Key Terms for Review

asymptotic giant branch (AGB) star

Chandrasekhar limit

cosmic ray

cosmic ray shower

glitch

helium shell flash

helium shell fusion

lighthouse model

magnetar

neutron degeneracy pressure

neutron star

nova (plural novae)

nucleosynthesis

photodisintegration

planetary nebula

primary cosmic ray

pulsar

secondary cosmic ray

Type Ia supernova

Type II supernova

white dwarf

X-ray burster

Review Questions

Question 13.1

Will the Sun shed most of its mass, and, if so, what is that event called?

  • a. yes, as a planetary nebula
  • b. yes, as a supernova
  • c. yes, as a white dwarf
  • d. yes, as a neutron blast
  • e. no

Question 13.2

A white dwarf is composed of primarily

  • a. neutrons
  • b. hydrogen and helium
  • c. iron
  • d. cosmic rays
  • e. carbon and oxygen

Question 13.3

What prevents a neutron star from collapsing?

  • a. hydrogen fusion
  • b. friction
  • c. electron degeneracy pressure
  • d. neutron degeneracy pressure
  • e. helium fusion

Question 13.4

What is the difference between a giant star and a supergiant star?

Question 13.5

Why is knowing the temperature in a star’s core so important in determining which nuclear reactions can occur there?

Question 13.6

What determines the temperature in the core of a star?

Question 13.7

What is a planetary nebula, and how does it form?

Question 13.8

What is the Chandrasekhar limit?

Question 13.9

What is a neutron star?

Question 13.10

Compare a white dwarf and a neutron star. Which of these stellar corpses is more common? Why?

Question 13.11

To test your understanding of the stages of stellar evolution, do Interactive Exercise 13.1 on the assigned Web site. You can print out your results, if required.

Question 13.12

What is the mass range of neutron stars?

Question 13.13

On an H-R diagram, sketch the evolutionary track that the Sun will follow between the time it leaves the main sequence and when it becomes a white dwarf. Approximately how much mass will the Sun have when it becomes a white dwarf? Where will the rest of its mass have gone?

Question 13.14

Why have searches for supernova remnants at visible wavelengths been less fruitful than searches at other wavelengths?

Question 13.15

To test your understanding of how stars “die,” do Interactive Exercise 13.2 on the assigned Web site. You can print out your results, if required.

Question 13.16

To test your understanding of neutron stars, do Interactive Exercise 13.3 on the assigned Web site. You can print out your results, if required.

Question 13.17

Why do astronomers believe that pulsars are rapidly rotating neutron stars?

Question 13.18

To test your understanding of rotating neutron stars, pulsars, and novae, do Interactive Exercise 13.4 on the assigned Web site. You can print out your results, if required.

Question 13.19

What is the difference between Type Ia and Type II supernovae?

Question 13.20

Compare a nova with a Type Ia supernova. What do they have in common? How are they different?

Question 13.21

Compare a nova and an X-ray burster. What do they have in common? How are they different?

Question 13.22

Describe what X-ray pulsars, pulsating X-ray sources, and X-ray bursters have in common. How are they different manifestations of the same type of astronomical object?

Advanced Questions

The answers to all computational problems, which are preceded by an asterisk (*), appear at the end of the book.

Question 13.23

What prevents thermonuclear reactions from occurring at the center of an isolated white dwarf? If no thermonuclear reactions take place in its core, why doesn’t that body collapse?

Question 13.24

Suppose you wanted to determine the age of a planetary nebula. What observations would you make, and how would you use the resulting data?

Question 13.25

Why is the rate of expansion of the gas shell in a planetary nebula often not uniform in all directions?

Question 13.26

What kinds of stars would you monitor if you wished to observe a supernova explosion from its very beginning? Look up the tabulated lists of the nearest and brightest stars in Appendix Tables E-5 and E-6. Which, if any, of these stars are possible supernova candidates? Explain.

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Question 13.27

To determine the period of a pulsar accurately, astronomers must take Earth’s orbital motion around the Sun into account. Explain why.

Question 13.28

* The distance to the Crab Nebula is about 2000 pc. When did the nebula actually explode?

Question 13.29

To test your overall understanding of stellar evolution, do Interactive Exercise 13.5 on the assigned Web site. You can print out your results, if required.

Discussion Questions

Question 13.30

Suppose that you discover a small glowing disk of light while searching the sky with a telescope. How would you determine whether this object was a planetary nebula? What else could this object be?

Question 13.31

Immediately after the first pulsar was discovered, one explanation offered was that the pulses were signals from an extraterrestrial civilization. Why did astronomers discard this idea?

Question 13.32

Describe how astronomers can determine whether a supernova at a known distance is Type Ia or Type II, assuming that they can see the supernova from the time it begins to brighten. There are at least two valid answers to this question.

Question 13.33

Describe how astronomers can determine whether a supernova at an unknown distance is Type Ia or Type II.

What If…

Question 13.34

Earth passed through an old supernova remnant? What would happen to Earth and life on it?

Question 13.35

The Sun were a B-type star, rather than a G-type star? Assuming that Earth orbiting the B-type star had the same composition and orbital distance that it has today, what would be different on Earth? If you answered this question in Chapter 11, you might want to see how the material in this chapter enhanced your previous answer.

Question 13.36

The Sun were an M-type star, rather than a G-type star? Assuming that Earth orbiting the M-type star had the same composition and orbital distance that it has today, what would be different on Earth?

Question 13.37

The Sun were expanding into a giant star today? How might we cope with this change?

Web Questions

Question 13.38

It has been claimed that the Dogon tribe in western Africa has known for thousands of years that Sirius is a binary star. Search the Web for information about these claims. What is their basis? Why are scientists skeptical, and how do they refute these claims?

Question 13.39

Search the Web for recent information about SN 1987A. Sketch the shape of the supernova remnant. Has a pulsar been detected yet in the center of this supernova remnant? If so, how fast is it spinning? Has the supernova debris thinned out enough to give a clear view of the neutron star?

Question 13.40

Search the Web for information about SN 1994I, a supernova that occurred in the galaxy M51 (NGC 5194). Why was this supernova unusual? Was it bright enough to have been seen by amateur astronomers?

Got It?

Question 13.41

How will the Sun end its “life cycle”? Will there be anything left of it after fusion in it ceases?

Question 13.42

Where was the iron in your blood formed?

Question 13.43

What are cosmic rays?

Question 13.44

When a star leaves the main sequence, where is the energy being generated that pushes its outer layers outward?

Observing Projects

Question 13.45

Planetary nebulae represent the late stages of the evolution of stars whose masses are similar to that of the Sun and are found throughout our Galaxy. You can use Starry Night ™ to explore the distribution of these objects in our sky and to view several of these spectacular nebulae. Set the view for your home location at some time in the evening with a field of view of about 100º. Open the Options pane and expand the Deep Space panel. Expand the NGC-IC Database list, click in its box to activate the display of the objects in this list, and click Off all entries in the list except Planetary Nebula. Use the hand tool to move around the sky. Note that these nebulae are mostly concentrated around the Milky Way in our sky. If you have access to a telescope, try to locate and observe several of these planetary nebulae, if possible on a clear, moonless night. Some of the more notable planetary nebulae include: Little Dumbbell (M76), NGC 1535, Eskimo, Ghost of Jupiter, Owl (M97), Ring (M57), Blinking Planetary, Dumbbell (M27), Saturn Nebula, and NGC 7662. If you do not have access to a telescope, use Starry Night ™ to examine in detail two of these planetary nebulae, M57, the Ring Nebula, and M27, the Dumbbell Nebula, and compare their shapes and sizes. Select Favourites > Explorations > Atlas and use the Find pane to center upon and magnify these two nebulae in turn. You can compare a ground-based image of M57 with a high-resolution image taken by the Hubble Space Telescope. First, open the Options pane, expand the Deep Space layer, and click on the Messier Objects to see an image taken with a ground-based telescope. Then replace this image by a space image by clicking in the Hubble Images box. Note that the Hubble image is displayed in a different alignment to that of the ground-based image. For each of these objects, note their Distance from the observer in the HUD, and then use the angular separation tool to measure the approximate angular radius of each of these nebulae. (a) How do you account for the difference in the shape of these two planetary nebulae? (b) What is the nature of the central star in each of these nebulae? (c) Calculate the physical size of these nebulae. (Hint: Translate angular size in arcseconds to radians and use the small-angle relationship; 1 radian = 206,265 arcseconds; 1 light-year = 9.46 × 1012 km)

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Question 13.46

The red supergiant Betelgeuse in the constellation Orion will explode as a supernova at some time in the future. Use the Starry Night™ program to investigate how the supernova might appear if the light from this explosion were to arrive at Earth tonight. Click the Home button in the toolbar to show the sky as seen from your location at the present time. Use the Find pane to locate Betelgeuse. If Betelgeuse is below the horizon, allow the program to reset the time to show this star. (a) At what time does Betelgeuse rise on today’s date? At what time does it set? (b) What is the apparent magnitude (mv) of Betelgeuse? (Hint: Use the HUD or the Info pane to find this information.) (c) If Betelgeuse became a supernova today, then at peak brightness it would be 11 magnitudes brighter than it is now. For comparison, mv = −4 for Venus at its brightest and mv = −12.6 for the full Moon. Would Betelgeuse be visible in the daytime? How would it appear at night? Do you think it would cast shadows? (d) Are Betelgeuse and the Moon both in the night sky tonight? (Use the Find pane to locate the Moon.) (e) If Betelgeuse were to become a supernova, how would the shadows cast by Betelgeuse differ from those cast by the Moon?

Question 13.47

Use the Starry Night™ program to examine the Veil Nebula, a large supernova remnant. Open Favourites > Explorations > Veil Nebula to see a view of the nebula high in the sky of Calgary, Canada, at midnight on August 1, 2013. (a) What significant feature do you notice about this supernova remnant in this 5-degree field of view? (b) Use the angular separation tool to measure the approximate angular distance between the components of this nebula. What angular distance separates these components? What form of optical aid is best suited to observing this object?

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WHAT IF…: A Supernova Exploded Near Earth?

The Flash

What would happen on Earth if a supernova occurred only 50 ly away? (Considering the titanic forces that supernovae release, it should come as no surprise that the high-energy electromagnetic radiation from such an explosion detonating much closer than this distance would immediately kill virtually all life on Earth.) Neutrinos would foreshadow by a few hours the visible flash and pending flood of X-rays and gamma rays from the supernova. The doomed star would then grow tremendously luminous, 50 times brighter than the Moon and only 8000 times less bright than the Sun. It would be brighter than the light from all other stars in the night sky combined.

The lethal X-rays and gamma rays would be the first causes of death on Earth. Within days, organisms from virtually all species of plants and animals would begin dying of radiation poisoning. Entire radiation-sensitive species might then be annihilated. The radiation would also cause many cancers and other internal diseases over succeeding years, leading to many more plant and animal deaths. At the same time, ultraviolet radiation reaching Earth’s surface would cause an astronomical jump in the rates of skin cancer and cataracts.

The first blast of ultraviolet radiation from the supernova would destroy Earth’s ozone layer within a matter of days, transforming the ozone primarily into atomic oxygen. With the removal of this protective barrier, ultraviolet radiation from both the supernova and the Sun would saturate Earth’s surface. After the intensity of ultraviolet radiation from the supernova diminished, sunlight would begin repairing the ozone layer, eventually returning it to normal levels.

The brightness and emission of all the electromagnetic radiation from the supernova would peak after a month. It would then fade, but the supernova remnant would be visible for millennia as an expanding cloud of gas and dust in the night sky. Fortunately for the life that survived the onslaught of high-energy radiation from the supernova, the remnant cloud would primarily emit harmless visible light.

The Aftermath

The most energetic particles from the supernova and its environs, the so-called cosmic rays, would also affect Earth’s surface. Moving at 90% of the speed of light, cosmic rays would arrive here only 5 to 10 years after the photons. Cosmic ray energies are much higher than those of any particles normally existing on Earth. For example, a typical cosmic ray has enough energy to light a small lightbulb for 1 second; it takes over 600,000 trillion electrons flowing through wiring in your house to do the same thing.

Some cosmic rays would slam into nitrogen or oxygen molecules in the air, shattering the air molecules and creating cosmic ray showers. The highest-energy cosmic rays from the supernova would reach Earth intact, as do the highest-energy cosmic rays from other sources today. These would break up atoms of the objects they strike on Earth’s surface, enhancing the earlier biological damage caused by the supernova’s electromagnetic radiation.

The bulk of the matter ejected into space by the supernova would travel at speeds of 16,000 km/s (60 million km/h), nearly 20 times slower than the emitted photons. Thus, the bulk of the supernova remnant would take at least 1000 years longer to reach us than did its initial radiation. By the time the bulk of the blast wave reaches the solar system, it would have become so thin and diffuse that it would probably do little damage to Earth’s atmosphere. However, we could expect this material to deposit a thin but exotic mix of elements into the upper atmosphere. All of this material would eventually fall to Earth and alter the chemistry of both the ocean and the soil.

The Outcome

The damage done to life by both the electromagnetic radiation and cosmic ray particles would disrupt the global food chain. In the oceans, large quantities of plankton and other microscopic organisms at the foundation of the chain would die off. As a result, many of the larger aquatic species that feed on these smaller organisms would starve. Similarly, on the surface, most plant life would wither, and many herbivorous animals would starve as a result. This would, of course, lead to the death and dislocation of animals throughout the food chain. Surviving plants and animals would undergo genetic mutations due to the supernova’s radiation. Most of these genetic changes would lead to the immediate death of the altered plant or animal, or their offspring, but a few changes would be beneficial and enable the mutated organisms to thrive.

How would the human race fare after the supernova? Our long-term survival would depend, in part, on whether surviving humans could coexist with surviving plants and animals, and with each other.

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