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Astronomy
HOMEWORK Chapter 13 - 9th Edition
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
a. yes, as a planetary nebula. The white dwarf is what is left nor become a neutron star.
2. A white dwarf is composed of primarily a. neutrons, b. hydrogen and helium, c. iron, d.
cosmic rays, e. carbon and oxygen.
e. carbon and xygen.
3. What prevents a neutron star from collapsing? a. hydrogen fusion, b. friction, c. electron
degeneracy pressure, d. neutron degeneracy pressure, e. helium fusion
d. neutron degeneracy pressure.
7. What is a planetary nebula, and how does it form?
Answer: A planetary nebula is a cloud of gas and dust blown off a low-mass (less than 8 M )
star near the end of its life. It forms because the radiation pressure from the star blows off the
star’s outer layers. Notice that a “planetary nebula” has nothing to do with planets; the term
is left over from the 1780s.
8. What is the Chandrasekhar limit?
Answer: The Chandrasekhar limit is the maximum mass that can be sustained by electron
degeneracy pressure. Its value is 1.4 M . It marks the mass limit of a White Dwarf.
9. What is a neutron star?
Answer: A neutron star is a very compact stellar remnant consisting almost entirely of neutrons. Typical mass 2 M , typical size 10 km, typical density 200 trillion times that of water.
10. Compare a white dwarf and a neutron star. Which of these stellar corpses is more common?
Why?
Answer:
White Dwarf
Neutron Star
Typical Radius (km)
5000
10
Progenitor
0.8 to 8 M star
8 to 25 M star
Density (compared to water)
106
1014
Composition
Carbon and Oxygen
Neutrons
White dwarfs are more common since lower mass stars are more common.
19. What is the difference between Type Ia and Type II supernovae?
Answer: Physically, the difference is that a Type II supernova occurs in a high-mass star
when the mass of the iron core reaches the Chandrasekhar limit; while a Type Ia supernova
occurs when a white dwarf, accreting from a companion, attains a mass of the Chandrasekhar
limit. Observationally, the Type II has hydrogen lines, while the Type Ia does not; and the
light curves (time histories of luminosity) are different.
As a result of these differences, and most important to astronomy as a whole, all Type Ia
supernovae are “identical,” so this is a “standard candle” and its distance can be determined
via the formula B = L/4πD 2 , where B is the apparent brightness. The ability to measure vast
distances is crucial to understanding the universe.
23. What prevents thermonuclear fusion from occurring at the center of a white dwarf? If no
thermonuclear reactions take place at its core, why doesn’t the star collapse?
Answer: A white dwarf consists of carbon and oxygen. Nuclear reactions involving these nuclei
require a temperature of 600 million K (pg 387). This is a lot hotter than the 10 million K
for hydrogen fusion because the carbon nuclei are heavier and have a charge of +6, which gets
squared. Oxygen, with a charge of +8, is even more difficult to fuse. The interior of a white
dwarf never gets this hot, so these nuclear reactions do not occur.
So, what does support it? Electron degeneracy pressure (EDP), our old friend. The core of
a white dwarf might be at several hundred million K, but this extremely high temperature
contributes only a small amount of the pressure, compared to EDP. As a result, as a white
dwarf cools down, the decrease in total pressure (EDP plus thermodynamic) is slight, so the
white dwarf does not shrink appreciably as it cools.
35. What if: The Sun were a B-type star, rather than a G-type star? Assuming that the Earth
orbiting the B-type star had the same composition and orbital distance as it has today, what
would be different on Earth? If you answered this question for Chapter 11, you might want to
see how material from this chapter (Chapter 13) enhanced your answer.
Answer: If it were a B-type main-sequence star, its mass would be around 10 to 15X the
Sun’s mass, so the year would be 3-4X shorter. Since the luminosity would be 5000-10,000X
its actual luminosity, it wouldn’t matter. Scorched cinder Earth. Moving Earth farther away
(80-100 AU, say) would recover our present temperature, but the dose of ultraviolet would
inhibit land life-forms. Chapter 13 pointed out that the lifetime of such a star would be only
15 million years. Earth hadn’t even formed in the first 15 million years. So: a B-type star isn’t
such a good idea.