Chapter 15

Using star counts, William and Caroline Herschel concluded that a. the Milky Way is made of stars. b. stars differ in mass. c. stars are not evenly distributed in our galaxy d. the Milky Way galaxy is disk-shaped. e. there are fewer massive stars than smaller stars.

What role did interstellar dust play in the quest to determine the shape and size of our galaxy? a. It magnified the light from stars, making them appear closer. b. It obscured some regions and made others appear dimmer and redder. c. It outlined the Milky Way's structure. d. It played no role; dust is not important on a galactic scale. e. It made stars appear bluer.

With which of the four fundamental forces must dark matter interact? Check all that apply. a. gravity b. electromagnetism c. weak nuclear force d. strong nuclear force e. none

The galactic disk is not characterized by a. spiral arms. b. a flat shape. c. stars, gas, and dust. d. mostly old, red stars. e. some massive, young stars.

In which component of the Milky Way do gas and stars bob slightly above and below the galactic plane? a. disk d. dark matter halo b. bulge e. bar c. stellar halo

Which statement about Population I and Population II stars is true? a. Population I stars are lower in metallicity b. Population I stars are found in the halo c. Population I stars are older d. Population II stars may be supermassive. e. Population II stars are older

A \(0.5-M_{\text {sun }}\) red giant star with high metallicity. a. can be younger than the Sun. b. is composed of primordial material. c. is probably a member of Population I d. may be found in a globular cluster. e. could not be located on a spiral arm.

Which of the following is not evidence that globular clusters formed very early in the history of the Milky Way? a. They all formed at the same time. b. They have low metallicity. c. Some have young, blue stars. d. Some have retrograde galactic orbits. e. They are nearly as old as the Universe.

What can be seen through Baade's Window? a. the nearest large spiral galaxy b. a spiral arm c. the central black hole d. the central region of the galaxy e. a globular cluster

Which statement about dark matter is true? a. It is denser toward the outer regions of the Milky Way. b. It represents 10 percent of total galactic matter. c. It does not respond to gravity. d. It ends at the average orbital radius of the globular clusters. e. Its nature is not yet understood.

Which of the following is not evidence of spiral arms in the Milky Way? a. the path of the Sun on the sky b. patterns of \(\mathrm{O}\) and \(\mathrm{B}\) stars c. \(\mathrm{CO}\) maps of the galaxy d. the distribution of hydrogen seen via the 21 -cm line e. the appearance of spiral arms in similar galaxies

Our Solar System orbits the center of the galaxy with approximately what period? a. 1 year b. 24 hours c. 200 million years d. 4.5 billion years e. 13 billion years

The term starburst refers to a. the death of a massive star. b. a collision between stars that annihilates both. c. a rapid increase in star formation. d. the merger of many stars into a black hole. e. the implosion of a Sun-like star.

Evidence for a central black hole in our galaxy includes all of the following except a. strong radio emission. b. stars orbiting rapidly around the potential black hole's position. c. the scarcity of brown dwarfs. d. a large apparent concentration of mass in a small volume of space. e. X-ray flares.

The age of the Milky Way Galaxy is about 13 billion years, but our Solar System is less than 5 billion years old. If the Sun had been one of the very first stars, how would the Sun and its evolution differ? Choose all that apply. a. The Sun would have lower metallicity. b. The Sun would already have reached the end of its fusion lifetime. c. The Sun would likely be in the bulge. d. There would be a lack of terrestrial planets because of the Sun's low metallicity. e. The Sun would be a more massive star.

Which statement(s) about galaxy groups is/are true? Choose all that apply. a. The largest galaxies tend to have satellite galaxies. b. Interactions between massive galaxies and their satellites tend to affect the satellites more. c. Galaxy mergers are rare in galactic evolution. d. Tidal streams can result from galaxy interactions. e. The Local Group is one of the largest galaxy groups.

True/False: Stars near the galactic center of the Milky Way are generally members of Population I.

In what ways did dust inhibit early astronomers from obtaining an accurate picture of our galaxy?

Where in the Milky Way is our Solar System located?

What are the so-called peculiar motions of disk stars?

Describe the differences in age, color, metallicity, and location of Population I and Population II stars. Can you explain why we would expect the age, color, and metallicity of stars to be interrelated?

How does the arrangement of globular clusters in our galaxy provide a clue to the timing of their formation?

How did the study of the galactic rotation curve help lead to the concept of dark matter?

Describe how star orbits near the center of the galaxy provide additional evidence for the existence of a massive black hole.

"You can't see the forest for the trees." How is this saying relevant to the study of the Milky Way? What are some other situations you've experienced when it has been difficult to discern the big picture because you were immersed in it?

The nomenclature for Populations I and II is somewhat counterintuitive, since Population II is older. What other examples of astronomical nomenclature have you learned that are similarly counterintuitive?

The average orbital radius of a star around a galactic black hole has an angular size of 0.25 arcsecond when observed from a distance of 6.2 kpc. What is the orbital radius in kilometers? in astronomical units?

The orbital radius of a star orbiting \(\operatorname{Sgr} \mathrm{A}^{*}\) is \(3.45 \times 10^{11}\) kilometers. Observed from a distance of \(7.46 \mathrm{kpc}\), what is its angular size in arcseconds?

Calculate the mass of a black hole, in solar masses, based on the orbit of a star with period 7.8 years and orbital radius \(2.8 \times 10^{11} \mathrm{km} .\)

Calculate the orbital velocity, in kilometers per second, of a star orbiting 15,000 pc from the center of a galaxy whose mass is 130 billion \(M_{\mathrm{Sun}}.\)

Star A orbits at 5,000 pc from the Milky Way's center; Star B orbits at 7,500 pc from the center. In the absence of dark matter, what would be the expected ratio of A's velocity to that of B? Assume that all of the luminous matter can be considered to reside in the galactic center.

How many times faster would a star's orbital velocity be at the Sun's position, compared with that of a star orbiting at 3.7 times the distance from the galactic center, assuming that dark matter is not a factor and that all the mass of the luminous matter can be considered to reside in the galactic center?

A star orbits in its galaxy at the same orbital radius as the Sun, but the mass of the galaxy is 2.6 times that of the Milky Way Galaxy. Assuming the ratio of the galactic mass lying within its orbit is the same as that of the Sun, how does the orbital velocity of this star compare to that of the Sun?

A globular cluster has an orbital radius of 25,000 pc. Using a galactic mass of \(1.0 \times 10^{12} M_{\text {Sun }}\) for both luminous and dark matter combined (and assuming that all the mass lies within the cluster's orbit), what is the orbital velocity of the globular cluster, in kilometers per second?