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1
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- Q1. Sirius has an apparent
magnitude of about -1.5 and an absolute magnitude of 1.4. Would you have
to wait more than or less than 30 years for a radio signal to arrive
from Sirius?
a. Less than 30 years.
b. More than 30 years.
- 30 years matches light travel time for about 10 parsecs or 30 light
years. If it were at 10 pc,
then M would equal m. If
farther away than 30 light years would need to have an absolute
magnitude greater than the apparent magnitude, which it does.
- Question 2 (1
point) Hadar, at a distance of about 145 parsecs, has an absolute
magnitude of -5.2. Which of the following is most likely Hadar's
apparent magnitude?
a.-8.8 b.-3.9 c.0.6 d.85.9
- m – M = -5 +5 log d = -5 + 5log 145 = -5 + 5(2.16)
- m = -5.2 +5.8 = 0.6
- Question 3 In an
H-R Diagram, stars with the largest radius are found in the ____ of the
diagram. (Fill in the blank.)
a.center b.upper left corner c. upper right corner
d.lower left corner
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2
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- Question 4 (1 point) Which of the following is *not* true?
You will want to refer to Appendix A -- the information is available in
tables A-7 and A-9.
a.Aside from the sun, Sirius (the "dog star") is the
brightest star in our sky. b. Aside from the sun, the nearest
star is Proxima Centauri. c. Aldebaran, the brightest star in
Taurus, is a relatively cool K5 star. d. Most of the nearby stars
are type A or B stars. e. Vega, a future North Star, is one of
the six brightest stars in the sky, about the same brightness
(magnitude) as Capella.
- Question 5 (1
point) If a star cooled by a factor of 2 but expanded by a factor of 10
the ratio of its new luminosity to its old luminosity would be
a.0.125 b.1.412 c.6.250
d.64.0 e.100.0
- L = 4πR2σT4 so then L (new) = (0.5)4(10)2
or 100/16
- L = 6.25 solar luminosities
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3
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- Question 6 (1 point) If the O6 V star in the Orion Nebula is
magnitude 5.4, how far away is the nebula? (Hint: Use spectroscopic
parallax and look at plots of the HR diagram showing absolute magnitudes
on one axis.) Choose the best answer below.
a.10 parsecs.
b.120 parsecs. c.300 parsecs d.1600 parsecs e.10000
parsecs
- an HR diagram, estimate M = -6 or so. So d = 10 (m-M+5)/5 is
roughly 10^3+ and we are
done since only one answer is between 1000 and 10000. – I’ll work on the
board.
- Question 7 (1
point) In my novel Star Dragon, the characters fly to a distant binary
star system, SS Cygni. SS Cygni is a type of binary system in which a
white dwarf star is consuming a main sequence K star. If the average
separation of the stars is 1.5 million km, and the orbital period is 6.6
hours, what is the combined mass of the stars? (Hint: it will help to
immediately convert 6.6 hours to years and 1.5 million km to AU.)
a.0.6 solar masses.
b.6 solar masses. c.1.8 solar masses. d.18 solar
masses e.1.3 solar masses.
- Either solve from basic considerations (mass of KV star and white dwarf)
or from Kepler’s Law:
Mass = a^3/P^2, with unit conversions to AUs and years. Get 1.8 solar masses. I’ll work on the board.
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4
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Question 8
(1 point) You observe two stars with the same absolute magnitude
and determine spectroscopically that one is type B star while the other
is a type G star. What can you conclude about the sizes of the
stars?
a.Nothing. b.The G star is larger than the B star.
c.The B star is larger than the G star. d.The stars are the same
size.- L is equal for the two stars, so (TG/TB)4(RG/RB)2
must equal one. The
temperature of the G star is cooler than that of a B star, so the first
term must be less than one.
There for the other term must be greater than one. This is also clear from looking
at lines of constant radius on an HR diagram, that a G star would have
to be a giant to have as much luminosity as a B star.
- Question 9 (1
point) A 3 solar mass star on the main sequence would have a luminosity
how many times larger than the sun's?
a.3 b.9 c.47d.94 e.1000
- Just some math here. L = M3.5,
and 33.5=47.
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5
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- Question 10 (1 point) Sunsets appear red for the same reason
that some stars in space appear red -- they are both seen through dust
particles. Why does this make them appear red?
a.The dust is heated and emits red light.
- b.The dust scatters blue light more than red light, such that more of
the red light passes directly though the dust.
- c.Dust polarizes light which always makes it redder.
- Question 11 (1
point) The Pleadies is an open star cluster not too far from us. It also
represents a textbook example of a type of nebula. Which type is
it?
a.An emission nebula b.A dark nebula c.A reflection
nebula d.A stellar nebula e.None of the above
Question 12
(1 point) Stars are born in regions which (before the new stars
can modify them) are
a.dense cold molecular clouds. b.hot ionized hydrogen
regions. c.low density regions between clouds
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6
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- Question 13 (1 point) What do we see in the Orion nebula
that indicates it is a region of new star formation?
a.Hot O stars
- b.Herbig-Haro Objects
- c.Dust Disks around stars
- d.All of the Above
- e.None of the Above
- Question 14 (1 point) The lifetime of high temperature main
sequence O and B stars is _____ the lifetime of low temperature K and M
stars. (Fill in the blank.)
- a.shorter than
- b.longer than
- c.about the same as
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7
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- Question 15 (1
point) In the movie Blade Runner, replicant Roy Batty was compared to a
candle: "A candle that burns twice as bright burns half as
long." Star lifetimes are more complex than this (i.e.,the
relationship is not linear). If the lifetime of a star is half that of
the sun, how luminous is it?
a.1 solar luminosity
b.2 solar luminosities c.0.5 solar luminosities
d.1.3 solar luminosities e.2.6 solar luminosities
- Need to use L = M3.5 and Lifetime = M -2.5
- First, Lifetime = ½ = (M) -2.5, solve to get M (1.3
solar masses)
- Plug into the other equation to get 2.6 solar luminosities. I’ll work on the board.
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Notes