Question 15.1

What is located in the nucleus of the Galaxy?

• a. a globular cluster
• b. a spiral arm
• c. a black hole
• d. the solar system
• e. a MACHO

Question 15.2

Which statement about the Milky Way Galaxy is correct?

• a. Our Galaxy is but one of many galaxies.
• b. Our Galaxy contains all stars in the universe.
• c. All stars in our Galaxy take the same time to complete one orbit.
• d. Most stars in our Galaxy are in the central bulge.
• e. None of the stars in our Galaxy move.

Question 15.3

What was the Shapley–Curtis debate all about? Was a winner declared at the end of the debate? Whose ideas turned out to be correct?

Question 15.4

How did Edwin Hubble prove that M31 is not a nebula in our Milky Way Galaxy?

Question 15.5

As seen in the northern hemisphere, why is the Milky Way far more prominent in July than in December? Planetarium software, such as Starry Night™, may be useful in answering this question.

Question 15.6

What observations led Harlow Shapley to conclude that we are not at the center of the Galaxy?

Question 15.7

Explain why globular clusters spend more time in the galactic halo than in the plane of the disk, even though their eccentric orbits take them across the disk of the Galaxy.

Question 15.8

How do hydrogen atoms generate 21-cm radiation? What do astronomers learn about our Galaxy from observations of that radiation?

Question 15.9

Why do astronomers believe that vast quantities of dark matter surround our Galaxy?

Question 15.10

What is synchrotron radiation?

Question 15.11

Why are there no massive O and B stars in globular clusters?

Question 15.12

What evidence indicates that a supermassive black hole is located at the center of our Galaxy?

Question 15.13

* Approximately how many times has the solar system orbited the center of the Galaxy since the Sun and planets were formed?

Question 15.14

Why do astronomers not detect 21-cm radiation from the hydrogen in giant molecular clouds?

Question 15.15

Determine the order of Doppler Shifts in Figure 15-8 from highest blueshift to lowest.

Question 15.16

Describe the rotation curve you would get if the Galaxy rotated like a rigid body.

Question 15.17

Compare the apparent distribution of open clusters, which contain young stars, with the distribution of globular clusters in the Milky Way. Why are open clusters also referred to as galactic clusters?

Question 15.18

* The visible disk of the Galaxy is about 100,000 ly in diameter and 2000 ly thick. If about five supernovae explode in the Galaxy each century, how often on average would you expect to see a supernova within 1000 ly of the Sun?

Question 15.19

Give reasons for the rapid rise in the Galaxy’s rotation curve (see Figure 15-19) at distances close to the galactic center.

Question 15.20

The solar system were located in a globular cluster 26,000 ly above the plane of the Galaxy’s disk? Make a drawing of what the Milky Way might look like from that location.

Question 15.21

The solar system were located at the edge of the visible galactic disk? How would the Milky Way appear to us?

Question 15.22

The solar system were located at the center of the Milky Way Galaxy? What would we see, and how would things be different on Earth?

Question 15.23

A globular cluster were crossing the disk of the Milky Way in our vicinity right now? What would we see, and how might things be different on Earth?

Question 15.24

To test your understanding of our Galaxy’s rotation, do Interactive Exercise 15.1 on the assigned Web site. You can print out your results, if required.

Question 15.25

Search the Web for information on the Shapley–Curtis debate. Who proposed the debate in the first place? Briefly outline the relevant scientific beliefs held by the two men. What points did they agree upon and what did they disagree on? Why was the debate considered inconclusive? Support your argument with examples, if possible.

Question 15.26

To test your understanding of the Milky Way’s structure, do Interactive Exercise 15.2 on the assigned Web site. You can print out your results, if required.

Question 15.27

Clouds of positrons (see Section 15-4 and An Astronomer’s Toolbox 10-1) have been discovered near the center of the Galaxy. Search the Web for information about this discovery. When was it made? How were the clouds discovered? What is thought to be the origin of these particles? What happens when ordinary electrons and positrons collide?

Question 15.2

Does the Sun move in the Milky Way? If so, describe its motion.

Question 15.29

About what fraction (or percent) of the matter in the Milky Way can astronomers see with current technology?

Question 15.30

Where in the Galaxy is the solar system located?

• a. in the nucleus
• b. in the halo
• c. in a spiral arm
• d. between two spiral arms
• e. in the central bulge

Question 15.31

Is the Milky Way in close contact with other galaxies? Explain.

Question 15.32

Use the Starry Night™ program to explore the alignment of the Milky Way with respect to our Earth-based coordinate system, and its appearance at different electromagnetic wavelengths. Select Favourites > Explorations > Milky Way to display a wide-field view of our spiral galaxy from the center of a transparent Earth at 6:30 a.m. on September 1, 2014. The view is similar to that seen by observers upon Earth and shows an edge-on view of this galaxy in a direction toward the galactic center, represented by the marked position of a foreground star, HIP86948. (a) You can use this view to estimate the alignment of the galactic plane with our reference plane in the sky, the Celestial Equator, which is the projection of the Earth’s equator onto the sky. Display the plane of the galaxy by clicking on View > Galactic Guides > Equator and then show the Celestial Equator by clicking on View > Celestial Guides > Equator. Estimate the angle between these two planes. (b) This visible-light image of the Milky Way shows significant structure, with many dark regions. These dark regions do not show the absence of material in these directions but the presence of dust and gas clouds obscuring the distant stars. Visible light is scattered and absorbed by these dense clouds. You can examine this galactic plane at other electromagnetic wavelengths. Open the Options menu and choose Stars > Milky Way… to display Milky Way Options. Use the brightness slider to show maximum brightness. Click the Wavelength box to expand the list of possible wavelengths and display the image of the galactic plane at each wavelength. At which wavelengths does the Galactic Center show up most prominently? Why do you think this region shows up brighter at these wavelengths?

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Question 15.33

Use the Starry Night™ program to measure the dimensions of the Milky Way Galaxy. Select Favourites > Explorations > Milky Way Galaxy to display a face-on view of a simulation of the Milky Way Galaxy from a distance of 0.128 Mly from the Earth. The position of the Sun is labeled, directly below the center of the Galaxy in the view on the screen. Note the spiral arm structure of the galaxy. (Recent research has revealed that our Galaxy appears to have a bar structure surrounding the galactic center.) (a) Click on the Cursor Selection tool to the left of the Toolbar and activate the Angular Separation tool to measure the angular separation between the Sun and the center of the Galaxy as seen from this vantage point, and find the corresponding distance in light-years (ly). (Both values are shown in the display beside the line drawn by the angular separation tool.) Convert the angular separation to a decimal number in degrees (1° = 60′ = 3600″), then find the scale of the image of the Galaxy in light-years/degrees. (b) Change the Cursor to the location scroller and use it to view the Galaxy edge-on and oriented vertically on the screen, with the Sun still below the center. To do this, place the location scroller tool at the center of the right-hand edge of the screen, hold down the mouse button (on a two-button mouse, hold down the left mouse button), and move the location scroller directly to the left, toward the center of the Galaxy. Then change to the angular separation tool and use it to measure the angular separation of the Sun from the center of the Galaxy. This value should be approximately the same as what you found in part a. (c) Use the angular separation tool to find the total angular diameter of the Milky Way Galaxy as seen from this viewpoint, measured from one end to the other of this edge-on view (Zoom out if necessary). Round off the measurement to the nearest degree and then use the scale that you calculated in part a to find the approximate diameter of the Galaxy in light-years. (d) In a similar fashion, use this viewpoint to measure the distance in degrees and light-years for the following quantities: the diameter (in the plane of the Galaxy) of the central bulge; the thickness (perpendicular to the plane of the Galaxy) of the central bulge; and the thickness (perpendicular to the plane of the Galaxy) of the disk of the Galaxy at the location of the Sun. (e) In the edge-on view of this simulated galaxy, note the effect of the dust and gas clouds that obscure the light from the more distant spiral arms. Compare this simulated view with that of the edge-on view of a distant galaxy by clicking on Home to return to your present sky and using the Find facility to display NGC4565. (Note: The image of this galaxy is slightly offset from the position found by Starry Night™.) How similar is this galaxy to the Milky Way galaxy, as represented by the Starry Night™ simulation?