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Astronomy
HOMEWORK Chapter 11 - 9th Edition
1. Stellar parallax measuements are used in astronomy to determine which of the following
properties of stars? a. speed; b. rotation rates; c. distances; d. colors; e. temperatures.
c. distance. The only direct way to determine distance.
3. Measurements of binary star systems are required to determine what property of stars? :
a. luminosity; b. apparent magnitude; c. distance; d. mass; e. temperature.
d. mass. The only direct way to determine mass.
15. Consider two identical stars, with one star 5 times farther away than the other. How much
brighter will the closer star appear than the more distant one?
Answer: Quoting from page 355, “Apparent brightness decreases inversely with the square
of the distance...” So, the ratio of the distances is 1/5, the the apparent brightness ratio is
52 = 25. The closer star looks 25 times as bright.
23. What is a radial velocity curve? What kinds of stellar systems exhibit such curves?
Answer: Radial velocity is simply the component of velocity in the line of sight. A radial
velocity curve is just a graph of this as a function of time. The systems which exhibit radial
velocity curves are systems of 2 or more stars orbiting each other, where the line of sight from
Earth is not perpendicular to the plane of the orbit.
35. a. What is the approximate mass of a main sequence star that is 10,000 times as luminous
as the Sun? b. What is the approximate lumniosuty of a main-sequence star whose mass is
one-tenth that of the Sun?
Answer: a. Reading Fig 11-14: Find 104 L on the vertical scale; trace horizontally until you
intersect the dots; then drop vertically to the horizontal (mass) axis. A main-sequence star
10,000 times as luminous as our sun will have a mass of about 10 M . Chap 12 will show that
explains its short life of only 10 million years.
b. A star with a mass of 10−1 M will have a luminosity of about 10−3 L . That’s why its life
will be 1000 billion years.
44. What if? 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 orbit distance as it does to-day, what
would be different here? We will pick up this question again in Chapter 13 for further insights.
Answer: Checking the HR-diagram (fig 11-7), Regulus is a good example. It’s 100 times the
luminosity of the Sun. Scorched Cinder Earth at 1 AU. Of course, the B-type might be a white
dwarf with about 1/200 the luminosity of the Sun. Snowball Earth. To expand the question,
suppose the Earth-Clone were moved to Saturn’s distance (10 AU) for Regulus, or to 0.1 AU
for the White Dwarf. At these distances, the inverse square would reduce or increase the energy
to about what it is for us. But, take a look at Fig 11-4 and note that the hotter star emits a
very large fraction of its energy in the UV. So, even though we adjusted the total energy to its
current value, there is MUCH more UV. Makes life hard for land creatures, anyway.