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Quiz 4
Physics 55: Introduction to Astronomy
Professor Henry Greenside
Monday, October 24, 2005
This quiz is closed book. You can use a scientific calculator but no other electronic device. If you have any
questions during the quiz, please do not hesitate to ask me.
The following formulas and data may be useful:
∆λ
v
=
,
c
λ
λmax =
L = luminosity = area × σT 4 ,
2, 900, 000
in nm.
T (kelvin)
I = apparent brightness =
L
.
4πd2
True or False Questions (1 point each)
Please circle “T” for true or “F” for false to indicate the truth of the following statements.
1. T / F If an object approaches the Earth with a constant speed, then the amount by which its
spectral lines are blueshifted does not change as the object becomes closer and closer.
2. T / F The Doppler effect explains why stars have different colors.
3. T / F A stationary excited atom will start to move after emitting a photon.
4. T / F A greenhouse effect occurs on Earth because the Earth’s atmosphere is transparent to infrared
radiation.
5. T / F The nebular theory of how our solar system formed suggests that many other stars should
have planetary systems.
6. T / F Earth’s atmosphere and oceans resulted from the impact of icy planetesimals that originated
from beyond the frost line.
7. T / F The Earth’s Moon did not form by the condensation and accretion of nebula material, as was
the case for the comparably big moons of the jovian planets.
8. T / F The spectrum of a spectroscopic binary star has spectral lines whose wavelengths vary
periodically in time.
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Multiple Choice Questions (4 points each)
Circle the letter that best answers each of the following questions.
1. If one has knowledge only of the Doppler shift of some remote object, then an astronomer
(a) will know exactly the total speed of the object.
(b) will underestimate the total speed of the object.
(c) will overestimate the total speed of the object.
2. If a star is moving away from the Earth, an astronomer looking only at its thermal radiation spectrum
will
(a) deduce its correct surface temperature.
(b) overestimate its surface temperature.
(c) underestimate its surface temperature.
3. Which of the following correctly orders the different kinds of light from shortest to longest wavelength?
(a) X-rays, gamma rays, ultraviolet, visible light, infrared, radio.
(b) gamma rays, X-rays, infrared, visible light, ultraviolet, radio.
(c) radio, infrared, visible light, ultraviolet, X-rays, gamma rays.
(d) some other order.
4. If the emission lines in the spectrum of one object are less blueshifted compared to the same spectral
lines of a second object, then the first object
(a) is moving away from us faster than the second object.
(b) is moving away from us slower than the second object.
(c) is moving toward us faster than the second object.
(d) is moving toward us slower than the second object.
5. The approximate chemical composition by mass of the solar nebula that collapsed to form our solar
system was
(a) 98 percent hydrogen and helium and 2 percent other elements.
(b) 50 percent hydrogen and helium and 50 percent other elements.
(c) 98 percent hydrogen and 2 percent helium.
(d) roughly equal proportions of hydrogen, helium, water, and methane.
6. Most of the extrasolar planets so far discovered
(a) are more massive than the Earth and orbit further than 1 AU from the star.
(b) are more massive than the Earth and orbit closer than 1 AU from the star.
(c) are less massive than the Earth and orbit further than 1 AU from the star.
(d) are less massive than the Earth and orbit closer than 1 AU from the star.
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7. If RM is the radius of the Moon, TM is the surface temperature of the Moon, RS is the radius of the
Sun, TS is the surface temperature of the Sun, and dM S is the distance from the Moon to the Sun,
then the amount of energy that the sunny side of the Moon receives each second is given by which of
the following expressions (circle the appropriate equation number):
2
4
4πRM
σTM
.
(1)
2
4
4πRM σTS .
(2)
2
πRM
4πRS2 σTS4 .
(3)
2
4πdM S
2
πRM
4πd2M S σTS4 .
(4)
2
4πRS
πRS2
2
4
4πRM
σTM
.
(5)
2
4πdM S
8. A monochromatic tunable laser is adjusted to emit its light at precisely the wavelength 121.6 nm at
which a stationary hydrogen atom will make a transition from its ground state to its first excited state.
As a result, when the laser beam passes through a flask of cold hydrogen gas, most of the beam is
absorbed and little light is received in a spectrometer on the far side of the flask. If the laser now
moves with a large constant speed toward the flask of cold hydrogen gas, then
(a) the spectrum will remain dark since all light is absorbed by the flask.
(b) a bright spectral line now appears with wavelength 121.6 nm.
(c) a bright spectral line now appears but with wavelength less than 121.6 nm.
(d) a bright spectral line now appears but with wavelength greater than 121.6 nm.
(e) two bright spectral lines appear, one with wavelength 121.6 nm and one with a different wavelength.
(f) one or more absorption lines appear in the spectrum.
Open Questions
(8 points) List four patterns that a theory of how the solar system formed should try to explain.
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Extra Credit
You can solve one of the following problems for extra credit.
1. (4 points) Describe briefly one of the pictures that appeared in the “Astronomy Picture of the Day”
website over the past five days. (There is a link to this site on the Physics 55 home page.)
2. (6 points) If the surface temperature of the Sun were to double, how much would the steady-state
surface temperature of the Earth change? (You need to justify your answer to get credit.)
3. (6 points) A laser that emits monochromatic light with wavelength λ is aimed at a mirror that is
approaching the laser with constant speed v. The mirror is oriented such that it reflects the laser beam
directly back toward the laser. Explain whether the wavelength of the reflected light, as measured by
a spectrometer sitting next to the laser, is redshifted, blueshifted, or the same compared to λ.
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