Question 1

Red and orange stars are found evenly spread throughout the galactic disk, but blue stars are typically found ________.

Accepted Answer

Blue stars are typically young and massive, and thus are commonly found in or near star-forming regions such as nebulae and clouds. Red and orange stars, on the other hand, are older and less massive, and can be found throughout the galactic disk.

Question 2

How did star formation likely proceed in the protogalactic cloud that formed the Milky Way?

Accepted Answer

The stars that formed first in the protogalactic cloud had random orbits and inclinations, which is characteristic of the halo stars in the Milky Way.

Question 3

Which of the following statements comparing halo stars to our Sun is not true?

Accepted Answer

Most stars in the halo are much older than the Sun, and therefore have completed their lifetimes already. The Sun, on the other hand, is still in the middle of its lifetime. Therefore, most stars in the halo are not in their final stages of life, as stated in choice A.

Question 4

Which of the following statements is not true of the object known as Sgr A* in the center of our Galaxy?

Accepted Answer

Sgr A* is not the brightest source of visible light in the direction of the galactic center. It is actually heavily obscured by dust and gas, and emits mostly in the infrared and radio parts of the spectrum.

Question 5

If you could watch a time-lapse movie of the interstellar medium over hundreds of millions of years, what would you see?

Accepted Answer

Watching a time-lapse movie of the interstellar medium over hundreds of millions of years would show gas that is constantly changing form between molecular clouds, atomic hydrogen, and hot, ionized bubbles and superbubbles. The gas is often moving at high speeds, particularly after one or more supernovae. This would make for an interesting and dynamic movie.

Question 6

Suppose you want to observe and study the radiation from gas inside an interstellar bubble created by a supernova. Which of the following observatories will be most useful?

Accepted Answer

Gas inside an interstellar bubble created by a supernova emits X-rays, which can be observed by the Chandra X-ray Observatory. The Keck I telescope is not sensitive to X-rays, and the SOFIA airborne infrared observatory is designed to observe infrared radiation, which is not emitted by the gas inside the bubble. The Hubble Space Telescope can observe visible and ultraviolet light, but not X-rays.

Question 7

What is the best evidence for an extremely massive black hole in the center of the Milky Way?

Accepted Answer

The orbits of stars in the center of the galaxy can be measured and used to calculate the mass of the central object. These measurements have shown that there is a mass of 3 to 4 million solar masses in a region no larger than our Solar System, which is best explained by the presence of an extremely massive black hole. The other options may provide supporting evidence, but the most direct evidence comes from the stellar orbits.

Question 8

All the following types of objects are found almost exclusively in the disk (rather than the halo) of the Milky Way except ________.

Accepted Answer

Globular clusters are found primarily in the halo of the Milky Way, not the disk.

Question 9

What evidence suggests that most of the mass of the Milky Way is in the form of dark matter?

Accepted Answer

The high orbital speeds of stars far from the galactic center suggest the presence of additional mass that is not accounted for by the visible matter, which is a characteristic of dark matter.

Answer: 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, which implies the existence of a large amount of dark matter.

Answer: B and D are not correct. Although dark matter emits no visible light, it does not emit radio emissions either, nor does it cause dark blotches that obscure our view of distant galaxies.

Question 10

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?

Accepted Answer

A) Halo stars, which orbit in the halo of the Milky Way, often have highly elliptical orbits that can bring them through the disk at high speeds, unlike the more circular orbits of disk stars. This explains why a star moving at such a high speed relative to the Sun is likely a halo star.

Question 11

Spiral arms appear bright because ________.

Accepted Answer

Spiral arms appear bright because they contain more hot young stars than other parts of the disk. These young stars are massive and bright, emitting a lot of light that is easily visible from a distance.

Question 12

If we could watch spiral arms from a telescope situated above the Milky Way over 500 million years, what would we see happen?

Accepted Answer

If we could watch spiral arms from a telescope situated above the Milky Way over 500 million years, we would see stars moving through the spiral arms, bunching up closer as they pass through. We would also observe young hot stars forming and eventually dying within the arms before having a chance to move out. This is because the spiral arms are regions of higher density where new stars can form due to the compression of gas and dust. They also act as a gravitational trap, keeping young stars within their boundaries for a short period before releasing them into the rest of the galaxy.

B, C, and D are not accurate because the winding up or fading away of spiral arms and the unwinding of the arms are not supported by current understanding of the dynamics and lifespan of spiral arms.

Quiz 14: Our Galaxy