Download Astronomy HOMEWORK Chapter 12 - 9th Edition 1. Consider a star

Astronomy
HOMEWORK Chapter 12 - 9th Edition
1. Consider a star behind a cloud of interstellar gas and dust as seen from our perspective. Which
of the following would you see? a. The star appears brighter than it would if the cloud were not
present, b. The star appears to be moving toward us, 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.
Answer: c. The star appears redder. Comment: the star would also appear dimmer. It would
become invisible if the cloud were really thick.
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⊙ .
Answer: c 0.08 M⊙ . Anything lower mass doesn’t ever heat the core hot enough to start Hydrogen
fusion, and is called a Brown Dwarf.
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
Answer: d. Fusion of Hydrogen in its core. The other fusions occur after the star has left the main
sequence. None of the other options are relevant as stellar heat sources.
6. Why do thermonuclear reactions not occur on the surface of a main-sequence star?
Answer: It isn’t hot enough. Fusion requires about 10 million K; the hottest main-sequence stars
are around 50 thousand K.
15. Explain how and why the turnoff point on the H-R diagram of a cluster is related to the cluster’s
age.
Answer: “Why” first. Nearly all stars in a cluster formed about the same time. High mass stars, in
the upper part of the Main Sequence, have shorter lifetimes. Lifetime on the Main Sequence increases
smoothly as mass decreases. So: the first stars to turn into Red Giants (and pass rapidly through
other stages) are the high-mass ones. So “How:” the turnoff point is determined (L and T ). These
values are correlated with a lifetime, and that’s the age of the cluster.
16. Why do astronomers believe that most globular clusters are made of old stars?
Answer: See the H-R diagram for M55, Fig 12-30, page 400 (9th ed.). Who stole the upper part of
the main sequence? What has happened is that stars in the upper part of the main sequence have
gone through their main sequence phase and moved on. Those which recently left are the trail of dots
going diagonally upward to the right. The only reasonable explanation is that nearly all these stars
formed long ago, and only those with long enough lifetimes on the main sequence are still on it. A
very few stars do appear on the upper part of the main sequence. These are called “blue stragglers”
and are the exception. They did form long after the great majority of the other stars in the cluster.
Furthermore, stars in globular clusters are nealy all “metal-poor,” indicating they formed early in the
history of the universe.
17. What are Cepheid variables, and how are they related to the instability strip?
Answer: Cepheids are stars which pulsate in brightness in a distinctive way due to a thermal instability. A higher-mass star becomes a Cepheid when its evolutionary path takes it across the instability
strip. The most important characteristic of Cepheids is that their pulsation period correltates with
their luminosity
22. How is a degenerate gas different from an ordinary gas?
Answer: In a degenerate gas, some electrons need to move faster so that all are not in the same state
(position and speed). This higher speed generates pressure independent of the temperature. In other
words, increasing the temperature a modest amount will cause a negligible increase in pressure.
26. How many 1.5 M⊙ stars do we need to equal the luminosity of one star of 10 times the mass,
15 M⊙ ?
Answer: Just read off table 12-2, pg 392 (9th ed.):
A star of 1.5 M⊙ has a luminosity of 5 L⊙ .
A star of 15 M⊙ has a luminosity of 10,000 L⊙ .
So, 10,000/5 = 2000. Two thousand lower-mass star to equal the LUMINOSITY of the high-mass
star.
35. What if: 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?
A 0.5 M⊙ star would have about 0.03 L⊙ (3% of the Sun’s luminosity). This makes things much
colder, maybe -70 Celsius. No need for freezers, lots of ice for winter sports, and the year would be
longer. Most likely none of this would matter because there would be no life.
If Earth were moved 6x closer (half of Mercury’s orbital distance), temperature would be around what
it is now. But, the Earth would probably get locked into synchronous rotation with its orbit, which
would tend to boil one side of the planet and freeze the opposite side.