Chapter 19
Conceptual Quiz
Part A
All the following types of objects are found almost exclusively in the disk (rather than halo) of the Milky Way EXCEPT ______.
Hint A.1
Study Section 19.3
ANSWER:

high-mass, red supergiant stars globular clusters young stars x-ray binaries
Part B
Applying the orbital velocity law to the a star orbiting 40,000 light-years from the center of the Milky Way galaxy allows us to determine ______.
Hint B.1
Study Section 19.4
ANSWER:

the percentage of the galaxy's mass that is made of dark matter. the mass of the Milky Way Galaxy that lies within 40,000 light-years of the galactic center. the mass of the black hole thought to reside in the center of the galaxy. the total mass of the Milky Way Galaxy.
Part C
By observing the 300 stars nearest Earth, we know that no star clusters have formed recently in our neighborhood because ______.
Hint C.1
Study Section 19.3
ANSWER:

there are no nearby stars of spectral type O or B. the stars have varying proportions of heavy elements. Clusters are typically born with thousands of stars. all of the stars are clumped together on one side.
Part D
How does the diameter of the disk of Milky Way Galaxy compare to its thickness?
Hint D.1
Study Section 19.2
ANSWER:

The diameter is about 100 times greater than the thickness. The diameter and thickness are roughly equal. The diameter is about 100,000 times greater than the thickness. The diameter is about 10 times greater than the thickness.
Part E
How does the interstellar medium affect our view of most of the galaxy?
Hint E.1
Study Section 19.1
ANSWER:

It absorbs all wavelengths of light. It produces so much visible light that it is opaque and blocks our view of anything beyond it. It has no effect on visible-light observations, but prevents us from studying the galactic center with radio waves or X-rays. It prevents us from seeing most of the galactic disk with visible, ultraviolet, and some infrared light.
Part F
How should we expect that the interstellar medium of the Milky Way will be different in 50 billion years?
Hint F.1
Study Section 19.2
ANSWER:

The total amount of gas will be about the same, but it will contain a much higher percentage of elements heavier than hydrogen and helium. Thanks to the recycling of the star-gas-star cycle, the interstellar medium should look about the same in 50 billion years as it does today. The total amount of gas will be much greater, since many stars will undergo supernovae between now and then. The total amount of gas will be much less than it is today.
Part G
How would you expect a star that formed recently in the disk of the galaxy to differ from one that formed early in the history of the disk?
Hint G.1
Study Section 19.2
ANSWER:

It should be much brighter. It should orbit the galactic center at a much higher rate of speed. It should have a higher fraction of elements heavier than hydrogen and helium. It should be higher in mass.
Part H
If we could see our own galaxy from 2 million light-years away, it would appear ______.
Hint H.1
Study Section 19.1
ANSWER:

to be a flattened disk with a central bulge and spiral arms. to fill the sky with widely spaced stars. as a faintly glowing band of light stretching all the way around the sky. like a single, dim star.
Part I
If you could watch a time-lapse movie of the interstellar medium over hundreds of millions of years, what would you see?
Hint I.1
Study Section 19.2
ANSWER:

Gas that changes only in very slow and steady ways, so that the movie would in fact be quite boring. Gas that is often moving at high speed, particularly after one or more supernovae, and constantly changing form between molecular clouds, atomic hydrogen, and hot, ionized bubbles and superbubbles. The entire disk of the Milky Way would pulsate in and out as it contracts to form stars and then blows out in supernovae and then contracts to form stars again and so on. The movie would alternate back and forth between being very bright when there is a lot of gas and very dark when there is very little gas.
Part J
Most nearby stars move relative to the Sun at speeds below about 30 km/s. Suppose you observe a nearby star that is moving much faster than this (say, 300 km/s). Which of the following is a likely explanation for its high speed?
Hint J.1
Study Section 19.2
ANSWER:

It is probably a halo star that is currently passing through the disk. It is a very young star, recently formed. It has been pushed to high speed by the shock wave from a nearby supernova. It is a very high mass star.
Part K
Red and orange stars are found evenly spread throughout the galactic disk, but blue stars are typically found ______.
Hint K.1
Study Section 19.3
ANSWER:

also evenly spread throughout the galactic disk. only in the central bulge. only near or in star-forming regions. in the halo.
Part L
Suppose a scientist holds a press conference at which he claims that 10% of the matter in the Milky Way is in the form of dust grains. In light of what we think we now know, does his claim seem reasonable? Why or why not?
Hint L.1
Study Section 19.2
ANSWER:

The 10% figure seems far too low, because current observations show that most of the mass of the galaxy is in the form of interstellar dust. It seems reasonable as long as we assume that red giant stars --- which produce dust grains in their stellar winds --- are more common than we thought. It is quite reasonable, because we already know that interstellar dust obscures our view through the disk of the galaxy. The 10% figure seems far too high because there are not enough of the kinds of elements that can take solid form in the galaxy.
Part M
After several star-gas-star cycles, there is
Hint M.1
Study Section 19.2
ANSWER:

Less gas and more heavy elements. More gas and less heavy elements. Less gas and less heavy elements. More gas and more heavy elements.
Part N
The most common form of gas in the disk of the Milky Way galaxy is _______.
Hint N.1
Study Section 19.2
ANSWER:

gas in stellar winds gas in hot bubbles atomic hydrogen gas molecular hydrogen
Part O
How did star formation likely proceed in the protogalactic cloud that formed the Milky Way?
Hint O.1
Study Section 19.4
ANSWER:

Gradually, the protogalactic cloud formed stars, starting from the center of the galaxy working outwards. The stars that formed last are on the far outskirts of the galaxy. Gas clouds along with the stars that formed first eventually settle into a galactic disk, circling the center of the galaxy. Gradually, the protogalactic cloud formed stars, starting from the outside regions of the galaxy. The stars that formed last are on the innermost orbits of the Galactic center. The stars that formed first orbit the center of the galaxy in any direction.
Part P
What observational evidence supports the galactic fountain model (which describes how gas cycles between the disk of the galaxy and regions high above the disk)?
Hint P.1
Study Section 19.2
ANSWER:

We have discovered that the entire galactic disk is being uniformly "rained on" by cool gas coming from the halo. We see hot gas high above the region of the disk near our solar system, along with cool gas that appears to be raining down from the halo. We have observed a lot of water molecules in the interstellar medium. We have discovered a jet of ionized gas shooting out of the bulge of our galaxy.
Part Q
Which of the following statements about how halo stars compare to our Sun is NOT true?
Hint Q.1
Study Section 19.3
ANSWER:

Most stars in the halo have cooler surface temperatures than the Sun. Most stars in the halo contain a much lower percentage of heavy elements than the Sun Most stars in the halo are in a more advanced stage of life than the Sun, such as in red giant stage. Most stars in the halo are less luminous than the Sun
Part R
Which of the following statements is NOT true of the object known as Sgr A* in the center of our Galaxy?
Hint R.1
Study Section 19.4
ANSWER:

It is thought to harbor a black hole of more than 2 million solar masses. It is a source of X-ray emission that we have observed with telescopes in space. It is a source of bright radio emission. Its accretion disk is so large that it shines brightly in visible light.
Part S
If we could watch spiral arms (imagine a telescope situated above the Milky Way) over 500 million years, what would we see happen?
Hint S.1
Study Section 19.2
ANSWER:

The spiral arms will eventually unwind, as centripetal forces send the stars flying outwards into intergalactic space. The spiral arms will eventually dissipate and fade away, since they are a temporary phenomenon that should only last for a million years or so. The spiral arms will seem to "wind up", to wrap more and more tightly around the center of the Galaxy. Stars will move through the spiral arms, bunching up closer as they pass through. Young hot stars will form and die within the arms before having a chance to move out.
Part T
What is the best evidence for an extremely massive black hole in the center of the Milky Way?
Hint T.1
Study Section 19.4
ANSWER:

We can see stars vanishing in the center of the Galaxy as they are sucked into the black hole and vanish forever from our sight. The center of our galaxy hosts a pulsar which is spinning so fast that it could only be a black hole. Huge amounts of X-rays are pouring out of the center of the galaxy, an incredibly high luminosity which could only be sustained by an accretion disk surrounding a super massive black hole. The 3-D motions of stars in the center of the Galaxy and Newton's version of Kepler's 3rd law indicate that the central object has a mass of 3-4 million solar masses in a region no bigger than our Solar System.