The Essential Cosmic Perspective, 8e (Bennett et al.)
Chapter 15 Our Galaxy
15.1 Multiple Choice Questions
1) What is the approximate diameter of the disk of the Milky Way?
A) 100 light-years
B) 1,000 light-years
C) 10,000 light-years
D) 100,000 light-years
E) 1,000,000 light-years
2) On a dark summer night in the northern hemisphere, we can see the Milky Way but we cannot
see the center of the Milky Way. Why not?
A) We are in the center of the Milky Way.
B) There are no stars in the center of the Milky Way, just a supermassive black hole.
C) Interstellar dust and gas absorbs and scatters visible light before it can reach us from the
center.
D) The center of the Milky Way does not emit enough visible light.
E) Stars are too densely spaced in the plane of our galaxy to see all the way to the center.
3) What kinds of objects lie in the halo of our galaxy?
A) open clusters
B) O and B stars
C) globular clusters
D) molecular clouds
E) all of the above
4) What kinds of objects lie in the disk of our galaxy?
A) open clusters
B) O and B stars
C) old K and M stars
D) gas and dust
E) all of the above
5) What makes up the interstellar medium?
A) open clusters
B) O and B stars
C) K and M stars
D) gas and dust
E) all of the above
6) If you were to take a voyage through the entire disk of the Milky Way, what kind of material
would you spend most of your time in?
A) empty space—a pure vacuum
B) hot bubbles of ionized hydrogen
C) star clusters
D) rarefied clouds of atomic hydrogen
E) cool, dense clouds of molecular hydrogen
7) How does the interstellar medium obscure our view of most of the galaxy in visible light?
A) Hydrogen gas produces so much visible light that the interstellar medium is opaque, blocking
our view of anything beyond it.
B) Dust reflects most light from distant regions of the galaxy back towards the source.
C) Molecules in the interstellar medium absorb all wavelengths of light.
D) The small mixture of dust grains in the interstellar medium absorbs visible light.
E) all of the above
8) How can we see through the interstellar medium?
A) by observing at wavelengths of radiation that are not absorbed by interstellar dust
B) by observing only the brightest stars in the galaxy
C) by observing the distribution of cool stars in the disk
D) by observing with telescopes above the Earth’s atmosphere, such as the Hubble Space
Telescope
E) We cannot see through the interstellar medium.
9) Harlow Shapley concluded that the Sun was not located at the center of the Milky Way
Galaxy by
A) looking at the shape of the “milky band” across the sky.
B) mapping the distribution of stars in the plane of the galaxy.
C) mapping the distribution of globular clusters in the galaxy.
D) mapping the distribution of gas clouds in the spiral arms.
E) looking at other nearby spiral galaxies.
10) Approximately how far is the Sun from the center of the galaxy?
A) 27 light-years
B) 270 light-years
C) 2,700 light-years
D) 27,000 light-years
E) 27 million light-years
11) Compared with stars in the disk, orbits of stars in the halo
A) are confined to a relatively thin plane.
B) are elliptical, with random orientations.
C) are elliptical, orbiting in the same direction.
D) do not have to be around the galactic center.
E) do not have to pass through the plane of the galaxy.
12) Approximately how long does it take the Sun to orbit the center of the Milky Way Galaxy?
A) 20,000 years
B) 200,000 years
C) 2 million years
D) 200 million years
E) 2 billion years
13) Why do we believe that most of the mass of the Milky Way is in the form of dark matter?
A) The orbital speeds of stars far from the galactic center are surprisingly high, suggesting that
these stars are feeling gravitational effects from unseen matter in the halo.
B) Although dark matter emits no visible light, it can be seen with radio wavelengths, and such
observations confirm that the halo is full of this material.
C) Theoretical models of galaxy formation suggest that a galaxy cannot form unless it has at
least 10 times as much matter as we see in the Milky Way disk, suggesting that the halo is full of
dark matter.
D) Our view of distant galaxies is sometimes obscured by dark blotches in the sky, and we
believe these blotches are dark matter located in the halo.
14) Approximately how long does it take the sun to go through a full cycle of “bobbing” up and
down in the plane of the galaxy?
A) a few hundred years
B) a few thousand years
C) a few million years
D) a few tens of millions of years
15) What causes the sun to “bob” up-and-down in its orbit around the galactic center?
A) gravitational attraction towards other nearby galaxies
B) gravitational attraction towards the center of the galaxy
C) the localized pull of gravity from stars within the disk
D) dark matter in the halo of the galaxy
E) dark matter in the disk of the galaxy
16) What elements do astronomers consider heavy elements?
A) elements that are heavier than iron
B) elements that are heavier than carbon
C) elements that are heavier than hydrogen
D) elements that are heavier than uranium
E) all elements besides hydrogen and helium
17) Where are most elements heavier than hydrogen and helium made?
A) in the interstellar medium
B) in the cores of stars and in supernovae
C) All were made in the Big Bang, when the universe first began.
D) in the cool atmospheres of red giant stars
E) in white dwarfs
18) How do stars like the sun primarily transport newly formed elements into the interstellar
medium?
A) when they explode in white dwarf supernovae
B) when they are in the protostar stage
C) when they form planetary nebulae
D) when they form red supergiants
E) Stars like the sun do not recycle any elements back into the interstellar medium.
19) How do high mass stars transport newly formed elements into the interstellar medium?
A) through stellar winds and supernova explosions
B) through supernova explosions and the formation of planetary nebulae
C) through protostar formation and supernova explosions
D) through stellar winds and the formation of planetary nebulae
20) How are interstellar bubbles of hot, ionized gas made?
A) by the collapse of a gas cloud to form stars
B) by the ejection of planetary nebulae from low-mass stars
C) by the winds of massive stars and supernovae explosions
D) by collisions between the Milky Way and satellite dwarf galaxies
E) by the rapidly rotating magnetic fields of pulsars
21) Sound waves in the interstellar medium
A) cannot travel since they are in a vacuum.
B) travel so slowly that they are undetectable.
C) can travel through the gas, but the very low density of the interstellar medium makes them
inaudible.
D) travel extremely quickly and are therefore very loud.
E) can travel through the halo interstellar medium but not the disk of the galaxy.
22) What is a shock wave?
A) a pressure wave that moves faster than the speed of sound
B) a pressure wave that moves slower than the speed of sound
C) a pressure wave that moves faster than the speed of light
D) an electromagnetic wave that can create electrical shocks
E) an electromagnetic wave created when electrons recombine with protons
23) What is the most common form of gas in the interstellar medium?
A) molecular hydrogen
B) molecular helium
C) atomic hydrogen
D) atomic helium
E) ionized hydrogen
24) What produces the 21-cm radio emission line that can be used to map the Milky Way
Galaxy?
A) atomic hydrogen
B) ionized hydrogen
C) molecular hydrogen
D) carbon monoxide
E) helium
25) Where do most dust grains form?
A) in supernovae
B) in the winds of red giant stars
C) in planetary nebulae
D) in molecular clouds
E) all of the above
26) Suppose you read somewhere that 10 percent of the matter in the Milky Way is in the form
of dust grains. Should you be surprised? If so, why?
A) There is nothing surprising about the 10 percent figure because dust grains are the material
from which stars are born.
B) Given how easily dust grains can form, 10 percent is a surprisingly low fraction.
C) The fraction of 10 percent is surprisingly high because dust grains can form only at low
temperatures.
D) The fraction of 10 percent cannot be correct. Dust grains are solid and only 2 percent of the
matter in the galaxy is made of anything besides hydrogen and helium.
27) What is the most abundant molecule in interstellar clouds besides molecular hydrogen?
A) molecular helium
B) water
C) carbon monoxide
D) ammonia
E) methane
28) The image of our galaxy in radio emission from carbon monoxide maps the distribution of
molecular clouds. Which of the following would give a similar picture of our galaxy?
A) 21-cm-line radio emission from atomic hydrogen
B) visible light, showing the edges of supernova bubbles
C) visible light, which is closest to how the night sky appears from Earth
D) X-rays from hot gas bubbles in the disk
E) infrared emission from interstellar dust grains
29) Over time, what is the net effect of the star-gas-star cycle in the Milky Way?
A) The total mass in the galaxy’s interstellar medium is gradually increased, and the new gas is
continually enriched in heavy elements.
B) The total mass in the galaxy’s interstellar medium is gradually reduced, and the remaining gas
is continually enriched in heavy elements.
C) The temperature of the interstellar medium varies between the cool 10-50 degrees (Kelvin)
required to form stars and the 2,000-30,000 degree (Kelvin) temperatures of the outer layers of
the stars that form. But over time, more and more of the interstellar medium remains cool.
D) The gas of the interstellar medium is continually depleted in elements heavier than helium.
E) There is no net effect to the cycle. Mass in the interstellar medium lost to star formation is
exactly replaced by stellar winds and supernova explosions.
30) Where does most star formation occur in the Milky Way today?
A) in the halo
B) in the bulge
C) in the spiral arms
D) in the galactic center
E) uniformly throughout the galaxy
31) How do high-mass stars make it more difficult for a molecular cloud to form more stars?
A) Strong winds push gas away.
B) Ultraviolet photons heat the gas.
C) Ultraviolet photons ionize the gas.
D) Radiation pressure pushes ionized gas away.
E) all of the above
32) The galactic center lies in the direction of which constellation?
A) Orion
B) the Big Dipper
C) Leo
D) Sagittarius
E) Taurus
33) Which of the following indicates the presence of high mass stars, and thus recent star
formation?
A) closely spaced low mass stars
B) ionization nebulae
C) globular clusters
D) planetary nebulae
34) Compared to our Sun, most stars in the halo are
A) young, red, dim, and have fewer heavy elements.
B) young, blue, bright, and have more heavy elements.
C) old, red, dim, and have fewer heavy elements.
D) old, red, dim, and have more heavy elements.
E) old, red, bright, and have fewer heavy elements.
35) Why are we unlikely to find Earth-like planets around halo stars in the galaxy?
A) Planets around stars are extremely rare in our galaxy.
B) Halo stars formed from gas containing few heavy elements, which are needed to create
terrestrial planets like the Earth.
C) Halo stars formed from gas containing lots of heavy elements and few of the light elements
needed to create terrestrial planets like the Earth.
D) Halo stars are all very low mass stars that cannot hold onto planets.
E) Halo stars formed directly from gas in the halo without forming a planetary disk.
36) Why has star formation ceased in the galactic halo?
A) All the halo gas consists of elements with more protons than helium.
B) The halo has no gas at all.
C) There are no white dwarf binaries to trigger star formation via their supernovae.
D) All of the galaxy’s cool gas settles to the galactic plane.
37) What evidence suggests that the protogalactic cloud that formed the Milky Way resulted
from several collisions among smaller clouds?
A) The stars in the halo of the Milky Way are organized into several dense clusters.
B) The Milky Way resembles an elliptical galaxy more than other spirals do.
C) Halo stars differ in age and heavy-element content, but these variations do not seem to depend
on the stars’ distance from the galactic center.
D) The bulge of the Milky Way is surrounded by many globular clusters, just as elliptical
galaxies are.
E) The Milky Way is the central galaxy of a dense cluster of galaxies.
38) How do we learn about the conditions at the center of our own galaxy, the Milky Way?
A) High-resolution photographs obtained by the Hubble Space Telescope have revealed the
galactic center using visible light.
B) Radio, infrared and X-ray light from the galactic center can be detected and used to determine
the conditions there.
C) The gas and dust in the Milky Way prevent any type of direct observation of the galactic
center, but theoretical models allow us to predict what is happening there.
D) We must look at the centers of other (external) galaxies and assume that they are similar to
the Milky Way.
39) Which of the following does not accurately describe what we observe toward the galactic
center?
A) At radio wavelengths, we see giant gas clouds threaded by powerful magnetic fields.
B) At infrared wavelengths, we see a massive star cluster.
C) At optical wavelengths, we see a cluster of old, red stars.
D) At X-rays, we see faint emission from an accretion disk around a black hole.
40) What evidence supports the existence of a very massive black hole at the center of our
galaxy?
A) We observe an extremely bright X-ray source at the center of our galaxy.
B) We can see gas falling into an accretion disk and past the event horizon of a black hole.
C) The motions of the gas and stars at the center indicate that it contains 4 million solar masses
within a region no larger than our solar system.
D) We observe a large, dark object that absorbs all light at the center of our galaxy.
E) all of the above
41) What is Sgr A*?
A) a source of bright X-ray emission coming from the entire constellation of Sagittarius
B) a source of bright radio emission in the center of our galaxy
C) a source of bright ultraviolet light near the center of our galaxy
D) the brightest star in the constellation Sagittarius
E) the bulge at the center of our galaxy
42) What evidence suggests that some of the stars in the halo of the Milky Way Galaxy came
from mergers with other galaxies?
A) Globular clusters are typically found only in the halo.
B) Some halo stars move together in organized streams.
C) Halo stars are among the oldest in the galaxy.
D) Halo stars have randomly oriented orbits.
43) What is thought to be the origin of enormous X-ray flares that occasionally come from the
direction of Sgr A*?
A) They are likely from X-ray binary systems that lie close to the galactic center.
B) They are likely from massive-star supernovae near the galactic center.
C) They are likely from comet-sized objects being torn apart by the tidal forces of the black hole
near the galactic center.
D) They are likely from supernova shock waves plowing into dense regions of the interstellar
medium near the galactic center.
15.2 True/False Questions
1) Open clusters and young stars are generally found only in the disk of the Milky Way and not
in the halo.
2) We can see most of the Milky Way with visible light.
3) Observing our galaxy at radio wavelengths allows us to see through the dust in the disk that
obscures our view.
4) The Milky Way looks the same in X-rays as it does at infrared wavelengths.
5) The Sun is located near the edge of our galaxy, approximately 100,000 light-years from the
galactic center.
6) Shapley used the distribution of globular clusters in our galaxy to determine that the Sun was
not at the center of the Milky Way.
7) All heavy elements are made during supernova explosions.
8) The star-gas-star cycle will continue forever because stars are continually recycling gas.
9) Supernovae explosions generate fast moving particles called cosmic rays that can penetrate
Earth’s atmosphere.
10) Most of the mass of the Milky Way is thought to be located in the halo of the galaxy in the
form of dark matter.
11) Most of the current star formation in the Milky Way occurs in its spiral arms.
12) Most of the mass of our galaxy is located at the galactic center in the form of a massive black
hole.
13) The massive black hole near the center of our galaxy is what holds the galaxy together and
prevents the outer parts from flying off into intergalactic space.
15.3 Process of Science Questions
1) Modified Newtonian Dynamics: The case for dark matter in the Milky Way can be viewed as a
failure of stars and gas far from the galactic center to obey Newton’s law of gravitational
attraction—that is, the stars and gas seem to orbit too quickly for the visible matter their orbits
enclose. Why do you think that astronomers choose to “save” Newton’s form of gravitational
attraction by postulating unseen dark matter, as opposed to regarding the observed orbital motion
as a falsification of Newtonian gravitation on the scale of the Milky Way? [Note: At the distance
scales involved, the gravitational forces in the galaxy are all fairly weak, much weaker than those
encountered in a neutron star or black hole, and hence Einstein’s theory of gravity (General
Relativity) gives the same predictions as Newton’s gravitational law].
2) Observational Bias: The history of human understanding of the structure of the Milky Way
starts with the bias that humans use visible light to see. Imagine you were a creature on another
planet in the disk of the Milky Way who used radio waves to “see” the universe. What would
have been your starting point for the structure of the Milky Way? What components would you
have found easier to detect? What components would have been harder?
3) A History of Water: One of the triumphs of 20th century astronomy is a convincing
explanation for the origin of the chemical elements. Consider a single water molecule in your
body, consisting of two hydrogen atoms and a single oxygen atom. Trace, from the earliest point
in time that you can, the history of these atoms. List all of the possible environments in which
these particular atoms could have spent time.
4) Inferring cloud properties using carbon-monoxide: At the low temperatures of molecular
clouds, carbon monoxide emits radio light more strongly than does molecular hydrogen. What
assumptions must be made to then use observations of carbon monoxide to infer the properties of
the cloud as a whole?
15.4 Short Answer Questions
1) Suppose you discovered a star made purely of hydrogen and helium. How old do you think it
would be? Explain.