Complete Test Bank Chapter.15 Our Galaxy The Milky Way

Chapter 15: Our Galaxy: The Milky Way

LEARNING OBJECTIVES

15.1 Galaxies Are Mostly Dark Matter

15.1a Explain how the amount of matter inferred from electromagnetic radiation can differ from that inferred from gravity.

15.1b Summarize the observational evidence for dark matter.

15.1c Compare and contrast the behavior of normal and dark matter.

15.1d Summarize the candidates for dark matter and the observational evidence supporting or refuting each candidate.

15.1e Describe the Doppler shifts we expect to observe as we look at stars in different parts of the galaxy.

15.2 Stars Form in the Spiral Arms of a Galaxy’s Disk

15.2a Summarize the evidence for spiral arms being regions of star formation.

15.2b Describe how spiral patterns are formed and maintained in spiral galaxies.

15.2c Explain how observations of 21-cm radiation trace out the spiral arms of the Milky Way Galaxy.

15.3 Components of the Milky Way Reveal Its Evolution

15.3a Explain why the heavy-element abundances in stars are snapshots of the chemical makeup of the gas out of which the stars formed.

15.3b Understand the process of chemical evolution in our galaxy over time.

15.3c Compare and contrast the components of the disk and the halo of the galaxy.

15.3d Explain how we distinguish between halo and disk stars and between stars in the thin disk and thick disk.

15.4 The Milky Way Offers Clues about How Galaxies Form

15.4a Summarize the evidence that the Milky Way formed when a huge clump of dark matter collapsed.

15.4b Describe the timeline of our galaxy’s formation, from collapse of the dark matter clump until today.

Working It Out 15.1

Working It Out 15.1a Calculate the mass of a galaxy knowing the orbital details for one star inside it.

Chapter 15: Our Galaxy: The Milky Way

MULTIPLE CHOICE

1. What is observed to happen to the orbital velocity of stars as you measure them at increasing distances from the center of the galaxy?

a. They move slower and slower. c. They vary greatly in a random fashion.

b. They move faster and faster. d. They essentially stay the same.

2. How much of the total mass of a typical galaxy, like the Milky Way, is composed of dark matter?

a. 10 percent c. 50 percent

b. 30 percent d. 90 percent

3. Which of the following is a weakly interacting massive particle (WIMP)?

a. small main-sequence M star c. unknown neutrino-like particles

b. planet d. All are valid choices.

4. The most likely dark matter candidate seems to be

a. black holes c. neutron stars

b. planets d. WIMPs

5. If a compact object crosses our line of sight to a distant star, the star’s light will be gravitationally lensed and we will observe a

a. brightening of the star in all wavelengths.

b. brightening of the star in only short wavelengths.

c. brightening of the star in only long wavelengths.

d. dimming of the star in all wavelengths.

6. How can massive compact halo objects (MACHOs) be directly detected?

a. They are a source of neutrinos.

b. They emit 21-centimeter (cm) radiation.

c. Distant stars are brightened by gravitational lensing.

d. They are evident in galactic rotational velocity curves.

7. Which of the following is a MACHO?

a. black hole c. white dwarf

b. neutron star d. All are valid choices.

8. What observed property of the Milky Way suggests that it contains a large amount of nonluminous matter, unlike stars?

a. rotation curve c. number and shape of the spiral arms

b. velocities of the open star clusters d. thickness of the disk

9. Most of the mass in our galaxy is in the form of

a. stars. c. dust.

b. gas. d. dark matter.

10. Our Milky Way Galaxy shares the following property in common with other galaxies.

a. Most of the mass in our galaxy is in the form of dark matter.

b. Most of the mass in our galaxy is in the form of dust.

c. Most of the mass in our galaxy is in the form of hydrogen.

d. Most of the mass in our galaxy is in the form of stars.

11. Observations of Doppler shifts toward the center of the galaxy show on one side hydrogen clouds are blueshifted,whereas on the other side hydrogen clouds are redshifted. This is the pattern of the rotation velocity of a

a. cube.

b. disk.

c. sphere.

d. The shape cannot be determined from the information given.

12. Which of the following methods is used to measure the velocity and direction of motion of gas in the galaxy?

a. Doppler effect

b. main-sequence fitting on a Hertzsprung-Russell (H-R) diagram

c. parallax

d. transit method

13. Which of the following curves represents the actual rotational velocities (vertical axis) as a function of radial distance (horizontal axis) from the center of a typical spiral galaxy?

a. c.

b. d.

14. Which of the following curves represents the predicted rotational velocities (vertical axis) as a function of radial distance (horizontal axis) from the center of a typical spiral galaxy if all of the galaxy’s mass was traced by luminous matter?

a. c.

b. d.

15. Luminous or normal matter has which of the following characteristics?

a. It is made of protons, neutrons, and electrons.

b. It can emit electromagnetic radiation.

c. It can absorb electromagnetic radiation.

d. All of these choices are valid.

16. Which of the following regions of the electromagnetic spectrum is best used to study structure in the Milky Way?

a. gamma rays c. ultraviolet

b. radio d. X-rays

17. Why is 21-cm radiation the best way to map the spiral arms in the Milky Way?

a. The molecular hydrogen gas that produces this emission is concentrated in the spiral arms.

b. These photons, which are produced by neutral hydrogen, penetrate the dense clouds of gas and dust in the disk.

c. This synchrotron emission is produced by supernovae, which are concentrated in the spiral arms.

d. Radio telescopes are easier to operate than optical telescopes, and observations can be made even during the daytime.

18. Twenty-one-centimeter radiation is important because it

a. allows us to study the deep interiors of stars.

b. allows us to image magnetic fields directly.

c. allows us to study neutral hydrogen in the interstellar medium.

d. is produced by every object in the universe.

19. What will happen to the spiral structure of a galaxy that was setup by a single disturbance?

a. Its pattern will become permanent.

b. The spiral arms will unwind after several rotations.

c. The spiral arms will completely wind themselves up after several rotations.

d. Any of these choices may occur.

20. What is likely the mechanism involved in sustaining a galaxy’s spiral shape?

a. bursts of stellar formation

b. gravitational interactions with other galaxies

c. gravitational interactions with an elongated bulge that rotates

d. All choices will produce stable spiral arms.

21. Which of the following is associated with the arms of a spiral galaxy?

a. density waves c. dust lanes

b. ongoing stellar formation d. All are valid choices.

22. Why do we know that at least one generation of stars formed and died before the Milky Way’s globular clusters formed?

a. All globular clusters reside in the disk of the Milky Way.

b. No globular cluster has zero heavy elements.

c. Globular clusters are 9 billion to 10 billion years old.

d. No globular cluster is older than 12 billion years.

23. Open star clusters primarily inhabit which part of spiral galaxies?

a. disk c. bulge

b. halo d. nucleus

24. Where are the most metal-rich stars found in the Milky Way?

a. in the disk near the Sun c. in the halo

b. the inner parts of the galaxy d. in globular clusters

25. Where in the Milky Way would we look to find lots of gas and dust?

a. disk

b. globular clusters

c. halo

d. The gas and dust is uniformly distributed in the Milky Way.

26. What objects are found in the thick disk of the Milky Way Galaxy?

a. younger stars and molecular clouds c. younger stars

b. older stars and molecular clouds d. older stars.

27. What objects are found in the thin disk of the Milky Way Galaxy?

a. younger stars and molecular clouds c. younger stars

b. older stars and molecular clouds d. older stars

28. Which of the following statements is a property of an open cluster?

a. contains hundreds to thousands of stars c. loosely gravitationally bound

b. includes young stars d. All choices are valid.

29. Which of the following statements is a property of a globular cluster?

a. consists of hundreds to thousands of stars

b. contains young stars

c. stars contain no heavy elements

d. tightly gravitationally bound

30. Cosmic rays are

a. photons with even higher energy than gamma rays.

b. high-velocity particles produced in novae.

c. primarily protons with very high energies.

d. synchrotron emission from strong magnetic fields.

31. What is the primary source of cosmic rays?

a. the Sun c. Sagittarius A*

b. novae outbursts d. supernovae explosions

32. In which part of the Milky Way would you find little or no neutral hydrogen, no current star formation, and stars that are older than 10 billion years?

a. halo c. galactic center

b. disk d. solar neighborhood

33. Globular clusters, compared with open clusters, generally

a. are located closer to the center of the Milky Way.

b. are younger.

c. have fewer amounts of heavy elements.

d. All choices are valid.

34. The chemical composition of a star’s atmosphere tells us

a. how much nuclear burning has gone on in the star.

b. the star’s evolutionary stage.

c. the chemical composition of the cloud from which the star formed.

d. All choices are valid.

35. In a star that has a chemical abundance similar to the Sun, what percentage of its mass is made of heavy elements?

a. 0.02 percent c. 2 percent

b. 0.2 percent d. 20 percent

36. In general, open clusters are found in the __________ of the Milky Way; globular clusters are found in the __________ of the Milky Way.

a. disk; disk c. disk; halo

b. halo; halo d. halo; disk

37. Magnetic fields in the Milky Way are concentrated in the disk because

a. halo stars are incapable of producing strong magnetic fields.

b. the fields are tied to the charged particles in dense molecular clouds.

c. gravity forces them to sink to the center of the disk.

d. supernovae explosions continually force them toward the middle of the disk.

38. Identify what is in this photo.

a. globular cluster near a molecular cloud

b. open cluster with remnant of cloud from which it formed

c. dwarf galaxy satellite of the Milky Way

d. Sagittarius A*

39. What observational evidence supports the idea that the Milky Way Galaxy was formed by a merger of several galaxies?

a. gamma-ray bubbles extending thousands of light-years above and below the galactic plane

b. outer halo stars orbiting in the opposite direction of the Galaxy’s rotation

c. synchrotron emission throughout the disk of the Galaxy

d. All choices are valid.

40. The Milky Way is part of the Local Group of galaxies. Which of the following statements best describes the Local Group?

a. a vast network of thousands of galaxies

b. the Milky Way and its orbiting companion galaxies

c. about 50 galaxies, including Andromeda and many dwarf galaxies

d. the Milky Way and its halo of globular clusters

41. What is ripping apart the Sagittarius dwarf spheroidal galaxy?

a. a supermassive black hole at its center

b. the energetic decay of a WIMP

c. a violent episode of star formation

d. the gravitational tidal force of the Milky Way

42. What is the evidence that the Milky Way must have formed when the gas within a huge “clump” of dark matter collapsed?

a. the number of nearby dwarf galaxies

b. the properties of globular clusters

c. the supermassive black hole at the center of the Milky Way

d. all of the above

43. The fact that the Milky Way has orbiting companion galaxies and is currently “eating” one of them is evidence that

a. all galaxies are blueshifted with respect to the Milky Way.

b. galaxies form by mergers of smaller protogalaxies.

c. the Milky Way is the center of the universe.

d. We cannot learn anything from this evidence.

44. Which of the following statements about the history of the Milky Way is correct?

a. The Milky Way was much larger in the past.

b. The Milky Way evolved from smaller clumps of protogalaxies.

c. The Milky Way has not evolved.

d. The Milky Way is expanding.

45. Because of the ages of globular clusters in the Milky Way, we think our galaxy’s early formation history was characterized by

a. one single cloud of gas gently collapsing and star formation proceeding slowly within it.

b. one single cloud of gas that rapidly collapsed and turned most of its gas into stars.

c. merging of protogalactic fragments that stimulated a high rate of star formation.

d. slow merging of protogalactic fragments after they had already turned most of their gas into stars.

1. Examine the following figure. The Sun is located a distance A 1.7 10⁹ astronomical units (AU) from the center of the Milky Way, and it takes 230 million years for the Sun to make one circular orbit. Using Kepler’s third law, compute the mass of the Milky Way interior to the Sun’s orbit.

2. Examine the following figure. The Sun is located a distance A from the center of the Milky Way and makes one circular orbit in the time P. Kepler’s third law can use A and P to compute a mass, but why is this not the entire mass of the Milky Way?

3. What is the main observational difficulty in observing the shape of the Milky Way?

4. What forms of electromagnetic radiation can be observed that allow us to probe the Milky Way Galaxy and determine its structure?

5. This radio image of the entire sky is shown here. Explain the significance of the concentrations of 21-cm radiation.

6. Explain why the stars of today in a spiral arm of a galaxy were not the same stars 20 million years ago.

7. Describe the differences between the ages and chemical abundances of globular and open star clusters.

8. Sketch an edge-on view of the Milky Way. Label the locations of the bulge, disk, and halo.

9. How do we know that globular cluster stars were not the first stars that formed in our galaxy?

10. Describe the population of globular star clusters in the Milky Way: How many are there, where do they reside in the Milky Way, and how do their ages and chemical abundances compare with the Sun?

11. Why can the observed chemical abundance of a star tell you something about its age?

12. Where would we find the stars with the richest abundance of massive elements? Where would we find those with the smallest amounts of massive elements? Why?

13. How does the H-R diagram of a globular star cluster show that it is very old? Provide a sketch of an H-R diagram for a globular cluster.

14. How does the H-R diagram of an open cluster show that itis very young? Provide a sketch of an H-R diagram for an open cluster.

15. Explain why the disks of galaxies glow from synchrotron radiation and describe how it relates to cosmic rays.

16. Explain why magnetic fields are confined to the disks of galaxies.

17. Describe the basic scenario that explains the formation and evolution of the Milky Way, including a description of how the halo and disk formed.

18. Examine the following figure. What are the Magellanic Clouds?

19. Examine the following figure. Where on Earth can one see the Magellanic Clouds?

20. Examine the following figure. Describe the characteristics of the Magellanic Clouds in terms of size, mass, composition, and motions.

21. Examine the following figure. How are the Magellanic Clouds related to the formation of the Milky Way Galaxy?