138 Bennett, Donahue, Schneider, Voit
Part IV: Stars
Instructor Guide for The Essential Cosmic Perspective, Eighth Edition 139
Chapter 11. Our Star
This chapter describes how the Sun works, laying the groundwork for the study of
stars in general by focusing on this all-important example.
Key Changes for the 8th Edition: For those who have used earlier editions of our
textbook, please note the following significant changes in this chapter:
Fully updated discussion of solar neutrino observations, including the
detection of proton-proton chain neutrinos.
Teaching Notes (by Section)
Section 11.1 A Closer Look at the Sun
We already introduced students to the Sun briefly in Chapter 6, during our tour
of the solar system in Section 6.1. Here, we take a more in-depth look at the Sun.
We begin by addressing one of the most basic of all astronomical questions: Why
does the Sun shine? This question immediately engages students because it relates
directly to their personal experience of the Sun and perhaps even to questions they
began asking as children. They are often surprised to find out how recently we
learned the answer, and the process of elimination that led to the right answer
provides a good example of scientific reasoning. We then use an imaginary
journey into the Sun to discuss its basic properties and structure.
Relating the history of ideas about how the Sun shines provides an early
opportunity to discuss gravitational contraction, a mechanism that will
140 Bennett, Donahue, Schneider, Voit
when gravity overwhelms pressure, and gravitational equilibrium occurs
when gravity is in balance with pressure. (Note: In using the term
gravitational equilibrium, we are consistent with terminology used by
Mitch Begelman and Martin Rees in their book ,
Scientific American Library Series [New York: W. H. Freeman, 1996].)
Section 11.2 Nuclear Fusion in the Sun
This section focuses on the process of nuclear fusion in the Sun and describes
how the balance between gravity and pressure acts as a thermostat in the solar
core, maintaining a constant fusion rate. Figure 11.8 is a useful aid to illustrate
this concept.
The topic of fusion in the solar core presents an opportunity to discuss the
role of mathematical modeling in science and to explain how we can be so
Instructor Guide for The Essential Cosmic Perspective, Eighth Edition 141
(Ironically, the best upper limits on the neutrino mass of about 0.23 eV come
not from detector experiments but from cosmological measurements, such as
those of the cosmic microwave background studied by the Planck mission
[Planck Collaboration, 2014, A&A, 571, A16; 2015, arXiv:1502.01589].)
Section 11.3 The Sun Earth Connection
This section discusses solar activity, including sunspots, and the ways in which
solar activity can affect Earth.
Answers/Discussion Points for Think About It/See It for Yourself
Questions
The Think About It and See It for Yourself questions are not numbered in the
book, so we list them in the order in which they appear, keyed by section number.
Section 11.1
(p. 290) At higher altitudes, there is less overlying air pressing down.
Because of the lower pressure, the atmosphere is less compressed.
Section 11.2
(p. 294) Heavier elements have more protons in their nuclei. Their greater
positive charge means they repel each other more strongly than do
hydrogen nuclei, which contain only 1 proton. The stronger repulsion
means that higher temperatures are required to overcome the repulsion and
make the nuclei fuse.
142 Bennett, Donahue, Schneider, Voit
Section 11.3
(p. 303) Because the Sun rotates faster at the equator, after a few days,
Solutions to End-of-Chapter Problems (Chapter 11)
Visual Skills Check
Review Questions
1. As something contracts, its gravitational potential energy is converted into
2. Gravitational equilibrium is a balance between the force of gravity pulling
3.
4. The corona is a region several million kilometers above the surface of the
Instructor Guide for The Essential Cosmic Perspective, Eighth Edition 143
5. Nuclear fission is the process of splitting an atomic nucleus into two.
6. Nuclear fusion requires high temperatures to cause the protons to collide at
high enough speeds that they get close enough to stick together (like Velcro)
7. four protons to form a single
helium nucleus. The actual process, called the proton-proton chain, requires
8. over time. This steadiness results because
hotter, the fusion rate would increase. This would produce more energy,
9. Photons take hundreds of thousands of years to get out of the Sun because
10. Mathematical models use the observed composition and mass of the Sun,
along with the laws of physics, to derive equations that describe the
11. Neutrinos are subatomic particles produced in nuclear reactions. They move
at nearly the speed of light and almost never interact with matter. Detectors
on Earth found only about one-third of the neutrinos predicted by models of
12. Solar activity refers to the changing features of the Sun, such as sunspots,
flares, prominences, and coronal mass ejections. The sunspots are regions of the
13. a temperature of about 5800 K. It looks mottled
because it is churning constantly with rising and falling gas. However, in
14. The chromosphere is best viewed with ultraviolet telescopes because it emits a
lot of light in that part of the spectrum, since its temperature is 10,000 K, and
15. The sunspot cycle is a cycle in which the average number of sunspots on the
Sun gradually rises and falls over a period of about 11 years. As the cycle
Instructor Guide for The Essential Cosmic Perspective, Eighth Edition 145
Does It Make Sense?
16. Before Einstein, gravitational contraction appeared to be a perfectly plausible
mechanism for solar energy generation. This statement does not make sense.
17. The solar wind usually flows outward from the Sun, but sometimes it turns
around and flows backward. This statement does not make sense. The solar
18. If fusion in the solar core ceased today, worldwide panic would break out
tomorrow as the Sun began to grow dimmer. This statement does not make
19. Astronomers have recently photographed magnetic fields churning deep
beneath the solar photosphere. This statement does not make sense. The
20. I wear a lead vest to protect myself from neutrinos. This statement does not
21.
more in about 5 years. Whether this statement makes sense or not is time
dependent. For example, it would have made sense in 2015, but it might not
22. News of a solar flare caused concern among businesses involved in
23. By observing solar neutrinos, we can learn about nuclear fusion deep in the
24.
25. Scientists are currently building an infrared telescope designed to observe
Quick Quiz
26. c. four individual protons
Process of Science
36. Scientists create computer models, using physical laws to predict the
conditions in the center of the Sun. These models are both tested and
informed by observations of the Sun and improved by better understanding
37. Scientists do not simply abandon their models unless they believe that the
counterevidence is so overwhelming that they must. Although early solar
neutrino experiments detected only a third of the neutrinos expected to
Instructor Guide for The Essential Cosmic Perspective, Eighth Edition 147
Group Work Exercise
38. Grading a discussion should involve evaluation of the process and the
students
convenience, we provide quick answers.
Short-Answer/Essay Questions
39. If fusion reactions suddenly shut off in the Sun, photons would continue to
40. We would be able to tell if fusion reactions had shut off because we would
41.
42. The Sun would look yellow and bright. The sunspots appear dark only in
contrast to their surroundings, but they are actually quite luminous.
43. Scientists use mathematical models and observations of solar vibrations and
44.
148 Bennett, Donahue, Schneider, Voit
violent behavior varies over time.
45. This is an essay question, but the key point should be that we are not
Quantitative Problems
46. that:
47.
max 2, 900, 000
T (Kelvin) nm
48. The energy that the Sun radiates into space each second is easily determined
from its luminosity (watts, or joules/s) to be 3.8 1026 joules. This energy
49. We will use the relationship between temperature and pressure:
50. a. The total amount of mass in the Sun is 2.0 1030 kg, 75% of which is
hydrogen and 13% of which becomes available for fusion. Thus, the
b. The Sun fuses 6 1011 kg of hydrogen per second and has 1.95 1029
kg avai
P
nk
T
150 Bennett, Donahue, Schneider, Voit
b. The flux of solar radiation at the surface of this imaginary sphere is the
luminosity of the Sun divided by the surface area of the sphere: