1. Which of the following describes a solar eclipse?
a. the earth is between the sun and the moon, and the earth is in the
moon's shadow
b. the earth is between the sun and the moon, and the moon is in the
earth's shadow
c. the moon is between the earth and the sun, and the moon is in the
earth's shadow
d. the moon is between the earth and the sun, and the earth is in the
moon's shadow
Choices a and b can be eliminated because, in them, shadows are pointing
toward the sun. Shadows cast by the sun must always point away from
the sun, so a and b are wrong. c describes a lunar eclipse, and d describes
a solar eclipse.
2. What is a light year?
a. a unit of time equal to 6 trillion seconds
b. the average distance between the Earth and the Sun
c. the distance light travels in a year
d. the time the Earth takes to orbit the Sun
If you remember nothing else from this class, I want you to remember that
a light year is a unit of distance, not time (Many movies
and TV shows get this wrong). A light year is the distance light travels
in a year, or about 9.5 trillion kilometers (6 trillion miles).
3. What is a parsec?
a. a unit of time equal to about 3.26 million seconds
b. a unit of angle equal to 1/3600 of a degree
c. the distance light travels in a year
d. the distance at which the parallax angle is 1 arcsecond
A parsec is another unit of distance that we used a lot in this class.
It's convenient if you are using trigonometric parallax to measure
distances (see question 4).
4. That famous observing team of Drs. Harden, Petrie, and Rozyczka measures
the parallaxes of the stars Sirius and Altair. They find that Sirius has
a parallax angle of 0.30 arcsecond, and Altair has a parallax angle of 0.15
arcsecond. What do they conclude?
a. Sirius is twice as far away as Altair
b. Altair is twice as far away as Sirius
c. Sirius is twice as large as Altair
d. Altair is twice as large as Sirius
Trigonometric parallax (or just "parallax") is the primary method used to
find distances to nearby stars. It works less and less well the farther
away the stars get (see question 5).
5. Drs. Harden, Petrie, and Rozyczka have discovered a Cepheid in a distant
galaxy, and want to determine exactly how distant that galaxy is. They
have measured the Cepheid's period and brightness. Can they determine its
distance?
a. yes, by using the period-luminosity relation to find the luminosity,
and with the luminosity and brightness they can find the distance
b. yes, by measuring its trigonometric parallax
c. yes, by measuring the Doppler shift of its spectrum
d. no, they would need to know its radius to do that
Cepheids are important because their periods and luminosities are related.
Once you know the apparent brightness and the luminosity of a star, you
can find its distance using the inverse-square law. Cepheids are quite
bright, and therefore well-suited to measuring the distances to other
galaxies. Trigonometric parallax will not work for measuring distances
outside our galaxy, and the Doppler shift of the spectrum would tell you
the Cepheid's velocity, not its distance from us.
6. The wavelength of blue light is about half that of red light. The
frequency of blue light is ______________ that of red light, and the
energy of a blue light photon is ______________ that of a red light
photon.
a. half, half
b. two times, four times
c. two times, two times
d. four times, four times
This is a "Q" class, so we had to have some math. The relevant equations
here are:
7. The Keck telescope has four times the diameter of the Hubble Space
Telescope. Ignoring the effects of Earth's atmosphere, the Keck telescope
has _____________ angular resolution and _____________ the
light-collecting power of the Hubble Space Telescope.
a. one fourth as good; one sixteenth
b. one half as good; four times
c. four times better; sixteen times
d. four times better; four times
More math. Angular resolution is better in a larger telescope, and
improves linearly with the diameter of the telescope (i.e. a telescope
twice as large will have angular resolution twice as good). Light-gathering
power is proportional to the area of the telescope, which is
8. Mars orbits the sun at a distance of approximately 1.5 AU. What is its
orbital period in years?
a. the cube root of 1.5, which is about 1.14
b. the square root of 1.5, which is about 1.22
c. the cube root of 1.5 squared, which is about 1.31
d. the square root of 1.5 cubed, which is about 1.84
This question is about Kepler's third law, relating the period of a
planet's orbit in years to its distance from the sun in AU's.
9. Proxima Centauri, the nearest star to the sun, is a Main Sequence star
that is cooler than the sun. Where on the H-R diagram would it be found?
a. above and to the right of the sun
b. above and to the left of the sun
c. below and to the right of the sun
d. below and to the left of the sun
We asked this one to make sure you remember the H-R diagram. The key
to this question is to remember that the temperature scale runs "backwards"-
i.e. hotter stars are on the left, cooler on the right. Also, remember
that the Main Sequence runs from the top left to bottom right.
For questions 10, 11, and 12: Alpha Centauri is a binary, consisting of
two Main Sequence stars, Alpha Centauri A and Alpha Centauri B. Alpha
Centauri A is a one-solar-mass star, and Alpha Centauri B is a
one-half-solar-mass star. They are separated by about 10 AU, the
distance between the sun and Saturn.
10. Alpha Centauri A is yellow. What color is Alpha Centauri B?
a. orange
b. yellow
c. blue
d. a one-half-solar-mass star is too cool to fuse hydrogen, so it doesn't
emit any visible light
A less massive Main Sequence star will be cooler than a more massive one.
If it's cooler, its surface color moves toward the red end of the spectrum.
11. Alpha Centauri A takes 30 years to orbit the binary's center of mass.
How long does Alpha Centauri B take to orbit the center of mass?
a. 15 years
b. 30 years
c. 60 years
d. 900 years
If you took the quiz that covered binary stars, you should remember this.
The center of mass of a binary must always be on the line connecting the
two stars, and the two stars are always the same distance from the center
of mass. Think of it as two stars on the ends of a stick, being twirled
like a baton. The two ends of the stick both take the same amount of time
to go around.
12. Which star will leave the Main Sequence first?
a. Alpha Centauri A, because it is more massive
b. Alpha Centauri B, because it is less massive
c. they will both leave at the same time, because they were born together
d. there is no way to tell, because the stars are touching each other and
exchanging mass
The more massive the star, the shorter its life. You know the stars are
not touching each other, because a one-solar-mass Main Sequence star is
much smaller than 10 AU in diameter (fortunately for us on Earth). In fact,
a red giant with one solar mass can only get to the size of the orbit of
Mars (1.5 AU), so the two stars will never be touching each other.
13. When does a protostar become a star?
a. when it begins to fuse hydrogen in its core
b. when its mass becomes greater than half a solar mass
c. when the peak of its spectrum moves into visible light
d. when the cloud of gas and dust around it is cleared away
Hydrogen fusion in its core is what defines a Main Sequence star. The
lower mass limit for stars is much lower than half a solar mass (it's about
1/10 of a solar mass). A star can have the peak of its blackbody spectrum
in the infrared and still be a star, and the cloud of gas and dust clears
away after the star begins to fuse.
14. Which phase in a low-mass star's evolution lasts longest?
a. protostar
b. Main Sequence
c. red giant
d. horizontal branch
This is an important fact about the lives of stars that you should
know. All of the other phases of evolution mentioned here are much
shorter than the Main Sequence phase.
15. What is the difference between a red giant and an asymptotic giant?
a. an asymptotic giant's interior is like a red giant's, but its surface
is hotter, so it's not red
b. high-mass stars become red giants; low-mass stars become asymptotic
giants
c. an asymptotic giant is a star that is traveling up the giant branch
and will eventually become a red giant
d. a red giant has a degenerate helium core; an asymptotic giant has a
degenerate carbon core
The red giant phase in a low-mass star's evolution is reached after
the supply of hydrogen in the core is exhausted. Then the star starts
burning helium into carbon as a horizontal branch star. After the helium
supply is exhausted, the star moves up the giant branch again, taking a
slightly different path. This is called the asymptotic giant branch.
16. The Pleiades and the Hyades are two star clusters in the constellation
of Taurus. The Pleiades has many bright blue stars but no bright red
stars, and the Hyades has many bright red stars and few bright blue stars.
What can we conclude?
a. the Pleiades is older than the Hyades
b. the Hyades is older than the Pleiades
c. the Pleiades is closer than the Hyades
d. the Hyades is closer than the Pleiades
Bright blue stars are Main Sequence stars. For a cluster to have them,
it must be young enough that those stars have not left the Main Sequence
yet. Bright red stars are red giants or asymptotic giants. For a cluster
to have those, it must be old enough that some stars have left the Main
Sequence. Thus, the Hyades is older than the Pleiades.
17. Drs. Harden, Petrie, and Rozyczka discover a binary system consisting
of a white dwarf of about 1.4 solar masses and a red giant of about 2
solar masses. They report that the red giant seems to be losing mass,
which is going onto the white dwarf. What will happen to this system
when the white dwarf exceeds the Chandrasekhar limit?
a. the white dwarf will collapse into a black hole
b. a Type I supernova
c. a Type II supernova
d. mass will start flowing back onto the red giant
A Type I supernova occurs when the mass of a white dwarf exceeds the
Chandrasekhar limit. In fact, the Chandrasekhar limit is about 1.4 solar
masses, so this system is going to have a Type I supernova relatively
soon.
18. Which of the following fusion reactions cannot produce
energy to keep a star from collapsing in on itself?
a. hydrogen into helium
b. helium into carbon
c. silicon into iron
d. iron into tellurium
Fusion of iron takes up energy. All the others produce energy. Hydrogen
fusion powers Main Sequence stars, helium fusion powers horizontal branch
stars, and silicon fusion powers very massive stars just before they
explode in a Type II supernova.
19. How big is a neutron star?
a. about the size of San Francisco (about 12 kilometers across)
b. about the size of the Earth
c. about the size of the Sun
d. about the size of the orbit of Mars
Neutron stars are incredibly dense. They have a mass of about 3
solar masses, in an object the size of San Francisco. This is the density
of atomic nuclei.
20. Why are black holes black?
a. because they are white dwarfs that have lost all their remnant heat
b. because they are so hot they only emit light in the ultraviolet
c. because their escape velocity is greater than the speed of light, so
any light they emit can't escape
d. because they are moving away from us so quickly, their light is
redshifted out of the visible range
A black hole is defined as an object whose escape velocity is greater
than the speed of light. Something inside the black hole could be emitting
all kinds of light, but the light cannot escape, so no observer outside
the black hole could ever see it.