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1
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- Question 1 (1 point) If the inner accretion disk around a
black hole has a temperature of 1 million K, at what wavelength will it
radiate the most energy?
a.1 million nm b. 3 nm c.
3 million nm d. 1 nm e.10 nm
- Wien’s Law: λ = 3000000 nm K/T
- λ = 3 nm (which is in the X-rays)
- Question 2 (1 point)
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 (z) of this quasar?
a. 0.5 b. 1 c. 1.5 d. 2 e. 2.5
- z = Δλ/λ and λ = 121.6 nm and Δλ = 425.6
nm – 121.6 nm
- so z = 304 nm /121.6 nm = 2.5
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2
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- Question 3 (1 point) Quasars can be 1000 times more luminous
than a 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, what would the apparent magnitude of the galactic core be?
a. -13.9 b.19 c. 0 d.-4.6 e.8.5
- We need to use the equation: m – M = -5 + 5 log d(pc)
- d = 8500 pc, M = -28.5, so: m + 28.5 = -5 + 19.6
- m = -13.9
- Question 4 (1 point)
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.
- v/c = ((z+1)2-1)/((z+1)2+1) = (2.25-1)/(2.25+1) =
(1.25)/(3.25) = 0.38
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3
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- Question 5 (1 point) 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 6 (1 point)
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 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 (AKA Gyrs), as in class.)
a. 5 billion yrs b.10 billion yrs c.15 billion yrs d. 20 billion
yrs
- Worked this in class before, so I’ll work it the other way. Under this assumption, Age = 1/H. If Age is 13.6 Gyrs for H = 72
km/s/Mpc, then Age for H = 100 km/s/Mpc can be written thus:
- Age(H=100)/Age(H=72) = (1/100 km/s/Mpc)/(1/72 km/s/Mpc)
- Age(H=100)/13.6 Gyrs = 72/100
- Age(H=100) = 10 Gyrs
- As usual, pick the closest answer.
We don’t always know numbers like H to precison.
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Notes