Question 16.1

What are the least massive galaxies in the universe?

1. normal spirals
2. barred spirals
3. giant ellipticals
4. dwarf ellipticals
5. irregulars

Question 16.2

Spiral density waves are directly responsible for which of the following?

1. flocculent spirals
2. grand-design spirals
3. supernovae
4. the collisions between galaxies
5. galactic cannibalism

Question 16.3

Which of the following statements about the motion of galaxies is correct?

1. All galaxies are moving apart.
2. Superclusters of galaxies are all moving apart.
3. Superclusters of galaxies are all moving toward each other.
4. The Milky Way Galaxy is at the center of the universe.
5. All clusters of galaxies in each supercluster are moving toward each other.

Question 16.4

What is the common name for the sonic boom created by lightning?

Question 16.5

In spiral galaxies, what spectral classes of stars are only found in spiral arms?

Question 16.6

What is the Hubble classification scheme? Which category includes the biggest galaxies? Into which category do the smallest galaxies fall? Which type of galaxy is the most common?

Question 16.7

In which Hubble types of galaxies are new stars most commonly forming? Describe the observational evidence that supports your answer.

Question 16.8

What is the difference between a flocculent spiral galaxy and a grand-design spiral galaxy?

509

Question 16.9

Briefly describe how the theory of self-propagating star formation accounts for the existence of spiral arms in some spiral galaxies.

Question 16.10

Briefly describe how the spiral density wave theory accounts for the existence of spiral arms in some spiral galaxies.

Question 16.11

How is it possible that galaxies in our Local Group still remain to be discovered? In what part of the sky are these galaxies located? What sorts of observations might reveal these galaxies?

Question 16.12

Can any galaxies besides our own be seen with the naked eye? If so, name one.

Question 16.13

What is the difference between a rich cluster and a poor cluster of galaxies? What is the difference between a regular cluster and an irregular cluster of galaxies?

Question 16.14

How can a collision between galaxies produce a starburst galaxy?

Question 16.15

Why do astronomers believe that considerable quantities of dark matter must exist in clusters of galaxies?

Question 16.16

Explain why the dark matter in galaxy clusters cannot be neutral hydrogen.

Question 16.17

What is the Hubble law?

Question 16.18

Some galaxies in the Local Group exhibit blueshifted spectral lines. Why are these blueshifts not violations of the Hubble law?

Question 16.19

What is a standard candle? Why are standard candles important to astronomers trying to measure the Hubble constant?

Question 16.20

What kinds of stars would you expect to find populating the space between galaxies in a cluster?

Question 16.21

*Suppose a spectrum of a distant galaxy showed that its redshift corresponds to a speed of 22,000 km/s. How far away is the galaxy in Mpc?

Question 16.22

*A cluster of galaxies in the southern constellation of Pavo (the Peacock) is located 100 Mpc from Earth. How fast, on average, are galaxies in this cluster receding from us? Why do different galaxies in the cluster show different velocities as measured from Earth?

Question 16.23

How would you determine what fraction of a distant galaxy’s redshift is caused by the galaxy’s orbital motion around the center of mass of its cluster?

Question 16.24

Discuss the advantages and disadvantages (for example, reliability, repeatability, etc.) of using the various standard candles to determine extragalactic distances.

Question 16.25

Discuss whether the various Hubble types of galaxies actually represent some sort of evolutionary sequence.

Question 16.26

Discuss the sorts of phenomena that can occur when galaxies collide. Do you think that such collisions can change the Hubble type of a galaxy? Explain your answers.

Question 16.27

From what you know about stellar evolution, the interstellar medium, and spiral density wave theory, explain the appearance and structure of the spiral arms of grand-design spiral galaxies.

Question 16.28

The solar system was located in an active star-forming region in a spiral arm, rather than on the edge of the Orion Arm? How would the solar system be different?

Question 16.29

The Milky Way collided with the Andromeda Galaxy? What might we experience on Earth? (This merger will occur in the distant future.)

Question 16.30

No distant galaxies showed any redshift or blue-shift? Discuss what that would imply about the universe.

Question 16.31

All distant galaxies showed a blueshift, rather than a redshift? Discuss what that would imply about the universe.

Question 16.32

To test your understanding of the global properties of galaxies, do Interactive Exercise 16.1 on the assigned Web site. You can print out your answers, if required.

Question 16.33

To test your understanding of the local group of galaxies, do Interactive Exercise 16.2 on the assigned Web site. You can print out your answers, if required.

Question 16.34

To test your understanding of the various types of galaxies, do Interactive Exercise 16.3 on the assigned Web site. You can print out your answers, if required.

Question 16.35

To test your understanding of star formation in the spiral density wave model, do Interactive Exercise 16.4 on the assigned Web site. You can print out your results, if required.

Question 16.36

What shape(s) do galaxies have?

Question 16.37

With most galaxies in the universe moving away from us equally in all directions, are we at the center of the universe? Explain.

Question 16.38

Are galaxies uniformly distributed throughout the universe? Explain your answer.

Question 16.39

Is the Milky Way Galaxy moving through the universe? Justify your answer.

Question 16.40

If you have access to a telescope with an aperture of at least 30 cm (12 in.) in a dark location, observe as many as possible of the spiral galaxies shown in the list of selected “Spiral Galaxies and Interacting Galaxies” in Appendix E. Many of these galaxies are members of the Virgo cluster, which can be seen from the northern hemisphere most conveniently during the spring. Because all galaxies are quite faint, be sure to schedule your observations for a Moonless night. The best view is obtained when a galaxy is high in the sky. While at the eyepiece, make a sketch of what you see. Can you distinguish any spiral structure in these galaxies?

Use Starry Night™ to examine each of the listed spiral galaxies in turn. If your telescope observations were successful, you can compare your sketches with the appearance of each of these spiral galaxies. If no telescope was available, you can still use Starry Night™ to examine these galaxies in detail. Click on Home and use the Find pane to locate and magnify each galaxy in turn. In each case, note whether the spiral is edge-on or face-on to observers upon Earth, and consider particularly the source of the light in the spiral arms to determine whether stars or diffuse nebulae are the major contributors to this light.

Question 16.41

Use the same session at the telescope and the same procedure as in the previous exercise to observe as many as possible of the interacting galaxies listed under “Spiral Galaxies and Interacting Galaxies” in Appendix E. In particular, attempt to make sketches of what you see while at the eyepiece. Can you distinguish hints of interplay between the galaxies?

Use Starry Night™ to examine each of the listed galaxies in turn. If your telescope observations were successful, you can compare your sketches with the appearance of each of these interacting galaxies. If no telescope was available, you can still use Starry Night™ to examine these intriguing galaxy collisions. Click on Home and use the Find pane to locate each galaxy in turn. In each case, look for evidence of collision and write a comment on how this collision manifests itself in the resulting shapes of the colliding galaxies. Can you see the difference between head-on and more distant collisions?

Question 16.42

Use Starry Night™ to visit a variety of galaxies and determine whether they are spiral, barred spiral, elliptical, or irregular. Click on Home to see the sky from your home location. Click on the Options tab, expand the Deep Space layer, and click Off all images except Messier Objects and Bright NGC Objects. Type Ctrl-H (Cmd-H on a Mac) or select View > Hide Horizon from the menu to remove the horizon. Also select View > Hide Daylight to remove daylight from the view. Use the Find pane to visit each of the galaxies listed below. For each object, type its name in the search box of the Find pane and press the Enter key. (Hint: To go to the galaxy without slewing, press the spacebar.) (a) Use the Zoom buttons to examine each galaxy in detail and then classify it as a spiral (S), barred spiral (SB), elliptical (E), or irregular (Irr), and the subclassification of each galaxy (e.g., Sa, E5).

(b) Use the Find pane and locate M51. What is the classification of this galaxy? (c) Examine this galaxy carefully and comment on anything unusual about its structure and/or its neighboring objects.

Question 16.43

Clusters of galaxies contain different numbers and distributions of galaxies and harbor significant amounts of the mysterious dark matter. In this exercise, you can use Starry Night™ to compare a few of these groupings and see the gravitational effect of dark matter. You can start by looking at one of the largest galaxy clusters, the Virgo cluster. Select Favourites > Explorations > Virgo Cluster–Milky Way from the menu. You are looking at this group of galaxies from a very large distance out in space, at about 66 Mly from the Sun. Our own Milky Way Galaxy is labeled at the bottom left of the view, across a void in space from this cluster. Use the location scroller to move around the Virgo cluster and consider its overall shape and its relationship to neighboring galaxies. (a) What is the general shape of the Virgo cluster? Zoom in toward this cluster until individual galaxies are shown and use the location scroller to help you to identify each classification of galaxy (elliptical, spiral, and irregular) within the group. Select File > Revert to return to the original view and identify several other groups of galaxies. Select one or two clusters of galaxies in turn, move the cursor over a galaxy within the selected group, right-click the mouse to open the object contextual menu, and select the Highlight option to identify this group. You can select the Centre option to move the selected cluster to the center of the view and examine the cluster’s extent across space. Again, use the location scroller and Zoom in to examine this cluster from various viewpoints. (b) Describe the distributions of their galaxies within the cluster, compared to that in the Virgo Cluster. For example, what are their shapes and relative sizes compared to Virgo and to each other? See if you can recognize the walls of galaxies surrounding large voids in space that link these concentrated regions of galaxies. (c) Click on the Home button to return to your sky. Click on the Find tab and ensure that the search box is empty. Click on the magnifying glass icon in the search box to open a dropdown list and click on Hubble Images. In the list of Hubble images, click on Gravitational Lens to center on this image of a cluster of galaxies known as CL0024+1654. Zoom in to a field of view of about 1 arcminute. This Hubble Space Telescope image shows a rich cluster of ordinary-looking yellowish galaxies surrounded by blue arcs of light. These are multiple images of a very distant galaxy, as seen through the gravitational “lens” of dark matter pervading the cluster of galaxies. The blue color of these images suggests that this distant galaxy is composed largely of young, blue stars. The distribution of this mysterious substance within galactic clusters can be inferred from these types of images of distant galaxies.