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1
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- Question 1 If interstellar dust
makes and RR Lyrae star look 1 magnitude fainter than it should, by how
much will we overestimate its distance?
a.1.6 times b.3.2 times c.1600 times d.5.2
times e.Actually we will underestimate its distance.
- Distance equations: m – M = -5 + 5log(d) and d = 10 (m-M+5)/5
where distance is in parsecs
- True distance = 10 (m-M+5)/5 if no dust.
- Dusty distance = 10 (m+1-M+5)/5
- Dusty distance/True Distance = 10 (m-M+6)/5 / 10 (m-M+5)/5
- = 10 (m-M+6)/5 –
(m-M+5)/5
- = 10 6/5 – 5/5
- = 10 1.2-1
- = 10 0.2
- = 1.6
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2
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- Question 2 Observationsof galaxies and clusters of galaxies
indicate that roughly ____ percent of the universe is dark matter.
a.5%b.25%c.50%d.75%e. 95%
- You have to look this one up, but it is a number you should know.
- Question 3 What is the most common type of galaxy, based on
what we see in our own Local Group?
a.irregular galalaxies b.flocculant spiral galaxies
c.grand design spiral galaxies d.giant ellipticals e.dwarf
ellipticals
- This one you should know from the figure of the Local Group in the
slides/text. I’d make sure I knew
this for the exam!
- Question 4 An elliptical galaxy could
- a.evolve into an irregular galaxy when it has used up all of its gas and
dust.
- b.be formed from the collision and merger of spiral galaxies.
- c.become a starburst galaxy if it were to move through the hot
intergalactic medium of a cluster.
- d.evolve from a single spiral galaxy when that spiral used up all of its
gas and dust.
- Elliptical galaxies have already used up their gas and dust, and it is
thought they can be formed from spiral mergers.
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3
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- Question 5 If Galaxy A is four times more distant than
Galaxy B, then according to the Hubble Law, the recessional velocity of
Galaxy A is larger than that of Galaxy B by what factor?
a.2 times. b.4 times. c.8 times. d.16 times. e.32 times.
- The Hubble Law is LINEAR. The
redshift v = Hxd. Four times the
distance, four times the velocity.
- Question 6 If a galaxy has a radial velocity (redshift) of
5000 km/s, how far away is it? Assume a Hubble Constant of 70
km/s/Mpc.
a.7 Mpc b.20 Mpc c.49 Mpc d.71 Mpc e.100 Mpc
- V = Hxd, so d = v/H = 5000 km/s/70 km/s/Mpc – the km/s cancel out to
give 5000/70 Mpc = 71 Mpc
- Question 7 If you
find a metal rich star it is more likely to be a member of which
Population?
a.The Population I (disk). b.The Population II (halo)
c.Either is equally likely.
- Metal rich stars are usually Population I stars found in the disk.
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4
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- Question 8 Quasars can be 1000 times more luminous than an
entire galaxy. The absolute magnitude of such a luminous quasar would be
about M = -28.5. If the black hole in the center of our galaxy became a
quasar, and obscuring gas and dust did not dim it, what would the
apparent magnitude of the galactic core be? Think about the answer and
what that would look like in the sky.
a.-13.9 b.19 c.0 d.-4.6 e.8.5
- Distance equation: m – M = -5 + 5logd where d is in parsecs
- So, we can plug in M and distance: m – (-28.5) = -5 + 5log8500
- m = -28.5 -5 + 5x(3.93) =
-33.5 + 19.6 = -13.9
- That’s about the same magnitude as the FULL MOON!
- Question 9 If we take a spectrum of a quasar and see that
the Lyman alpha line, observed in the laboratory at a wavelength of
121.6 nm, appears at a wavelength of 425.6 nm, what is the redshift of
this quasar?
a.0.5 b.1 c.1.5 d.2 e.2.5
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5
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- Question 10 A quasar is observed to have a redshift z=0.5.
What recessional velocity does this correspond to?
a.4% of the speed of light. b.38% of the speed of light.
c.50% of the speed of light. d.77% of the speed of light.
e.99% of the speed of light.
- Use the relativistic Doppler equation: v/c = [(z+1)2
-1]/[(z+1)2+1]
- V = 0.38 c
- Question 11 If the expansion rate is constant (no
acceleration or deceleration) and H equals 100 km/sec/Mpc (not the real
value) then what would be the age of the universe? (Hint: you can either
derive the answer directly from the appropriate equations, or you can
find the answer by comparing this value of H to the more realistic one
of 72 km/sec/Mpc which implies 13.6 billion years.)
a.5 billion years b.10 billion years c.15 billion years d. 20
billion years
- This is just the simple-minded solution, time = 1/H. Working it out on the slide gives the
right answer, 10 billion years, which is 72% of 13.6 billion years.
- Also note that even for “constant” expansion the Hubble constant would
be larger in the past.
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6
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- Question 12 The cosmic background radiation comes from a
time in the evolution of the universe when
- a. "inflation" was occuring.
- b. electrons began to recombine with nuclei to form neutral atoms.
- c. gamma rays had enough energy to destroy nuclei.
- d. gravity began to pull material together to form galaxies.
- Question 13 IF we were to observe that the Hubble constant,
H, was larger for objects at greater distances, then that would imply
that
- a. the expansion rate of the universe is accelerating.
- b. the expansion rate of the universe is slowing. (MISGRADED!)
- c. the expansion rate is constant with time.
- OK, the easiest way to think of this is to realize that the Hubble law
says that v = Hxd, so at larger distances, the slope would become
STEEPER (remember, H is just the slope of the plot of d vs. v). A steeper slope means that the
velocity changes more quickly as the distance changes. Larger distances mean earlier
times. So, the expansion rate was
faster in the past than it is now.
That would mean that the expansion rate must be slowing. But it is an ill-posed question I
inherited from another professor since you can get this behavior for all
sorts of universes. It’s
complicated. For instance, for
constant expansion rate, t = 1/H, so in the early universe (i.e. large
distances), the universe was younger and H had to be bigger!
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Notes