC: Applying OBJ: Compute and compare orbital and spin angular momentum.

1. What is the ratio of the orbital angular momentum of Jupiter to its spin angular momentum? Jupiter’s orbit has a semimajor axis of 5 AU and period of 12 years, and Jupiter has a rotation period of 0.4 day and a radius of 70,000 km.
1. 650,000
2. 26,000
3. 920
4. 38
5. 5

ANS: B         DIF: Difficult       REF: Working It Out 7.1 MSC: Applying OBJ: Compute and compare orbital and spin angular momentum.

1. If an interstellar cloud having a diameter of 1016 m and a rotation period of 1 million years were to collapse to form a sphere that had the diameter of our Solar System, approximately 40 AU, what would its rotation period be? Assume the cloud’s total mass and angular momentum did not change.
1. 1 million years
2. 600 years
3. 1 year
4. 6 years
5. 4 months

ANS: E         DIF: Difficult       REF: Working It Out 7.1 MSC: Applying OBJ: Compute and compare orbital and spin angular momentum.

1. Consider a small parcel of gas in the cloud out of which the Sun formed that initially was located in the accretion disk at a distance of 10 AU from the Sun and rotating around it with a speed of 10 km/s. If this parcel of gas eventually found its way to a distance of 1 AU from the Sun without changing its orbital angular momentum, what would be its new rotation speed?
1. 100 km/s
2. 1 km/s
3. 001 km/s
4. 10 km/s
5. 1,000 km/s

ANS: A         DIF: Difficult       REF: Working It Out 7.1 MSC: Applying OBJ: Compute and compare orbital and spin angular momentum.

1. If an astronomer on a planet orbiting a nearby star observed the Sun when Neptune was transiting in front of the Sun, how would the Sun’s brightness change? Note that the radius of Neptune is 2.5 × 107
1. The Sun’s brightness would decrease by 0.1 percent.
2. The Sun’s brightness would increase by 0.1 percent.
3. The Sun’s brightness would increase by 1 percent.
4. The Sun’s brightness would decrease by 1 percent.
5. The Sun’s brightness would not change at all.

ANS: A         DIF: Difficult       REF: Working It Out 7.3 MSC: Applying OBJ: Estimate the size of a planet by considering how much of its parent star’s light it occults. SHORT ANSWER

1. Explain the nebular hypothesis, and describe two observations that support it.

ANS: In the nebular hypothesis, a rotating cloud of interstellar gas collapsed and flattened to form a disk from which the Sun and planets formed. The observation of disks around protostars and young stars provides evidence in support of this idea, as does the fact that all planets orbit the Sun in the same direction. DIF: Easy  REF: Section 7.1  MSC: Understanding OBJ: Illustrate the nebular hypothesis for solar system formation.

1. Explain why astronomers believe that the formation of planets is a natural by-product of star formation.

ANS: To explain the structure of the solar system (the direction and inclination of planetary orbits), astronomers believe that planets must form out of disks of material. These disks would bear a striking resemblance to disks that have been observed around a number of young stars. DIF: Easy  REF: Section 7.1  MSC: Understanding OBJ: Illustrate the nebular hypothesis for solar system formation.

1. How do meteorites tell us about how the solar system formed?

ANS: Studying meteorites can tell us about the chemical composition of the solar nebula, as well as the process of planetesimal formation. Some meteorites show a clumpy, nonuniform, composition similar to concrete. This leads astronomers and geologist to conclude that small things stuck together to grow larger and form planetesimals. DIF: Difficult  REF: Section 7.1 MSC: Understanding OBJ: Describe how astronomers and geologists arrived at the same conclusions about Earth’s origins from different pieces of evidence.

1. What does conservation of angular momentum mean?

ANS: It means the angular momentum of a system cannot be changed via internal forces; it can be changed only by external forces. DIF: Easy  REF: Section 7.2  MSC: Understanding OBJ: Define the bold-faced vocabulary terms within the chapter.

1. What evidence do we have that the accretion disk that formed the Solar System was initially much more dense near its center?

ANS: The Sun contains 99 percent of all the mass of the Solar System. DIF: Easy  REF: Section 7.2  MSC: Applying OBJ: Illustrate how accretion disks transfer angular momentum so that stars and planets can collapse.

1. Explain why an accretion disk forms around a protostar when an interstellar cloud collapses.

ANS: As the cloud collapses, the rate of rotation increases so that it halts the collapse of the cloud toward its axis of rotation, but not parallel to its axis of rotation. DIF: Medium  REF: Section 7.2   MSC: Understanding OBJ: Illustrate how accretion disks transfer angular momentum so that stars and planets can collapse.

1. What happens to a slowly rotating cloud as it collapses to form a stellar system?

ANS: Due to the conservation of momentum, the cloud’s rotation increases, eventually flattening into an accretion disk. DIF: Medium  REF: Section 7.2   MSC: Remembering OBJ: Explain conservation of angular momentum.

1. What is the difference between refractory and volatile materials?

ANS: Refractory materials are capable of withstanding high temperatures without melting or being vaporized, whereas volatile materials are not. DIF: Easy  REF: Section 7.3   MSC: Remembering OBJ: Distinguish between refractory and volatile materials.

1. Explain why there are significant amounts of methane and ammonia in the atmospheres of Uranus and Neptune but not nearly as much in the atmospheres of Jupiter and Saturn.

ANS: Ammonia and methane are volatile materials that are only found in the far outer Solar System where temperatures are very low. At the radii of Jupiter and Saturn, the nebula was hotter than that at Uranus and Neptune, which are farther from the Sun. DIF: Medium  REF: Section 7.3  MSC: Applying OBJ: Compare and contrast primary and secondary atmospheres.

1. Why does an accretion disk heat up?

ANS: As material falls onto the disk, gravitational potential energy is converted to kinetic energy. The kinetic is converted to thermal energy when the material collides with other material in the disk. DIF: Medium  REF: Section 7.3   MSC: Remembering OBJ: Use conservation of energy to argue why material falling on an accretion disk heats the disk up.

1. The primary atmosphere of Earth consisted of what type of chemical elements and from where did it originate? What chemical elements did the secondary atmosphere of Earth consist of and from where did it originate?

ANS: The primary atmosphere consisted mostly of hydrogen and helium, similar to the material that formed the solar nebula. The secondary atmosphere of Earth consisted mostly of carbon dioxide that was outgassed from the interior due to volcanic activity DIF: Medium  REF: Section 7.3 MSC: Remembering OBJ: Compare and contrast primary and secondary atmospheres.

1. Explain the primary reasons why the inner solar nebula was hotter than the outer solar nebula.

ANS: The inner solar nebula was hotter than the outer solar nebula because the inner regions converted more of their potential energy into kinetic energy and heat when the original cloud collapsed to form the Solar System. In addition, when the Sun began to shine, it heated the inner Solar System more than the outer Solar System. DIF: Medium  REF: Section 7.3 MSC: Understanding OBJ: Use conservation of energy to argue why material falling on an accretion disk heats the disk up.

1. Why did the terrestrial planets lose their primary atmospheres?

ANS: The planets have too little mass and therefore too little gravitational force to keep in the hot light gases that were present in the protoplanetary disk. DIF: Medium  REF: Section 7.3 MSC: Remembering OBJ: Compare and contrast primary and secondary atmospheres.

1. How do astronomers explain the basic difference in composition between the inner planets and the outer planets?

ANS: The inner planets (Mercury, Venus, Earth, and Mars) formed in the region of the Solar System where only refractory materials could exist in solid form, therefore they are composed mostly of rocks and metals. The outer planets (Jupiter, Saturn, Uranus, and Neptune) formed out where even volatile materials could exist in solid form. As a result, in addition to rocks and metals, the Jovian planets and their moons are largely composed of ices as well. The solar wind reinforced these differences by clearing the inner Solar System of light gases during the planetary formation process. DIF: Medium  REF: Section 7.3 MSC: Understanding OBJ: Relate the temperature of an accretion disk to the presence of different types of materials (e.g., refractory, volatile, organic, ice) within the disk.

1. Why did the planetesimals in the asteroid belt never coalesce into a planet?

ANS: The gravity of Jupiter was so strong that the material could not coalesce. DIF: Easy  REF: Section 7.4   MSC: Understanding OBJ: Describe how planetesimals become planets.

1. Why might a newly discovered comet contain clues to the composition of the early solar nebula?

ANS: Comets are believed to be made of ice and dust similar in composition to the early solar nebula. A newly discovered comet might be on its first orbit of the Sun, and, as it heats and melts, the gases it emits can tell us about the chemical composition of the solar nebula. DIF: Easy  REF: Section 7.4 MSC: Understanding OBJ: Describe how planetesimals become planets.

1. What are craters in the solar system evidence of?

ANS: Craters in the solar system are evidence of a time when the solar system had many small planetesimals that were colliding to form our present solar system. DIF: Medium  REF: Section 7.4 MSC: Remembering OBJ: Describe how planetesimals become planets.

1. How did the formation of our Moon differ from the formation of the Galilean moons of Jupiter?

ANS: Astronomers believe that the formation of our Moon occurred due to a collision of a Mars-sized object with the early Earth. The remains of the planet eventually coalesced into our Moon. By contrast, the Galilean moons are believed to have formed naturally with the rest of the Solar System. Astronomers believe that the Jovian system formed its own mini accretion disk, out of which the Galilean moons formed around Jupiter, much as the planets formed around the Sun. DIF: Medium  REF: Section 7.4  MSC: Applying OBJ: Describe how planetesimals become planets.

1. Approximately how massive are most of the extrasolar planets that have been discovered using the Doppler effect, and which planet in our Solar System is similar in mass? Why is the Doppler effect method more likely to find massive (rather than low-mass) planets and planets that are close to their stars?

ANS: The planets found are mostly smaller than Neptune, 2 to 10 times Earth’s mass. The Doppler effect is more likely to find massive planets because the Doppler shift of their parent star will be larger, because the gravitational pull is proportional to the mass. Also, it is easier to find a planet closer in because the force of gravity is stronger as it is inversely proportional to the square of the semimajor axis. Thus, more massive planets and planets closer to their star are easier to detect with the Doppler shift. DIF: Easy  REF: Section 7.5   MSC: Remembering OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. Explain why most of the extrasolar planets that astronomers first detected were so-called “hot Jupiters.”

ANS: Originally technology did not allow us to detect smaller planets. The easiest planets to detect are massive planets, which cause their parent stars to wobble the most, in close orbits, which cause their parent stars to wobble faster. DIF: Medium  REF: Section 7.5  MSC: Applying OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. Have any Earth-sized, terrestrial, extrasolar planets been detected? If so, explain what method(s) is(are) used.

ANS: The Kepler mission has been finding Earth-sized planets using the transit method to detect them and using the Doppler shift method to follow up and measure the radii, masses, and densities and to characterize them as either terrestrial or giant planets. DIF: Medium  REF: Section 7.5 MSC: Remembering OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. In addition to the percentage reduction in light, is anything else needed to determine the size of the transiting planet?

ANS: The radius of the star is also needed to calculate the size of the transiting planet. DIF: Medium  REF: Section 7.5   MSC: Understanding OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. Explain how astronomers use the Doppler effect to detect the presence of extrasolar planets.

ANS: As a planet orbits a star, the gravitational attraction between the planet and star causes them to orbit a common center of mass. To an outside observer, this causes the star to appear to “wobble.” As it does so, it periodically moves toward us and then away from us. These radial motions produce a Doppler effect in the spectra of the star. By measuring the Doppler effect, astronomers can infer the mass of the planet and its distance from the star. DIF: Medium  REF: Section 7.5  MSC: Applying OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. What property of an extrasolar planet can be determined directly from the Doppler effect data shown in the figure below? What other properties of the planet can then be determined?

ANS: The period of repetition of the Doppler shifts is also the orbital period of the planet. The orbital distance and mass of the planet can then be calculated from the maximum orbital velocity observed along with Newton’s generalized version of Kepler’s third law, if the mass of the central star and the inclination of the orbit can be determined. DIF: Medium  REF: Section 7.5  MSC: Applying OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. Briefly explain the five different observational methods we use to detect extrasolar planets. How many extrasolar planets have been discovered to date?

ANS: The five different observational methods we use to detect extrasolar planets are (1) using the Doppler shift to detect the motion of its parent star, (2) detecting transits when a planet moves in front of its parent star and dims it, (3) detecting microlensing events when the planet moves across the line of sight of its parent star and brightens it, (4) directly imaging the planet as it orbits its star, and (5) the astrometric method of precise measurement of a star’s position. Slightly more than 2000 extrasolar planets have been detected as of the writing of this textbook. DIF: Medium  REF: Section 7.5  MSC: Applying OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. What evidence do we have that planetary systems are common in the universe?

ANS: Astronomers have directly observed disks around young stars. Astronomers have now detected more than 2000 planets. DIF: Medium  REF: Section 7.5 MSC: Remembering OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. What is planet migration?

ANS: The force of gravity from all the nearby objects can move planets from the orbit they form in. This can cause planets to move inward or outward in the disk. DIF: Medium  REF: Section 7.5 MSC: Remembering OBJ: Describe how planetary migration accounts for hot Jupiters being located very close to their host stars.

1. What are some limitations of the radial velocity method of exoplanet detection?

ANS: The star needs to be bright. Technological limitations prevent us from detecting Earth-massed planets. The star has to be moving toward or away from Earth; therefore, for a star system that is face-on (we see its pole directly) it is impossible to use this technique to detect a planet. DIF: Medium  REF: Section 7.5 MSC: Understanding OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. What are some limitations of the transit method of exoplanet detection?

ANS: This method can only detect planets in systems that are aligned so that the plane of the star system is edge on, meaning the planet will pass in front of the star. The diameter of the planet relative to the star, needs to be large enough to cause a measurable decrease in the brightness of the star. DIF: Medium  REF: Section 7.5   MSC: Understanding OBJ: Summarize the five methods that astronomers use to detect extrasolar planets.

1. Compare the orbital angular momentum of Earth and Jupiter. Which is larger and by how much? (Note that Jupiter’s mass is 318 times that of Earth, the semimajor axis of Jupiter’s orbit is 5.2 AU, and Jupiter’s orbital period is 12 years.)

1. Which of these shows the correct order of collections of galaxies, starting with the smallest and ending with the largest?
1. group, supercluster, cluster
2. cluster, supercluster, group
3. group, cluster, supercluster
4. cluster, group, supercluster
5. supercluster, group, cluster

ANS: C         DIF: Easy              REF: 23.1 MSC: Remembering OBJ: Compare and contrast groups, clusters, superclusters, and walls.

1. The most common type of galaxy found in a galaxy cluster is a ___________ galaxy.
1. spiral
2. giant elliptical
3. giant irregular
4. dwarf
5. barred spiral

ANS: D         DIF: Easy              REF: 23.1 MSC: Remembering OBJ: Compare and contrast groups, clusters, superclusters, and walls.

1. What does the large-scale structure of the universe look most like?
1. a sponge with many large holes
2. a loaf of wheat bread with many tiny holes
3. a plate of flat noodles
4. a jar of marbles
5. a pizza with evenly distributed pepperoni

ANS: A         DIF: Easy              REF: 23.1 MSC: Remembering OBJ: Compare and contrast groups, clusters, superclusters, and walls.

1. What is our cosmic address?
1. Earth, Solar System, Milky Way, Local Group, Laniakea Supercluster
2. Earth, Solar System, Andromeda, the Great Attractor, Local Group
3. Earth, Local Group, Solar System, Milky Way, Cosmic Microwave Background
4. Earth, Local Group, Solar System, Milky Way, Laniakea Supercluster
5. Earth, Solar System, Andromeda, the Great Attractor, Laniakea Supercluster

ANS: A         DIF: Easy              REF: 23.1 MSC: Remembering OBJ: Compare and contrast groups, clusters, superclusters, and walls.

1. Peculiar velocities are produced by
1. erroneous redshifts.
2. eclipsing binary stars.
3. interstellar winds.

ANS: B         DIF: Easy              REF: 23.1 MSC: Understanding OBJ: Describe how Hubble’s law helps astronomers map the structure of mass in the universe.

1. The most common galaxy collections are called _______ and most of their members are _______ galaxies.
1. clusters; spiral
2. groups; dwarf
3. superclusters; elliptical
4. voids; peculiar
5. walls; lenticular S0

ANS: B         DIF: Easy              REF: 23.1 MSC: Remembering OBJ: Compare and contrast groups, clusters, superclusters, and walls.

1. How many times would our Local Group fit along the distance that separates it from the Virgo Cluster?
1. 5
2. 10
3. 200
4. 1000
5. 2

ANS: A         DIF: Easy              REF: 23.1 MSC: Remembering OBJ: Compare and contrast groups, clusters, superclusters, and walls.

1. Understanding the distribution of galaxies in the universe requires the construction of a 3D map where for each galaxy we need to know its position and
1. morphological type.
2. star formation rate.

ANS: B         DIF: Easy              REF: 23.1 MSC: Applying OBJ: Describe how Hubble’s law helps astronomers map the structure of mass in the universe.

1. The peculiar velocity of a galaxy describes its motion relative to the
1. Local Group.
2. center of the Milky Way.
3. center of the universe.
4. Great Attractor.
5. cosmic microwave background.

ANS: E         DIF: Easy              REF: 23.1 MSC: Remembering OBJ: Compare and contrast groups, clusters, superclusters, and walls.

1. The figure below shows a small section of the Virgo galaxy cluster. The indicated galaxy can be classified as

ANS: B         DIF: Easy              REF: 23.1 MSC: Applying OBJ: Compare and contrast groups, clusters, superclusters, and walls.

1. Virgo galaxy cluster is at an estimated distance of 54 million ly. What is the average recession speed of the cluster, assuming a Hubble constant of 70km/s/Mpc?
1. 1,200 km/s
2. 32,000 km/s
3. 300 km/s
4. 10,000 km/s
5. 3800 km/s

ANS: A         DIF: Medium        REF: 23.1 MSC: Applying OBJ: Describe how Hubble’s law helps astronomers map the structure of mass in the universe.

1. Scientists estimate the following quantities for the Coma cluster of galaxies: the total mass of the cluster is Mtotal ~3 × 1015 M, the total mass of all stars Mstars ~ 3 × 1013 M, and the mass of the hot X-ray−emitting gas Mgas ~ 1 × 1014 M. What is corresponding percentage of dark matter within the Coma cluster?
1. 4 percent
2. 25 percent
3. 72 percent
4. 96 percent
5. There is no dark matter contribution.

ANS: D         DIF: Medium        REF: 23.1 MSC: Applying OBJ: Summarize the evidence for dark matter dominating the mass of groups and clusters.

1. Which of the following is not true about the Great Attractor?
1. It is about 75 Mpc away.
2. It has a mass of several thousand times the mass of the Milky Way.
3. It is at the center of the Laniakea supercluster.
4. It is in fact a black hole the size of a galaxy.
5. It has gravitational effects on the motion of galaxies and groups of galaxies.

ANS: D         DIF: Medium        REF: 23.1 MSC: Remembering OBJ: Compare and contrast groups, clusters, superclusters, and walls.

1. Choose the incorrect statement about the gas that fills the space between galaxies in a cluster?
1. It is very hot and glows in X-rays.
2. Its total mass far exceeds the combined mass of all the stars in the galaxy members of the cluster.
3. Its particles move so fast that they easily escape the gravity of the cluster.
4. It originates in stellar winds and/or it is stripped away from colliding galaxies.
5. Its temperature gives us clues about the total gravitational mass of the cluster.

ANS: C         DIF: Medium        REF: 23.1 MSC: Remembering OBJ: Summarize the evidence for dark matter dominating the mass of groups and clusters.

1. Which of the following does not provide direct evidence for the existence of dark matter?
1. the rotation curves of spiral galaxies
2. the motions of galaxies in clusters
3. the temperature of the diffuse intergalactic gas within clusters
4. the gravitational lensing produced by galaxy clusters
5. the changing fraction of peculiar galaxies as a function of redshift

ANS: E         DIF: Medium        REF: 23.1 MSC: Analyzing OBJ: Summarize the evidence for dark matter dominating the mass of groups and clusters.

1. Which of the following is not a way in which astronomers detect dark matter in clusters of galaxies?
1. by determining the amount of mass necessary to gravitationally collapse clouds of gas to form the number of stars present
2. by determining the amount of mass necessary to gravitationally hold onto the hot gas within galaxy clusters
3. by determining the amount of mass necessary to gravitationally hold a cluster of galaxies together
4. by determining the amount of mass necessary to gravitationally lens images of distant objects
5. None of the above; all of these are ways that astronomers detect dark matter in galaxy clusters.

ANS: A         DIF: Medium        REF: 23.1 MSC: Analyzing OBJ: Summarize the evidence for dark matter dominating the mass of groups and clusters.

1. Which of the following components makes up the largest amount of normal matter in a typical large cluster of galaxies?
1. supermassive black holes
2. stars
3. cold gas
4. hot gas
5. neutron and white dwarf stars

ANS: D         DIF: Difficult       REF: 23.1 MSC: Remembering OBJ: Summarize the evidence for dark matter dominating the mass of groups and clusters.

1. If a galaxy is a member of a large cluster of galaxies, like the Coma cluster, the galaxy would have a typical velocity of 1,000 km/s. If the cluster is 10 Mpc in diameter, how long would it take the galaxy to cross from one side of the cluster to another?
1. 10,000 years
2. 1 million years
3. 10 million years
4. 100 million years
5. 10 billion years

ANS: E         DIF: Difficult       REF: 23.1 MSC: Applying OBJ: Compare and contrast groups, clusters, superclusters, and walls.

1. What has the dominant role in defining the large-scale structure of the universe?
1. supernovae from the first generation of stars
2. gravity
3. matter/antimatter annihilation
4. magnetic fields
5. electric force

ANS: B         DIF: Easy              REF: 23.2 MSC: Understanding OBJ: Describe how gravitational instabilities created the seeds of large-scale structure.

1. Structure formation in our universe
1. occurs for the largest structures first.
2. occurs for the smallest structures first.
3. begins on all spatial scales at the same time.
4. begins after clusters form.
5. begins with planets.

ANS: B         DIF: Easy              REF: 23.2 MSC: Understanding OBJ: Illustrate the steps that led to large-scale structure formation.

1. Structure formation in the universe proceeds hierarchically, meaning that
1. large objects collapse and then fragment to form smaller objects.
2. large objects form at the same times as smaller objects.
3. small objects collapse and then merge to form larger objects.
4. only small objects form and are stable over time.
5. normal matter collapses first and dark matter collapses later.

ANS: C         DIF: Easy              REF: 23.2 MSC: Understanding OBJ: Illustrate the steps that led to large-scale structure formation.

1. Quantum fluctuations in the early universe
1. were the seeds that grew into today’s galaxies.
2. are the reason dark matter exists.
3. were made of small black holes.
4. had no effect on the current structure of the universe.
5. can be observed with radio telescopes.

ANS: A         DIF: Easy              REF: 23.2 MSC: Understanding OBJ: Describe how gravitational instabilities created the seeds of large-scale structure.

1. The “Lambda-CDM” model combines the properties of __________ to explain the formation of structure in the universe.
1. black holes and neutron stars
2. dark energy and cold dark matter
3. star formation and angular momentum
4. nucleosynthesis and hot dark matter
5. gravity and nuclear forces

ANS: B         DIF: Easy              REF: 23.2 MSC: Remembering OBJ: Describe how gravitational instabilities created the seeds of large-scale structure.

1. Our current ideas on galaxy formation suggest that the visible parts of galaxies
1. form first and are incorporated into dark matter halos later.
2. form only in the densest parts of dark matter halos.
3. can tell you the total size of the dark matter halo.
4. can tell you everything about the formation history of that galaxy.
5. spread out over distances larger than those of dark matter halos.

ANS: B         DIF: Easy              REF: 23.2 MSC: Understanding OBJ: Relate the presence of dark matter to the formation of galaxies in the early universe.

1. Why can’t dark matter halos collapse to be the same size as the visible parts of galaxies?
1. Dark matter can’t dissipate its energy through radiation from collisions.
2. Dark matter is made mostly of mini−black holes.
3. Dark matter has much more angular momentum.
4. Dark matter annihilates when it begins to get that dense.
5. Dark matter particles are too large to collapse that much.

ANS: A         DIF: Medium        REF: 23.2 MSC: Applying OBJ: Relate the presence of dark matter to the formation of galaxies in the early universe.

1. The figure below shows snapshots taken from the Bolshoi simulation of the formation of the large-scale structure in the universe. Which one of these images shows the current structure of the universe?
1. A
2. B
3. C
4. D
5. All of these occur at the same redshift, just in different regions of the universe.

ANS: B         DIF: Medium        REF: 23.2 MSC: Applied OBJ: Illustrate the steps that led to large-scale structure formation.

1. Which of the following is not a possible candidate for dark matter?
1. axions
2. positrons
3. photinos
4. neutrinos
5. WIMPS

ANS: B         DIF: Medium        REF: 23.2 MSC: Remembering OBJ: Summarize the observational evidence that dark matter cannot be composed of ordinary matter.

1. Scientists think that neutrinos cannot be the dominant form of dark matter in the universe. Why?
1. Neutrinos are an example of hot dark matter that could form large superclusters but not smaller structures.
2. Neutrinos interact too strongly with ordinary matter.
3. Neutrinos would decay over time and disappear, causing galaxies to fall apart.
4. Neutrinos would not gravitationally lens background galaxies.
5. Neutrinos are charged particles.

ANS: A         DIF: Medium        REF: 23.2 MSC: Understanding OBJ: Compare and contrast the properties and possible constituents of cold and hot dark matter.

1. The small density variations that subsequently led to the formation of large-scale structure in the universe are thought to have formed
1. from quantum fluctuations during inflation.
2. due to the supernovae of the very first stars.
3. at the end of the recombination epoch.
4. as particle-antiparticle pairs annihilated.
5. as supermassive black holes powered the first quasars.

ANS: A         DIF: Medium        REF: 23.2 MSC: Remembering OBJ: Describe how gravitational instabilities created the seeds of large-scale structure.

1. The density of normal matter in the early universe was ______________ in the present epoch universe.
1. much higher than the density of normal matter
2. much lower than the density of normal matter
3. about the same as the density of normal matter
4. much higher than the density of dark matter
5. about the same as the density of dark matter

ANS: C         DIF: Medium        REF: 23.2 MSC: Understanding OBJ: Summarize the observational evidence that dark matter cannot be composed of ordinary matter.

1. Which of the following is true about neutrinos?
1. They are an example of cold dark matter.
2. They are an example of hot dark matter.
3. They have been theoretically predicted, yet never detected.
4. They must be much more massive than the dark matter candidate called
5. They account for all the dark matter in the universe.

ANS: B         DIF: Medium        REF: 23.2 MSC: Analyzing OBJ: Compare and contrast the properties and possible constituents of cold and hot dark matter.

1. Which of the following statements about dark matter is incorrect?
1. Cold and hot dark matter play differently on the formation of small and large-scale structure.
2. Cold dark matter has the dominant role in the formation of individual galaxies.
3. Dark matter most likely consists of elementary particles with no electric charge.
4. Physicists use particle accelerators to look for hypothesized dark matter candidates.
5. Hot dark matter has the dominant effect in the formation of individual galaxies.

ANS: E         DIF: Medium        REF: 23.2 MSC: Analyzing OBJ: Compare and contrast the properties and possible constituents of cold and hot dark matter.

1. If dark matter consisted of ordinary particles like protons and neutrons,
1. the fraction of light elements produced in the Big Bang nucleosynthesis would have been severely different from what scientists observe.
2. it wouldn’t have affected gravitationally the large scale structure of the universe.
3. it would have prevented the expansion of the universe.
4. scientists would easily be able to measure the decay of proton in the lab.
5. it would manifest differently in individual galaxies and in large superclusters of galaxies.

ANS: A         DIF: Difficult       REF: 23.2 MSC: Understanding OBJ: Summarize the observational evidence that dark matter cannot be composed of ordinary matter.

1. Dark matter is essential in understanding the formation of the large scale structure in the universe in that it
1. was clumpier than normal matter in the early universe.
2. consists exclusively of antimatter particles.
3. has a repulsive effect, just like the dark energy.
4. explains the physics of black holes.
5. cannot be made of elementary particles.

ANS: A         DIF: Difficult       REF: 23.2 MSC: Understanding OBJ: Relate the presence of dark matter to the formation of galaxies in the early universe.

1. The best hypothesis about the nature of dark matter is that it consists of particles with no electric charge. Why would such particles have no electric charge?
1. They cannot be more massive than an electron.
2. If they did, they would emit photons as they move in external magnetic fields.
3. Charged particles would have been all annihilated in particle-antiparticle collisions in the early universe.
4. No elementary particle has electric charge.
5. Charged particles formed only later inside stars.

ANS: B         DIF: Difficult       REF: 23.2 MSC: Evaluating OBJ: Summarize the observational evidence that dark matter cannot be composed of ordinary matter.

1. How do the properties of the CMB give support to the existence of dark matter?
1. The CMB has the same temperature as the cold dark matter.
2. The faint glow of the CMB was actually produced by dark matter particles as they annihilated normal matter particles.
3. The opaque CMB is essentially hiding the dark matter that existed earlier in the universe.
4. The CMB is too smooth to account for the structure we observe in the universe.
5. The observed spatial scale of CMB clumpiness perfectly matches that of the dark matter.

ANS: D         DIF: Difficult       REF: 23.2 MSC: Analyzing OBJ: Describe how gravitational instabilities created the seeds of large-scale structure.

1. In the early universe, why were inhomogeneities in the distribution of normal matter much smaller than inhomogeneities in the dark matter?
1. Normal matter is pushed away by supernova explosions.
2. Magnetic fields smoothed the distribution of charged particles in the normal matter but not in dark matter.
3. Dark matter particles were more massive than and cooled off before normal matter, thus dark matter fluctuations had a longer time over which to grow.
4. Dark matter was 10 times more massive than normal matter.
5. Radiation pressure affected normal matter but not dark matter.

ANS: E         DIF: Difficult       REF: 23.2 MSC: Applied OBJ: Illustrate the steps that led to large-scale structure formation.

1. The figure below shows snapshots taken from the Bolshoi simulation of the formation of the large-scale structure in the universe. Which one of these images represents the state of the universe at the highest redshift?
1. A
2. B
3. C
4. D
5. All of these occur at the same redshift, just in different regions of the universe.

ANS: C         DIF: Difficult       REF: 23.2 MSC: Applying OBJ: Illustrate the steps that led to large-scale structure formation.

1. Telescopes like ALMA (working in the range 0.4-3 mm) and JWST (working in IR, in the range 0.6-28.5 micro-m) would help astronomers close a gap of observations that corresponds to the window
1. z= 2−3, when the star formation rate peaked.
2. z= 10−1000, between the epochs of recombination and reionization.
3. z= 0−5, when there was a big change in the relative fraction of galaxies with peculiar morphologies.
4. z> 1100, to directly observe the inflation epoch.
5. such telescopes are actually designed to only explore planetary systems within the boundaries of our Milky Way.

ANS: B         DIF: Difficult       REF: 23.2 MSC: Remembering OBJ: Relate the presence of dark matter to the formation of galaxies in the early universe.

1. The future James Webb telescope is designed to observe the most distant galaxies in the universe. It will observe them in
1. X-ray.

ANS: B         DIF: Easy              REF: Working It Out 23.2 MSC: Applying OBJ: Calculate the observed wavelength of highly redshifted light.

1. The most distant galaxies detected at z ≈ 10 are best observed with __________ wavelengths of light.
1. infrared
2. visible
3. X-ray
4. gamma ray

ANS: A         DIF: Easy              REF: Working It Out 23.2 MSC: Applying OBJ: Calculate the observed wavelength of highly redshifted light.

1. At what redshift would a quasar’s emission line emitted at 121.6 nm show up at the “normal” wavelength of 486.1 nm of the Balmer Hβ line?
2. 1
3. 2
4. 3
5. 5
6. 7

ANS: C         DIF: Medium        REF: Working It Out 23.2 MSC: Applying OBJ: Calculate the observed wavelength of highly redshifted light.

1. What would be the observed wavelength for the Balmer Hβ line emitted at 486.1 nm by a quasar at redshift z = 5?
2. 9 µm
3. 0 nm
4. 0 cm
5. 6 nm
6. 2 µm

ANS: A         DIF: Medium        REF: Working It Out 23.2 MSC: Applying OBJ: Calculate the observed wavelength of highly redshifted light.

1. Reionization of the neutral gas in the universe occurred due to the
2. decay of dark matter particles.
3. emission of neutrinos by the first stars that formed.
4. release of jets of charged particles from supermassive black holes.
5. radiation from the first stars, supernovae, and black holes that formed.
6. positron and electron annihilations.

ANS: D         DIF: Easy              REF: 23.3 MSC: Understanding OBJ: Describe the properties of the very first generation of stars and whether these are based on observations or hypotheses.

1. The first stars formed in the universe had ___________ compared with the stars formed today.
2. higher heavy element content and higher mass
3. higher heavy element content and lower mass
4. lower heavy element content and higher mass
5. lower heavy element content and lower mass
6. higher mass and longer lifetimes

ANS: C         DIF: Easy              REF: 23.3 MSC: Factual OBJ: Explain why the first generation stars were expected to be very massive, while subsequent generations could have high or low mass.

1. If astronomers discover a new ultrafaint galaxy, where would it most likely to be found?
2. on its own, away from other galaxies
3. a few billion light years away from Earth
4. at high redshift
5. in a large galaxy cluster
6. orbiting the Milky Way

ANS: E         DIF: Easy              REF: 23.3 MSC: Analyzing OBJ: Describe the properties of the first generation of galaxies and whether these are based on observations or hypotheses.

1. Which of the following does not describe the current view on the very first stars?
2. They formed inside dark matter minihalos.
3. There was no dust available to help the process of star formation.
4. They must have been tens of times more massive than our Sun.
5. The material from which they formed contained no elements more massive than lithium.
6. The formed in large clusters with numerous members.

ANS: E         DIF: Easy              REF: 23.3 MSC: Analyzing OBJ: Describe the properties of the very first generation of stars, and whether these are based on observations or hypotheses.

1. Which of the following best explains the difference between the heavy-element abundances seen in the first stars and those seen in stars that we observe today?
2. Stars today have more heavy elements, because modern stars have higher masses, allowing them to create more heavy elements through nuclear fusion.
3. Stars today have more heavy elements, because the gas that formed the current stars was enriched by the higher mass elements formed in previous generations of stars.
4. Stars today have a fewer heavy elements, because they have been around long enough to use up the larger mass atoms.
5. Stars today have a smaller abundance of heavy elements, because they haven’t been around long enough to make as many of the larger atoms.
6. The stars that astronomers observe now are the first generation of stars.

ANS: B         DIF: Medium        REF: 23.3 MSC: Understanding OBJ: Describe the properties of the very first generation of stars and whether these are based on observations or hypotheses.

1. Which of the following is not true about ultrafaint dwarf galaxies?
2. They may be the remains of the first galaxies or first minihalos.
3. They may offer support for the bottom-up scenario.
4. They have been predicted but never been observed.
5. They are dominated by dark matter.
6. They have fewer stars than globular clusters do.

ANS: C         DIF: Medium        REF: 23.3 MSC: Understanding OBJ: Describe the properties of the first generation of galaxies and whether these are based on observations or hypotheses.

1. The processes of galaxy and star formation differ in all but one of the following aspects. Which one?
2. Star formation follows a “top-down” sequence whereas galaxy formation involves a “bottom-up” sequence.
3. Dark matter is essential in the formation of galaxies, but it is not involved in the formation of stars.
4. Galaxy angular momentum originates in interactions of clumps whereas star angular momentum is caused by the turbulence of the original molecular clouds.
5. The formation of a star is a very slow process whereas a galaxy forms in a very short timescale due to the huge difference in gravitational forces at play.
6. When stars form, they acquire most of the mass of the collapsing system whereas in a Milky Way− like galaxy much of the matter remains distributed in a disk.

ANS: D         DIF: Medium        REF: 23.3 MSC: Evaluating OBJ: Compare and contrast the processes of star and galaxy formation.

1. Each of these statements describes the steps that occur during star formation. Which of these is not also true for galaxy formation?
2. Angular momentum leads to the formation of a disk.
3. For both stars and galaxies, the largest objects form first, with smaller objects coming together later.
4. A gas cloud radiates energy, allowing it to collapse further than when it was hotter.
5. Gravitational instability leads to collapse.
6. The original large gas cloud splits into smaller fragments, because areas with higher density also have greater gravitational pull.

ANS: B         DIF: Difficult       REF: 23.3 MSC: Analyzing OBJ: Compare and contrast the processes of star and galaxy formation.

1. Which of these lists shows the correct chronological order of the events listed, starting with the earliest and ending with the most recent?
2. reionization, dark matter halos collapse, recombination, first galaxies are formed
3. dark matter halos collapse, reionization, first galaxies are formed, recombination
4. reionization, first galaxies are formed, dark matter halos collapse, reionization
5. recombination, dark matter halos collapse, first galaxies are formed, reionization
6. first galaxies are formed, dark matter halos collapse, reionization, recombination

ANS: D         DIF: Difficult       REF: 23.3 MSC: Analyzing OBJ: Describe the properties of the very first generation of stars, and whether these are based on observations or hypotheses.

1. Low-mass stars could form in the second generation because
2. massive elements produced in the first stars coalesced into dust grains.
3. they formed in minihalos of cold dark matter.
4. gravitational waves led to the fragmentation of dark matter minihalos into microhalos.
5. there was a lot more raw material available for star formation after the demise of the first stars.
6. different types of fundamental forces controlled the formation of the first and second generation.

ANS: A         DIF: Difficult       REF: 23.3 MSC: Understanding OBJ: Explain why the first generations of stars were expected to be very massive, while subsequent generations could have high or low mass.

1. Choose the answer that does not correctly describe the second generation of stars.
2. They could have form with low mass, because they formed in cooler environments.
3. They contain small amounts of many massive elements.
4. A few of the low-mass second-generation stars have been identified in the halo of the Milky Way.
5. Their formation process is very different fromm that of the first generation stars.
6. The low-mass second-generation stars are very hot and luminous and thus easy to detect.

ANS: E         DIF: Difficult       REF: 23.3 MSC: Understanding OBJ: Explain why the first generations of stars were expected to be very massive, while subsequent generations could have high or low mass.

1. Panel (b) of the figure below shows the enhanced infrared glow obtained after nearby stars and galaxies are subtracted from a standard Spitzer Space Telescope exposure. Scientists ascribe this faint signature to
2. the CMB.
3. the first stars and galaxies formed.
4. GRBs and quasars.
5. our Sun.

ANS: B         DIF: Difficult       REF: 23.3 MSC: Understanding OBJ: Describe the properties of the first generation of galaxies and whether these are based on observations or hypotheses.

1. What do astronomers predict will be the final state of our universe?
2. a Big Crunch in which everything collapses back in on itself
3. an ever-expanding universe filled with nothing but hydrogen and helium gas
4. a universe that stops expanding and is filled with nothing but white dwarfs, neutron stars, and black holes
5. an ever-expanding universe filled with photons and elementary particles
6. a universe that stops expanding once enough stars become black holes

ANS: D         DIF: Easy              REF: 23.4 MSC: Understanding OBJ: Describe the expected stages of future evolution of our universe.

1. We expect the kinds of galaxies that we see at redshift of z ≈ 4 to be
2. much like what we see today.
3. smaller and much more irregular looking than today.
4. smaller versions of what we see today.
5. far more numerous but with fewer spiral galaxies.
6. larger versions of what we see today.

ANS: B         DIF: Easy              REF: 23.4 MSC: Applying OBJ: Summarize the observational evidence that galaxies formed by hierarchical merging.

1. Compared with what we see today, galaxies in the past were
2. more ordered and more likely to have spiral structure.
3. more ordered and less likely to have spiral structure.
4. messier and more likely to have spiral structure.
5. messier and less likely to have spiral structure.
6. exactly the same as they are today.

ANS: D         DIF: Easy              REF: 23.4 MSC: Applying OBJ: Relate the formation of supermassive black holes to galaxy evolution and star-formation rates.

1. Which of these statements about galaxy clusters is true?
2. Galaxy clusters do not require dark matter in order to form.
3. Galaxy clusters are the largest structures in the universe.
4. Small galaxy clusters form first and then merge together to form larger galaxy clusters.
5. Galaxy clusters are evenly distributed throughout the universe.
6. There are such large distances between galaxy clusters that they never actually run into each other.

ANS: C         DIF: Easy              REF: 23.4 MSC: Analyzing OBJ: Compare and contrast the results of cosmological simulations to measurements of cosmic structure.

1. Which probably formed last in the course of the evolution of the universe?
2. a typical proton inside a water molecule on the Earth
3. a helium atom in the surface of the Sun
4. a typical star that is a member of a globular cluster star in our Milky Way
5. the Milky Way
6. the Virgo Supercluster

ANS: E         DIF: Easy              REF: 23.4 MSC: Applying OBJ: Compare and contrast the results of cosmological simulations to measurements of cosmic structure.

1. Cosmologists estimate that the last stars will form about ________ years from now.
2. 109
3. 1014
4. 1065
5. 1098
6. 10100

ANS: B         DIF: Easy              REF: 23.4 MSC: Remembering OBJ: Describe the expected stages of future evolution of our universe.

1. Scientists estimate that, in the distant cosmic future, before the universe becomes filled exclusively with elementary particles, the last large concentrations of mass will be
2. white dwarfs.
3. neutron stars.
4. black holes.
5. brown dwarfs.
6. ultrafaint galaxies.

ANS: C         DIF: Easy              REF: 23.4 MSC: Remembering OBJ: Describe the expected stages of future evolution of our universe.

1. If dark energy is embedded within the vacuum of space, then the best places to study it would probably be the
2. spiral galaxies.
3. interacting, peculiar galaxies.
4. cosmic walls and filaments.
5. cosmic voids.
6. clusters of galaxies.

ANS: D         DIF: Easy              REF: 23.4 MSC: Applying OBJ: Compare and contrast the results of cosmological simulations to measurements of cosmic structure.

1. The largest cosmic supervoid ever discovered spans an estimated length of about 1.8 Gly. How many Milky Way galaxies would fit side-to-side within this apparently empty region in space?
2. 8
3. 18
4. 180
5. 18,000
6. 8 × 109

ANS: D         DIF: Easy              REF: 23.4 MSC: Applying OBJ: Compare and contrast the results of cosmological simulations to measurements of cosmic structure.

1. Which of the following is not likely to happen when two spiral galaxies collide?
2. A more massive elliptical galaxy might form out of the merger.
3. The two supermassive black holes at their centers could form a binary black hole system.
4. Individual stars collide to create many supernovae.
5. A burst of star formation will occur in the merged galaxy.
6. The cold gas in the merged galaxy might be blown away by supernovae.

ANS: C         DIF: Medium        REF: 23.4 MSC: Understanding OBJ: Summarize the observational evidence that galaxies formed by hierarchical merging.

1. By measuring the star formation rate in galaxies as a function of their redshift, we have learned that the average star formation rate in galaxies peaked approximately _________ years ago.
2. 1 billion
3. 3 billion
4. 5 billion
5. 7 billion
6. 11 billion

ANS: E         DIF: Medium        REF: 23.4 MSC: Remembering OBJ: Relate the formation of supermassive black holes to galaxy evolution and star-formation rates.

1. How has the fraction of galaxies with peculiar morphologies evolved from z ≈4 to the present epoch?
2. About 10 percent of today’s galaxies are peculiar, whereas at z ≈4 the fraction increases to more than half.
3. The fraction has been the same 50 percent over the last 4−5 billion years.
4. At the present epoch there are far more interacting galaxies than when the universe was about 9−10 billion years old.
5. The fraction has been unchanged 10 percent over the last 4−5 billion years.
6. There are no peculiar galaxies today, whereas at z ≈4 the fraction is close to 10 percent.

ANS: A         DIF: Medium        REF: 23.4 MSC: Evaluating OBJ: Summarize the observational evidence that galaxies formed by hierarchical merging.

1. Which of the following may not necessarily be indicative of a “bottom-up” scenario for galaxy formation?
2. Galaxies observed at very high redshift are typically 20 times smaller than the Milky Way.
3. There are many more peculiar galaxies at high redshift compared with those at low redshift.
4. Our own galaxy, the Milky Way, is classified as a barred spiral.
5. Quasars exist even at redshifts exceeding z =
6. The star formation rate has changed over cosmological time.

ANS: C         DIF: Medium        REF: 23.4 MSC: Analyzing OBJ: Summarize the observational evidence that galaxies formed by hierarchical merging.

1. Ignoring the effect of redshift, we expect the galaxies that we see at a redshift of z = 3 will be _____________ than galaxies today.
2. more irregular and redder
3. larger and redder
4. smaller and bluer
5. smaller and redder
6. larger and bluer

ANS: C         DIF: Medium        REF: 23.4 MSC: Applied OBJ: Relate the formation of supermassive black holes to galaxy evolution and star-formation rates.

1. Which of the following is not a reason that supercomputer cosmological simulations like Bolshoi are extremely valuable?
2. They produce images aesthetically suitable for the general public.
3. Comparing simulations to actual observational data sets constraints on fundamental parameters of the universe.
4. They can trace the evolution of both visible and invisible matter in the universe.
5. They can make predictions that can be cross-checked against observations.
6. They allow the study of a sequential process of galaxy evolution.

ANS: A         DIF: Medium        REF: 23.4 MSC: Analyzing OBJ: Compare and contrast the results of cosmological simulations to measurements of cosmic structure.

1. Which of the following is not a consequence of intergalactic encounters and mergers?
2. the rate of star formation
3. the activity of the galactic supermassive black holes
4. the formation of supermassive black holes
5. the changing proportions of various morphological types with the age of the universe
6. the cosmological recession of galaxies

ANS: E         DIF: Medium        REF: 23.4 MSC: Understanding OBJ: Relate the formation of supermassive black holes to galaxy evolution and star-formation rates.

1. The expansion of the universe will eventually render the CMB invisible because its photons will
2. have wavelengths that will exceed than the size of the observable universe.
3. be swamped by the light from the ever-increasing star-formation rates in the universe.
4. not have enough energy to escape the gravity of degenerate stellar remnants.
5. all combine and again produce pairs of massive particles.
6. not escape the strong gravity of supermassive black holes the size of galaxy clusters.

ANS: A         DIF: Medium        REF: 23.4 MSC: Analyzing OBJ: Describe the expected stages of future evolution of our universe.

1. Which of these statements about black holes in the early universe is not true?
2. Supermassive black holes affected the star formation rates in early galaxies.
3. The growth of supermassive black holes is likely linked with galaxy mergers.
4. The first generation of stars had high enough masses to leave black holes behind after exploding as supernovae.
5. The black holes in the early universe should have been larger than those seen today.
6. Supermassive black holes may have formed from mergers of smaller black holes.

ANS: D         DIF: Difficult       REF: 23.4 MSC: Analyzing OBJ: Relate the formation of supermassive black holes to galaxy evolution and star-formation rates. SHORT ANSWER

1. Describe the large-scale structure of the universe.

ANS: The structure of the universe is somewhat like a sponge; galaxies and clusters of galaxies lie along walls and filaments that surround cosmic voids in space that contain much fewer galaxies. DIF: Easy    REF: 23.1         MSC: Analyzing OBJ: Compare and contrast groups, clusters, superclusters, and walls.

1. In the context of the galaxy clusters, rank the proportions of stellar, intergalactic (intra-cluster) X-ray− emitting gas, and dark matter mass.

ANS: In general the mass of dark matter in galaxy clusters is much larger than the mass of the intra-cluster X-ray−emitting gas. Even though this hot gas is diffuse and thus low in density, the colossal volume it fills within the cluster makes for a huge amount, which can exceed up to five times the combined luminous mass of all the stars that belong to the numerous galaxy members. The combined mass of stars and hot gas does not typically exceed 10 percent of the total mass of the system, the rest being ascribed to the (unseen) dark matter. DIF: Easy    REF: 23.1         MSC: Remembering OBJ: Summarize the evidence for dark matter dominating the mass of groups and clusters.

1. Studying galaxies of various morphological types, scientists have estimated equivalent mass-to-light ratios (M/L) of about 1−30 in solar units (1 M/ 1 L), being lower for spiral and higher for elliptical galaxies. In galaxy clusters, however, the typical total mass-to-light ratio is estimated in the range 300−600 (M/L). What is the implication of these numbers?

ANS: The much higher M/L ratio for galaxy clusters indicates that on a larger scale there is a huge contribution from the invisible (dark matter). The M/L is a very simple way to reveal the dark matter contribution on different scales within the large-scale structure of the universe. DIF: Easy    REF: 23.1         MSC: Understanding OBJ: Summarize the evidence for dark matter dominating the mass of groups and clusters.

1. Explain what the arcs seen in the Hubble Space Telescope image shown below represent.

ANS: The bluish arcs represent the distorted image of a background galaxy, located behind the cluster of bright galaxies seen at the center of the image. The mass of these galaxies distorts the spacetime and acts as a gravitational lens, changing the path of light from the otherwise hidden background galaxy. Based on how bright the arcs appear, one estimates that an amplification of light is also involved; in other words, the lensed galaxy appears more luminous than it really is. The hubblesite.org image archive provides the following additional information: the background galaxy is about 10 billion ly away; the foreground cluster coined RCS2 032727-132623 is twice as close to us. DIF: Easy    REF: 23.1         MSC: Applying OBJ: Summarize the evidence for dark matter dominating the mass of groups and clusters.

1. Scientists quantified the following amounts of mass for the Coma cluster of galaxies: the total mass of the cluster is Mtotal ~3 × 1015 M, the total mass of all stars Mstars ~ 3 × 1013 M, and the mass of the hot X-ray−emitting gas Mgas ~ 1 × 1014 M. What is corresponding percentage of dark matter within the Coma cluster?

ANS: The total mass Mtotal = Mstars + Mgas + MDark Matter. Therefore the percentage of dark matter is given by MDark Matter / Mtotal ~ (3.3 × 1015 − 0.03 × 1015 − 0.1 × 1015) M/ 3.3 × 1015 M= 96% DIF: Medium                    REF: 23.1     MSC: Applying OBJ: Summarize the evidence for dark matter dominating the mass of groups and clusters.

1. Use Hubble’s law to explain how measurements of redshift help astronomers map out the large-scale structure of the universe.

ANS: Hubble’s law (v = H0 / D) states that, due to the expansion of the universe, the farther away a galaxy is from us, the faster it is receding. Astronomers measure a galaxy’s redshift imprinted in its spectral signature. The redshift is indicative of the changing scaling factor of the expanding universe; thus, it is a direct measure for the recession velocity. The velocity is further used to determine the distance to that galaxy, via Hubble’s law. DIF: Medium    REF: 23.1     MSC: Understanding OBJ: Describe how Hubble’s law helps astronomers map the structure of mass in the universe.

1. Describe two ways in which you could measure the mass of a galaxy cluster.

ANS: Here are three possible answers.

• Determine the mass needed to bind the hot intra-cluster X-ray−emitting gas to the cluster.
• Determine the velocities of galaxies in the cluster, and use their kinetic energy to estimate the total mass of the cluster.
• Determine the mass using the gravitational lensing of background galaxies.

DIF: Medium    REF: 23.1     MSC: Applying OBJ: Summarize the evidence for dark matter dominating the mass of groups and clusters.

1. How do we know how fast the Milky Way is moving relative to the local universe? What are we moving toward, and what do we think is mostly responsible for this motion?

ANS: The Doppler shift of the CMB tells us how fast we are moving relative to the CMB, which is the adopted rest-frame relative to the universe at large. We are moving toward a mass overdensity that is named the Great Attractor because of gravitational attraction. The Great Attractor represents the center of mass of a large superclsuter called Laniakea. The Milky Way and the Local Group are positioned at the outer edge of this supercluster. DIF: Difficult    REF: 23.1     MSC: Conceptual OBJ: Compare and contrast groups, clusters, superclusters, and walls.

1. What is the fundamental difference between the two types of dark matter candidates? List an example of each type.

ANS: Cold dark matter particles move relatively slowly (hence the description cold), whereas hot dark matter particles consist of particles that move very rapidly (hence the description hot). Two preferred candidates for cold dark matter are elementary particles called axions (very low mass) or photinos (very massive). Hot dark matter could consist of neutrinos, predicted and confirmed experimentally. DIF: Medium    REF: 23.2     MSC: Understanding OBJ: Compare and contrast the properties and possible constituents of cold and hot dark matter.

1. Why do astronomers think that cold dark matter (as opposed to hot dark matter) is the primary component of dark matter in galaxies?

ANS: Slowly moving particles (cold dark matter) require less mass to keep them bound gravitationally, whereas fast-moving particles can easily become unbound and be lost to space unless a much larger mass would hold them close via gravity. Only slow particles in cold dark matter could collapse to form the smaller, galaxy-size structures. The fast particles of hot dark matter could play a role in the formation of much larger structures like clusters and superclusters. Cosmological (computer) simulations support and confirm the scenario of cold dark matter dominating within galaxies. DIF: Medium    REF: 23.2     MSC: Understanding OBJ: Describe the collapse of cold dark matter into a galaxy halo after recombination.

1. How are the observed properties of the CMB leading to the idea that dark matter plays a crucial role in the formation of structure in the universe?

ANS: The CMB signature is very homogeneous, with random variations of the order of 10-5. Cosmologists show that these variations are too small to be conducive to the formation of galaxies and clusters, as we observe today. The models require many large-scale clumps to start with, and they require that these clumps later be amplified by gravity. Something else must be involved in the process, and that something is dark matter. It was clumpier than normal matter in the early universe, and it couldn’t have been smoothed out (homogenized) by radiation in the early universe, simply because dark matter does not interact with photons. DIF: Medium    REF: 23.2     MSC: Understanding OBJ: Describe how gravitational instabilities created the seeds of large-scale structure.

1. Explain why the physics of formation of the second generation stars is even more complex than that of the first generation.

ANS: The formation of stars in the second generation followed a very different “recipe” compared with that of the first-generation stars; the presence of dust grains (formed from heavy elements synthesized inside the first stars), extra molecules more complex than just H2, turbulence in the dusty gas that collapsed to form collections (clusters) of stars, and magnetic fields add to the complexity of the formation process. DIF: Medium    REF: 23.2     MSC: Evaluating OBJ: Describe the properties of the very first generation of stars and whether these are based on observations or hypotheses.

1. Based on the figure and chart shown below, roughly estimate the range of redshift defining (i.e., bracketing) the epoch of reionization.

ANS: Reading off the chart, during the 400 million to 900 million year interval, we would estimate (by interpolation within the tabulated markers) the redshift range to be 6 < z < 11. DIF: Medium    REF: 23.3     MSC: Applying OBJ: Describe the properties of the first generation of galaxies, and whether these are based on observations or hypotheses.

1. Explain why a galaxy can collapse to a much smaller size than its dark matter halo.

ANS: Dark matter collapses down until the random motions of its particles match the gravity. Dark matter cannot collapse any further because it does not dissipate its thermal energy and angular momentum by collisions or via emitted radiation. Normal matter, however, can collapse further as gas clouds collide and with one another and also release photons, dissipating their energy and angular momentum. DIF: Difficult    REF: 23.2     MSC: Applying OBJ: Relate the presence of dark matter to the formation of galaxies in the early universe.

1. What are some other possible candidates (macroscopic objects) for dark matter besides elementary particles?

ANS: Possible candidates could be imagined considering that they would be very faint and thus impossible to account for, especially at large distances. They could be degenerate stellar remnants (black holes, neutron stars, white dwarfs) or, transitioning into the domain of the faintest stars and failed stars (e.g., red dwarfs, brown dwarfs), planets, small fragments like the planetesimals, etc. The major issue is that such objects should exist in extremely large numbers to account for the large amount of “missing” mass. Or maybe we don’t have a complete understanding of how gravity operates on large scales, different from what we observe in tabletop experiments. DIF: Difficult    REF: 23.2     MSC: Creating OBJ: Compare and contrast the properties and possible constituents of cold and hot dark matter.

1. What is the only identified form of dark matter?

ANS: Neutrinos are the only type of dark matter that has been observed experimentally. They would belong to the so-called hot dark matter category, because of their almost luminal motion. They would not match the WIMP description; although the are indeed weakly interacting particles, their masses are probably very low, much lower than the expected mass rage for the massive particles within WIMP group. They have been around since the earliest times of the universe, being byproducts of high energy phenomena (nuclear fusion, nuclear decay, etc.). Scientists have not measured their masses yet, although they have good reason to suspect that it is many orders of magnitude less than the mass of an electron. Neutrinos are very elusive and hard to detect due to their poorly interacting nature. DIF: Difficult    REF: 23.2     MSC: Analyzing OBJ: Compare and contrast the properties and possible constituents of cold and hot dark matter.

1. What would be the observed wavelength for the Lyman-alpha emission at 121.6 nm from the distant galaxy at redshift z =68?

ANS: λobserved = (1 + z) λemitted = (1 + 8.68) × 121.6 nm = 1177 nm, which is in the infrared portion of the spectrum. (Note that the galaxy emitted a UV photon and it is “received” as an IR photon.) DIF: Easy    REF: Working It Out 23.2 MSC: Applying OBJ: Calculate the observed wavelength of highly redshifted light.

1. An astronomer wants to study the epoch of reionization, which occurred roughly in the range of 6 < z < What range of wavelength must the astronomer be able to detect corresponding to light emitted at 500nm?

ANS: λobserved = (1 + z) λemitted This means that λobserved would range between (1 + 6) × 500 nm = 3,500 nm = 3.5 µm and (1+11) × 500 nm = 60,000 nm = 60 µm. DIF: Medium    REF: Working It Out 23.2 MSC: Applying OBJ: Calculate the observed wavelength of highly redshifted light.

1. In the immediate aftermath of the recombination epoch, neutral hydrogen produced 21-cm photons. What would be the corresponding observed wavelength (at the present epoch) for such photons emitted at redshift z ~ 100?

ANS: λobserved = (1 + z) λemitted = (1 + 100) × 21 cm = 2,121 cm or about 21 m. DIF: Medium    REF: Working It Out 23.2 MSC: Applying OBJ: Calculate the observed wavelength of highly redshifted light.

1. The Hubble Space Telescope is equipped with an infrared camera WFC3/IR installed on it in 2009 during the last servicing mission, sensitive to photons up to about 1.7 µ It is estimated that the youngest (i.e., most distant) galaxy detected in the Hubble XDF (eXtreme Deep Field; see the images in the figure shown below) existed just 450 million years after the Big Bang. What is the emitted wavelength from that galaxy that was still detectable with the aforementioned infrared camera? (Note that the redshift is z ~ 10.2 when the universe has the indicated very young age.)

ANS: λobserved = (1 + z) λemitted; λemitted = λobserved / (1 + z) = 1.7 µm / (1 + 10.2) = 152 nm, which is in the UV domain. Extra note: The future James Webb Space Telescope (JWST) will be aimed at the XDF and it will find even fainter and likely younger galaxies that existed when the universe was just a few hundred million years old. JWST will be sensitive up to 28−µm photons, making it suitable to pierce into the XDF even deeper, capturing the epoch when the very first stars and galaxies formed and brought an end to the Dark Ages. DIF: Medium    REF: Working It Out 23.2 MSC: Applying OBJ: Calculate the observed wavelength of highly redshifted light.

1. The Mid-Infrared Instrument (MIRI) that will be attached to the James Webb Space Telescope covers the wavelength range of 5 to 28 microns. Calculate over what redshift range it can detect the 500 nm photons emitted by galaxies.

ANS: λobserved = (1 + z) λemitted, which means that z =λobserved / λemitted − 1. The closer limit is set by z = 5000 nm/500nm − 1 = 9 and the more distant limit is set by z = 28,000 nm/500nm − 1 = 55. Such observations would close the currently existing gap between 400,000 and 400 million years along the universe’s age sequence. DIF: Difficult    REF: Working It Out 23.2 MSC: Applying OBJ: Calculate the observed wavelength of highly redshifted light.

1. What was temperature and wavelength of the cosmic background blackbody signature at the epoch of reionization, about 400 million years after Big Bang?

ANS: The wavelength of photons obeys the “stretching” effect λobserved = (1 + z) λemitted. The CMB is probably the best example of a blackbody signature and therefore it evolves according to Wien’s law, λT = constant. This implies that as the wavelength gets longer, T is getting lower, i.e., cooler. So, Tobserved (1 + z) = Temitted. So, as 400 million years of age corresponds to z ~ 11, one can infer that corresponding temperature was Temitted = (1 + 11) × 2.73K = 32.8 K and the wavelength of the thermal signature was λemitted = 2.9 × 10-3 m K / 32.8 K = 88.4 µm, which was already shifted into the IR. DIF: Difficult    REF: Working It Out 23.2 MSC: Applying OBJ: Calculate the observed wavelength of highly redshifted light.

1. Explain why dark matter dominates the halos of galaxies but does not play a role in the formation of small structures (stars, planets, etc.) within galaxies.

ANS: Dark matter cannot collapse along with the normal matter to form galaxies, and it dominates the halos of the galaxies. Stars and planets form within the galaxies, where dark matter plays no role in assembling them. DIF: Medium    REF: 23.3     MSC: Understanding OBJ: Compare and contrast the processes of star and galaxy formation.

1. What stage in the evolution of the universe is coined as “the epoch of reionization,” and what is the significance of this name?

ANS: The epoch of reionization signifies the end of the “Dark Ages” of the universe, estimated to have covered the segment of age 400−800 million years. It begins with the time when the UV light from the first (massive) stars removed electrons from neutron hydrogen atoms, ionizing them. The process continued as more stars and even galaxies started to form. This is called reionization to emphasize that it is the second episode of this kind in the history of the universe, the first occurring before the CMB was liberated. DIF: Medium    REF: 23.3     MSC: Analyzing OBJ: Describe the properties of the very first generation of stars and whether these are based on observations or hypotheses.

1. What was the cooling mechanism that allowed the first stars to form?

ANS: The first stars must have been massive (tens of solar masses), mostly because the only “coolant” in the collapsing process was probably the molecular hydrogen. There were no other molecules or dust particles to cool the gas, as it was the case for the second and subsequent generations of stars. DIF: Medium    REF: 23.3     MSC: Evaluating OBJ: Explain why the first generations of stars were expected to be very massive, while subsequent generations could have high or low mass.

1. Explain and contrast the top-down vs. bottom-up scenarios for star, galaxy, and galaxy cluster formation, respectively.

ANS: The formation of galaxies is a hierarchical process in which the larger structures are building by merging smaller ones (“bottom-up,” from small to big). So, the first structures to form are the result of some collapsing clumps of mater and subsequently they merge into larger, more massive objects like galaxies and even clusters of galaxies). Individual stars, on the other hand, form as a result of fragmentation of large clumps of collapsing molecular clouds in what is called “top-down” (from big to small). DIF: Medium    REF: 23.3     MSC: Understanding OBJ: Compare and contrast the processes of star and galaxy formation.

1. One of the most distant galaxies observed to date shows a redshift z =68, whereas one of the most distant candidates (not fully confirmed yet) is a galaxy at a redshift of about z ~ 10.7. Why are such discoveries, rare for now, so exciting?

ANS: Scientists currently have little direct information from the so called Dark Ages period of the universe and from the subsequent transition into the epoch of reionization, which extends for several hundred million years after the Big Bang. Such observations probe the earliest stages when the first galaxies formed. Note that z = 8.68 would correspond to about 600 million years of age of the universe and the z ~ 10.7 would imply an even younger stage, about 400 million years after the Big Bang. DIF: Medium    REF: 23.3     MSC: Understanding OBJ: Describe the properties of the first generation of galaxies and whether these are based on observations or hypotheses.

1. Outline a few fundamental differences between the formation of the very first stars and that of the second or subsequent generations.

ANS:

• The first stars must have formed inside dark matter minihalos, which also contained a mixture of light elements: hydrogen, helium, and very little lithium. The second-generation-and-later stars formed within clouds of interstellar gas and dust enriched with more massive elements, dark matter having little to no effect in their formation.
• The first stars must have been massive (tens of solar masses), mostly because the only “coolant” in the collapsing process was probably the molecular hydrogen. Stars in later generations could form with less mass because dust (formed from massive elements synthesized by first stars) was an effective coolant, along with molecules in the cold molecular clouds.
• The first stars likely didn’t form in numerous groupings, whereas later generations star involved formation in clusters.

DIF: Difficult    REF: 23.3     MSC: Evaluating OBJ: Explain why the first generations of stars were expected to be very massive, while subsequent generations could have high or low mass.

1. How do astronomers explain the formation of elliptical galaxies?

ANS: Most elliptical galaxies likely formed by mergers between smaller galaxies. As a result of such a merger, any structure that was present in the galaxies would have disappeared, leaving behind an elliptical shape. In the burst of star formation that the merger induces, the gas is expelled by supernovae explosions and leads to the cessation of star formation. DIF: Easy    REF: 23.4         MSC: Applying OBJ: Describe the hierarchical formation scenario that built up fragments into today’s galaxies.

1. Explain how computer simulations of structure formation and observations of the structure in the universe today can help astronomers determine the nature of dark matter.

ANS: Only simulations with specific types and amounts of dark matter result in the formation of the types of structures that actually exist in the universe. If the dark matter parameters that were used in the simulation cannot result in the structures we see today, then the universe could not actually be dominated by those “failed” types of dark matter. Such simulation cannot tell scientists what dark matter is, but they help by ruling out the unlikely candidates. DIF: Medium    REF: 23.4     MSC: Understanding OBJ: Compare and contrast the results of cosmological simulations to measurements of cosmic structure.

1. Based on the snapshot of a Bolshoi cosmological simulation in the figure below, roughly estimate the size of the largest void captured in the frame. Note that each side of the square field in the simulation represents 89 Mpc.

ANS: A fast visual estimate (white line for guiding the eye) would suggest that 2.5 × length of blue line ~ 89 Mpc. So, the widest void would span about 35 Mpc or 115 Mly. DIF: Medium    REF: 23.4     MSC: Applying OBJ: Compare and contrast the results of cosmological simulations to measurements of cosmic structure.

1. Explain why the Bullet Cluster is considered a strong example for the existence of dark matter, based on the figure below.

ANS: The Bullet Cluster represents the collision of two galaxy clusters. A smaller one (on the right side in the figure) has moved through the bigger one (shown on the left) at high speed. The image shows distinct and relevant parts of this cosmic natural laboratory: 1) the visibly bright “tiny” individual galaxy members, along with 2) the superheated intra-cluster /intergalactic X-ray−emitting gas (red), and 3) the total mass distribution mapped via gravitational lensing effects. As the Bullet cluster has pierced through the larger “target” cluster, the intra-cluster gas is responding and slowing down because of collision. The total amount of matter with measured gravitational effects on the light from background galaxies (shown in blue) seems to have stayed with the galaxies and not suffered from this collision a spatial displacement. It is this decoupled behavior between gas and the rest of the mass that supports the notion that the total mass is dominated by poorly interacting particles called dark matter. DIF: Difficult    REF: 23.4     MSC: Analyzing OBJ: Summarize the observational evidence that galaxies formed by hierarchical merging.

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