Question 12.1

Consider a star behind a cloud of interstellar gas and dust and at rest as seen from our perspective. Which of the following statements is correct?

• a. The star appears brighter than it would if the cloud were not present.
• b. The star appears to be blue-shifted.
• c. The star appears redder than it would if the cloud were not present.
• d. The star would be invisible at all wavelengths.
• e. The star would always appear green.

Question 12.2

What is the lowest mass that a star can have on the main sequence?

• a. There is no lower limit.
• b. 0.003 M_⊙
• c. 0.08 M_⊙
• d. 0.4 M_⊙
• e. 2.0 M_⊙

Question 12.3

What is the source of energy that enables a main-sequence star to shine?

• a. Friction between its atoms
• b. Fusion of hydrogen in a shell that surrounds the core
• c. Fusion of helium in its core
• d. Fusion of hydrogen in its core
• e. Burning of gases on its surface

Question 12.4

What are giant molecular clouds, and what role do they play in star formation?

Question 12.5

Why are low temperatures necessary for dense cores to form and contract into protostars?

Question 12.6

Why do thermonuclear reactions not occur on the surface of a main-sequence star?

Question 12.7

What is an evolutionary track, and how do such tracks help us interpret the H-R diagram?

Question 12.8

Draw the pre–main-sequence evolutionary track of the Sun on an H-R diagram. Briefly describe what occurred throughout the solar system at various stages along this track. (You may find it useful to review Chapter 11.)

Question 12.9

On what grounds are astronomers able to say that the Sun has about 5 billion years remaining in its main-sequence stage?

Question 12.10

What will happen inside the Sun 5 billion years from now when it begins to evolve into a giant?

Question 12.11

How is the evolution of a main-sequence star with less than 0.4 M_⊙ fundamentally different from that of a main-sequence star with more than 0.4 M_⊙?

Question 12.12

Draw the post–main-sequence evolutionary track of the Sun on an H-R diagram up to the point when the Sun becomes a helium-fusing giant. Briefly describe what might occur throughout the solar system as the Sun undergoes this transition.

Question 12.13

What does it mean when an astronomer says that a star “moves” from one place to another on an H-R diagram?

Question 12.14

What is the helium flash and what causes it?

Question 12.15

Explain how and why the turnoff point on the H-R diagram of a cluster is related to the cluster’s age.

Question 12.16

Why do astronomers believe that most globular clusters are made of old stars?

Question 12.17

What are Cepheid variables, and how are they related to the instability strip?

Question 12.18

What occurs in Cepheid stars that is analogous to the vapor raising the lid on a pot of boiling water?

Question 12.19

What are RR Lyrae variables, and how are they related to the instability strip?

Question 12.20

What is a Roche lobe, and what is its significance in close binary systems?

Question 12.21

What are the differences between detached, semidetached, contact, and overcontact binaries?

Question 12.22

How is a degenerate gas different from an ordinary gas?

Question 12.23

If you took a spectrum of a reflection nebula, would you see absorption lines, emission lines, or no lines? Explain your answer.

Question 12.24

Why is it useful to plot the apparent magnitudes of stars in a single cluster on an H-R diagram?

Question 12.25

What might happen to the massive outer planets when the Sun becomes a giant?

Question 12.26

* How many 1.5 M_⊙ main-sequence stars would it take to equal the luminosity of one 15 M_⊙ star?

Question 12.27

* How many times longer does a 1.5 M_⊙ star fuse hydrogen in its core than does a 15 M_⊙ star?

Question 12.28

Why does a shock wave from a supernova produce relatively few high-mass O and B stars compared to the number of low-mass A, F, G, K, and M stars produced?

Question 12.29

How would you distinguish a newly formed protostar from a giant, given that they occupy the same location on the H-R diagram?

Question 12.30

What observational consequences would we find in H-R diagrams for star clusters as a result of the universe having a finite age? Could we use these consequences to establish constraints on the possible age of the universe? Explain your answers.

Question 12.31

Discuss the possibility of life-forms and biological processes occurring in giant molecular clouds. In what ways might conditions favor or hinder biological evolution?

Question 12.32

Is there any evidence that Earth has ever passed through a star-forming region in space?

Question 12.33

The solar system passed through a giant molecular cloud? How would this encounter affect Earth and life on it?

Question 12.34

The Sun began entering its giant phase today? What would happen to Earth and to life on it?

Question 12.35

Earth were orbiting a 0.5 M_⊙ star at a distance of 1 AU? What would be different for Earth and life on it? What effects would moving Earth closer to the lower-mass Sun have?

Question 12.36

The Sun were a variable star? How would this change life on Earth, and, assuming we could live in orbit around such a star, how might it change our perspective of the cosmos?

Question 12.37

To test your understanding of where stars are formed, do Interactive Exercise 12.1 on the assigned Web site. You can print out your results, if required.

Question 12.38

To test your understanding of variable stars, do Interactive Exercise 12.2 on the assigned Web site. You can print out your results, if required.

Question 12.39

To test your understanding of close binary star systems, do Interactive Exercise 12.3 on the assigned Web site. You can print out your results, if required.

Question 12.40

Which “last” longer, high-mass or low-mass stars? Explain your reasoning.

Question 12.41

Is Jupiter a “failed” star? Why or why not?

Question 12.42

Which star, if either, is at the center of a binary star system? Explain your reasoning.

Question 12.43

Did the Sun form shortly after the universe came into existence? Explain your answer.

Question 12.44

Are stars still forming today? Explain your answer.

Question 12.45

Use a telescope to observe at least two of the following interstellar gas clouds: M42 (Orion), M43, M20 (Trifid), M8 (Lagoon), M17 (Omega). You can easily locate them by using the coordinates in the list below, by using the Starry Night™ program, or by looking at star charts published in such magazines as Astronomy and Sky & Telescope. For each nebula, can you identify the stars responsible for the ionizing radiation that causes the nebula to glow? Draw a picture of what you see through the telescope and compare it with a photograph of the object. Which components of the nebula are not visible through your telescope, when compared to photographs in textbooks or the images displayed in Starry Night™? Why do you think that these components are not visible through your telescope?

Question 12.46

Several star clusters can be seen relatively easily through a good pair of binoculars. You can locate them with the aid of your Starry Night™ program, by using the star charts published in such magazines as Astronomy and Sky & Telescope, or by using the list of coordinates below. Observe as many of these clusters as you can. Use a telescope, if available, to look at them. Note the overall distribution of stars in each cluster. Can you see any of these clusters with the naked eye? What difference do you note between binocular and telescopic images of individual clusters?

Question 12.47

Use the Starry Night™ program to examine the Milky Way Galaxy. Open Favourites > Explorations > Star forming regions. The view looks toward the center of the Milky Way from the center of a transparent Earth. The view also displays the galactic equator and the constellations. Observe the mottled appearance of the Milky Way caused by regions of opaque dust and gas. Click on the Options tab, expand the Deep Space layer, and click the checkbox to turn Nebulae On. Click on the + sign to the left of Nebulae to expand this layer. Click Off all the checkboxes for the Nebula options except that for Dark Nebula. Numerous dark nebulae visible in Earth’s night sky are shown with green outlines or markers. (a) Use the hand tool and zoom controls to look around the view and describe the distribution of these dark nebulae in the sky. (b) Now click Off the Dark Nebula option in the Options pane and click On the Emission Nebula. Again use the hand tool and zoom controls to look around the sky and observe the distribution of these nebulae and describe your observations. (c) Use the Find tool to locate and identify several of the more prominent emission nebulae: M8, M20, NGC3372, M42, and NGC7000 (the North America Nebula). Magnify each of these nebulae in turn and describe some of the details of your observations, such as color, shape and structure. (d) Zoom out again to return to the wide-field view and use the hand tool to find the tight knot of emission nebulae between the constellations of Ursa Major and Draco. Zoom in on this region to a field of view about 1° wide. You will notice that the outlined emission nebulae belong to the galaxy M101 (the Pinwheel Galaxy), which is about 27 million light-years from Earth. Describe the distribution of these emission nebulae within this galaxy and compare it to your observations of the distribution of emission nebulae in the Milky Way. (e) Given the distance to M101, what does the fact that these emission nebulae in this galaxy are visible from Earth suggest about their properties?

403

Question 12.48

Use Starry Night™ to examine the H-R diagram of the Pleiades star cluster. This group of stars was formed relatively recently in astronomical time. Select Favourites > Explorations > Pleiades to display this cluster of young stars in the view. Click on the Status tab to display the H-R diagram of all stars in this field of view around the Pleiades. This distribution is representative of the general population of stars, as seen in the Hipparcos satellite data in Figure 12-31. However, if you restrict the distance to display only the stars within this localized cluster, a different pattern emerges. Click on the Distance cut-off checkbox in the H-R Options layer of the Status pane to restrict the distance to a range between 320 and 420 ly. (a) Where on the HRD do you now find the majority of the stars of this cluster? (b) In view of the existence within this cluster of very hot stars with high output of energy, what does this tell you about the age of this cluster compared to the general population of stars?