Astr 222 Practice Midterm #1 Answers
- Describe what is meant by Population I objects
and Population II objects. Give examples.
Good answer: Population II
objects are older, metal-poor stars and globular clusters like those
found in the Galaxy's
halo. Population I refers to more metal-rich objects like disk
the Sun, or young open star clusters.
Weak answer: Populations II stars are old stars, while the Sun is a
Population I star.
- Describe the Magellenic Clouds.
Good answer: The Magellenic
Clouds are two
dwarf irregular galaxies orbiting around the Milky Way. They are small
or smaller) and low mass (a few percent or less of the Galaxy's mass)
lie ~ 50-60 kpc away. They orbit our Galaxy with a period of a few
Weak answer: The Magellenic Clouds are dwarf galaxies that orbit the
- Why do we believe there is a lot of dark matter
in the galaxy?
Good answe: If we look at the
rotation curve of the Galaxy -- the orbital speed as a function of
radius -- we find that the rotation curve stays flat well beyond the
point where there are no more stars. Since V2 ~ GM/R, M ~ RV2/G
so that if the rotation curve is flat there must be more and more mass
in the Galaxy as we go to large
radius. But since we have run out of stars, that mass must be "dark
Weak answer: The galaxy rotates to fast to be held together by the stars' gravity
- Describe what is meant by the "luminosity
of galaxies. In this context, what is L*? Sketch what this function
function describes the relative numbers of galaxies of different
-- i.e., how many bright galaxies, how many faint galaxies. There are a
lot more faint galaxies than bright ones. L* is the galaxy luminosity
the number of galaxies drops quickly, and is about 2x1010
Here is a sketch:
answer: The luminosity function tells you how many bright galaxies
there are are. (no sketch, or unlabelled sketch)
- Why does the X-ray variability of the galactic
center place a limit on the size of the object at the center?
Good answer: In order for the
of an object to vary appreciably, it must coordinate itself so that the
entire object varies its light output coherently. In other words, one
needs to know what the other is doing, so they can both "get bright" or
fade. But the fastest that information (ie "time to get bright!") can
conveyed from one side to the other is at the speed of light, and that
a time t=R/c. So if an object varies over a time t, it must have a size
is no larger than ct.
Weak answer: Big things cannot change their brightness quickly.
- You are studying a distant star cluster, and
that the stars appear too red for their spectral type -- their colors
too red by 0.25 in B-V color. You also find that there is a Cepheid
variable star in this cluster, with a period of 10 days and a mean
apparent V magnitude of 7.0. How far away is the cluster?
If the stars are "too red" by
0.25, that means E(B-V)=0.25. But since AV=3.2E(B-V), stars
are too red by 0.25 are extincted by 3.2x0.25=0.8 magnitudes. So if we
correct for dust, the mean apparent V magnitude is actually 6.2 (ie
7.0-0.8). If the
Cepheid has a period of 10 days, then it has an mean absolute magnitude
M=-2.8xlog(10)-1.43=-4.23. Then we can get distance from m-M=5logd-5,
or d=1200 pc.
- If your telescope can reliably measure the
brightnesses of stars down to 20th magnitude, what is the fathest away
you could detect a Cepheid variable? Remember that the most luminous
Cepheids have period
of about 100 days.
If a luminous Cepheid has a
period of 100 days, it has an absolute magnitude of -7.0. If you detect
them down to 20th magnitude, that means m-M=20-(-7)=27, or a distance
of d=2.5 million parsecs (Mpc). And yes, we have measured Cepheids out
that far (and farther)!
Sorry, no answers to the essay questions, since one of them will be on the exam!