AST 105 HW #5 Solution
Week of September 21st, 2015
Note: All Problems are from The Cosmic Perspective (7ed)
Chapter 14
Review Problems
1. Briefly describe how gravitational contraction generates energy. When was it important in the Sun’s
history? Explain.
Answer:
Gravitational force attracts matter toward a core causing an object to contract. This phenomenon is called
gravitational contraction. About 4.5 billion years ago, gravitational contraction of the Sun made the core of
the Sun hot and dense enough to start nuclear fusion reaction. The heat energy generated by the fusion
reaction keeps the core of the Sun stable against the gravitational force.
2. What two forces are balanced in gravitational equilibrium? What does it mean for the Sun to be in
energy balance?
Answer:
While gravitational force pushes matter toward its core, pressure force move matter away. In gravitational
equilibrium, the gravitational force and the pressure force balance out. Gravitational equilibrium can also be
understood in terms of energy balance. In the equilibrium, the heat energy generated by the core balances
out the gravitational potential energy.
6. Why does nuclear fusion require high temperatures and pressures?
Answer:
Nuclear fusion needs two positively charged nuclei to come close enough to fuse. Normally, nuclei are not
close to each other, and electromagnetic repulsive force prevents them from getting very close. At high
temperature, nuclei attains enough kinetic energy to overcome this repulsive barrier. At high pressures, the
rate of nuclear fusion reaction increases due to increased density of nuclei.
9. Why has the Sun gradually brightened with time?
Answer:
Fusion reaction converts four hydrogen nuclei to single helium nucleus. The total number of particles in the
Sun, therefore, gradually decreased over time, causing the Sun to shrink. This slowly shrinkage made the
core of the Sun hotter and denser and increased the rate of fusion reaction. Higher the rate of fusion,
brighter the Sun shines.
13. What do we mean by solar activity? Describe some of features of solar activity, including sunspots,
solar prominences, solar flares, and coronal mass ejections.
Answer:
Solar activity, or solar weather, are short-lived phenomena on the Sun. It includes sunspots, solar
prominences, solar flares and coronal mass ejections. Many of solar activity are related to activity of the
Sun’s magnetic field. If the magnetic field prevents hot gas from entering into a region of the Sun’s surface,
the region become colder and darker than surrounding area. This region is called a sunspot. As for solar
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prominence, the magnetic field creates an arc of hot gas above the Sun’s surface. Solar flare happens when
the energy stored in the magnetic field is suddenly released.
Coronal mass ejections are sudden release of charged particles from the Sun’s corona.
17. What is the sunspot cycle? Why is it sometimes described as an 11-year cycle and sometimes a 22-year
cycle? Are there longer term changes in solar activity?
Answer:
The sunspot cycle is variation in the number of sunspots on the surface of the Sun. This cycle has average
period of 11 year. It, however, sometimes has 22 year period, because the magnetic field of the Sun flips
every 11 years or so.
Test Your Understanding
Decide whether the statement makes sense (or is clearly true) or does not make sense (or is clearly false).
Explain clearly; not all of these have definitive answers, so your explanation is more important than your
chosen answer.
19. Before Einstein, gravitational contraction appeared to be a perfectly plausible mechanism for solar
energy generation.
Answer:
The statement is false. Before Einstein, scientists tried to explain solar energy generation using only
gravitational contraction. The energy generated by the gravitational contraction can only support the Sun to
shine for 22 million years, but scientists knew the Sun was older than that. They needed new mechanism for
solar energy generation, which Einstein provided.
20. A sudden temperature rise in the Sun’s core is nothing to worry about, because conditions in the core
will soon return to normal.
Answer:
The statement is true. Gravitational equilibrium will stabilize the temperature of the Sun’s core.
21. If fusion in the solar core ceased today, worldwide panic would break out tomorrow as the Sun began
to grow dimmer.
Answer:
The statement is false. Even if fusion in the solar core ceased today, the Sun will shine steadily for very long
time.
22. Astronomers have recently photographed magnetic fields churning deep beneath the solar
photosphere.
Answer:
The statement is false. We can only measure magnetic fields by looking how they interact the matter. One
cannot directly photograph magnetic fields. Also, it is not possible to photograph something beneath the
photosphere.
23. I wear a lead vest to protect myself from solar neutrinos.
Answer:
The statement is false. Neutrinos have very low chance of interacting with matter. It can pass through a light
year of lead, and does not interact with any of the lead atoms.
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24. If there are few sunspots this year, we should expect many more in about 5 years.
Answer:
The statement is true. The sunspot cycle has average period of 11 years or sometimes 22 years. Assuming
we are at the lowest point of the cycle, the number of sunspots should increase next 5 years.
25. News of a major solar flare today caused concern among professionals in the fields of communication
and electrical power generation.
Answer:
The statement is true. The charged particles released from solar flare can interrupt communication and
electrical power generation.
26. By observing solar neutrinos, we can learn about nuclear fusion deep in the Sun's core.
Answer:
This statement is true. By studying solar neutrinos, we can learn about the rate of nuclear fusion inside the
Sun.
27. If the Sun’s magnetic field somehow disappeared, there would be no more sunspots on the Sun.
Answer:
This statement is true. Sunspots occur because the Sun’s magnetic field prevents hot gas from entering into
them. If the magnetic field disappear, there can’t be any more sunspots.
28. Scientists are currently building an infrared telescope designed to observe fusion reactions in the Sun’s
core.
Answer:
The statement is false. Photons generated via nuclear fusion are generally in X-ray / Gamma-ray spectrum.
Therefore, infrared telescope is probably useless if one wants to study nuclear fusion.
Quick Quiz
Choose the best answer to each of the following. Explain your reasoning with one or more complete
sentences.
29. Which of these groups of particles has the greatest mass?
(a) a helium nucleus with two protons and two neutrons
(b) four electrons
(c) four individual protons
Answer:
c) Proton and neutron have the same mass. However, when they combine to form a nucleus, some of their
mass are converted to energy.
33. Why do sunspots appear darker than their surroundings?
(a) They are cooler than their surroundings.
(b) Thye block some of the sunlight from the photosphere.
(c) They do not emit any light.
Answer:
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a) Sunspots are actually hot. They have average temperature of 4000 Celsius. They appear darker only
because they are cooler than surrounding photosphere, which has average temperature 6000 Celsius.
38. Which of these things poses the greatest hazard to communication satellites?
(a) photons from the Sun
(b) solar magnetic fields
(c) particles from the Sun
Answer:
a) Photons are not charged, so they do interrupt the communication satellites. And the solar magnetic
fields do not reach the Earth’s atmosphere.
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