Note: The word "brightness" is used in this quiz to mean how bright a star appears in the sky. The word "luminosity" refers to how bright a star is (independent of its distance from us).
1.What happens when a stationary observer views a moving light
source?
a. the speed, frequency, and wavelength of the light all change
b. the frequency of the light doesn't change, but its wavelength and
speed do
c. the speed of the light doesn't change, but its frequency and wavelength
do
d. the wavelength and brightness of the light change, and the speed and
frequency stay the same
The speed of the light coming to the observer from the moving light source
does not change. The wavelength does change, and speed of light = wavelength
* frequency, so the frequency must change too.
2. Because of the Doppler shift, a light source emits light that is
_________ when it is moving ____________ than when it is at rest.
a. redshifted (at longer wavelengths); away from us
b. redshifted (at longer wavelengths); across our line of sight
c. redshifted (at longer wavelengths); toward us
d. blueshifted (at shorter wavelengths); in any direction
If an object is moving away from us, its light appears redshifted. If it is
moving toward us, its light appears blueshifted. If it is moving across our
line of sight, the light is not Doppler-shifted.
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 one year
d. the distance at which the parallax angle is 1 arcsecond
This is a definition that you need to know to understand what's going on
when we talk about distances.
4. The trigonometric parallaxes of Stars A and B are measured. Star A has
a parallax angle of 1 arcsecond, and Star B has a parallax angle of 0.5
arcsecond. Which of the following is true?
a. Star A is twice as far away as Star B
b. Star B is twice as far away as Star A
c. Star A is twice as large as Star B
d. Star B is twice as large as Star A
If we know a star's parallax angle, we can measure its distance. A small
parallax angle means the star is far away, a large one means it is nearby.
5. We move two stars, Vega and the sun, so that both are 10 parsecs from
Earth. We find that Vega's magnitude is 0 (zero), and the sun's is +5.
Now move the stars back to their original places. Suppose that Vega has
a planet orbiting it at a distance of 1 AU. How bright will Vega appear
from that planet?
a. 100 times as bright as the sun
b. 5 times as bright as the sun
c. 1/5 as bright as the sun
d. 1/100 as bright as the sun
For this question, you need to remember that the magnitude scale runs
"backwards" (so a star with magnitude 0 is brighter than one with magnitude
+5), and that a difference of 5 magnitudes is equal to a factor of 100
in brightness.
6. What do you need to know to determine a star's luminosity?
a. brightness and color
b. brightness and distance
c. color and distance
d. distance and radius
If you know how bright a star appears, and how far away it is, you can use
the inverse square law to find how luminous it is.
7. Why do hot O stars have weaker hydrogen lines in their visible-light
spectra than cooler A stars do?
a. O stars contain a lower percentage of hydrogen than A stars
b. The hydrogen in O stars' atmospheres is all in the ground state,
and absorption lines starting from the ground state aren't in the visible
c. O stars do have hydrogen lines in the visible, but they're emission lines
d. Most hydrogen atoms in O stars are ionized, so they can't absorb
photons
O stars are hot enough to ionize most of their hydrogen, which means that the
electrons have escaped from the protons. If there are no electrons orbiting
protons, there cannot be any emission or absorption lines.
8. In order to make an H-R diagram, you need to know what two properties
of stars?
a. temperature and brightness
b. color and temperature
c. H and R
d. temperature and luminosity
You have to be careful about the difference between temperature and
luminosity here. Brightness depends on a star's luminosity and its
distance from us. A star's temperature obviously does not depend on its
distance from us, so plotting temperature versus brightness doesn't tell
us anything about stars. Color and temperature are related by Wien's law,
so plotting them wouldn't tell us anything about stars, either. H and R
are the initials of Hertzsprung and Russell, who first came up with the
diagram. Plotting the temperature versus luminosity for stars tells us a
great deal about the properties of stars.
9. What is the Main Sequence on the H-R diagram?
a. the clump of stars in the upper right part of the diagram
b. the clump of stars in the lower left part of the diagram
c. the diagonal line running from the top left to bottom right
d. all the stars brighter than the sun
The stars in the upper right of the H-R diagram are red giants.
The stars in the lower left of the H-R diagram are white dwarfs.
10. The star Spica is hotter and more luminous 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
For this question, you have to remember that hot stars are on the left of
the H-R diagram, and more luminous stars are at the top.
11. Which of the following is the mass and luminosity of a possible Main
Sequence star?
a. mass = 1 solar mass, luminosity = 10 solar luminosities
b. mass = 2 solar masses, luminosity = 11 solar luminosities
c. mass = 2 solar masses, luminosity = 0.5 solar luminosities
d. mass = 10 solar masses, luminosity = 1 solar luminosity
This is a real Main Sequence star (it's Altair). We know that choices a
and d are wrong, because stars with 1 solar mass have 1 solar luminosity.
Choices c and d must be wrong, because the mass-luminosity relationship
tells us that a more massive star is more luminous than a less massive
one.
12. The star Capella is the same color as the sun, but it is much more
luminous. What can be inferred from this?
a. Capella is bigger than the sun and has a lower surface temperature
b. Capella is bigger than the sun and has a higher surface temperature
c. Capella is smaller than the sun and has the same surface temperature
d. Capella is bigger than the sun and has the same surface temperature
Capella has to have the same surface temperature as the sun, because it is
the same color. If it is more luminous, it must be because it has
a larger surface area.
13. Stars A and B are the same size. The temperature of Star A is twice
the temperature of Star B. Which of the following is true?
a. Stars A and B have the same luminosity
b. Star A is twice as luminous as Star B
c. Star A is 4 times as luminous as Star B
d. Star A is 16 (24) times as luminous as Star B
Use the Stefan-Boltzmann law here, which states that if two blackbodies have
the same surface area, their luminosities are proportional to the fourth
power of their temperatures.
For questions 14 and 15: Sirius A is twice as massive as its white dwarf companion, Sirius B.
14. The equation for the location of the center of mass is M1*R1 = M2*R2. Where is their center of mass?
a. at the center of Sirius A
b. halfway between Sirius A and Sirius B
c. between the two stars, but closer to Sirius A
d. between the two stars, but closer to Sirius B
The center of mass of the two stars must be between them, and it will be
closer to the more massive star.
15. The orbital period of Sirius B is 50 years. What is the orbital period
of Sirius A?
a. 25 years
b. 50 years
c. 100 years
d. 200 years
The center of mass must be between the stars at all times. That means they
have to take the same amount of time to go around it.
16. If you know the velocities of the stars in a binary system, but you do
NOT know the distance from the stars to us, which of the following can you
find?
a. the luminosities of the stars
b. the masses of the stars
c. the distance between the stars
d. the radii of the stars
If you know the velocities of the stars, you can use Kepler's third law to
find the masses, as we discussed in class and in the Web notes.
17. How can you tell that the star you are observing is actually an
eclipsing binary?
a. large telescopes reveal stellar disks eclipsing each other
b. the moon eclipses the star at least twice a year
c. sometimes the star appears red, and sometimes it appears blue
d. its brightness varies regularly
An eclipsing binary periodically goes dimmer as one star blocks part of the
light from the other star. Eclipsing binaries do not always consist of
one red star and one blue star, so choice c is wrong. The moon can eclipse
stars located near the ecliptic, but eclipsing binaries can be anywhere in
the sky, so choice b is wrong. In general, telescopes cannot reveal the disks
of stars- those pictures you see are artists' conceptions of what the stars
look like.
18. What is the energy source for a star like the sun?
a. burning a fuel such as coal (combining it with oxygen)
b. gravitational potential energy released by the contraction of the sun
c. nuclear fission of uranium into thorium and helium
d. nuclear fusion of hydrogen into helium
We considered all of these possibilities in class. Choices a and b won't
work because they don't allow the sun to shine for as long as we know it
has, and choice c doesn't work because the sun doesn't have very much
uranium in it.
19. Which of the following elements is most common in the sun?
a. carbon
b. uranium
c. hydrogen
d. helium
The sun is about 3/4 hydrogen and 1/4 helium, with traces of other things.
20. What must you do to fuse two protons?
a. overcome the nuclear force, which makes the protons repel each other
b. collide the protons with two neutrinos, then with each other
c. overcome the electric force, which makes the protons repel each other
d. wait until the gravitational attraction of the protons makes them collide
This is a question about the "volcano" diagram we saw at the end of class
on Thursday. Protons are positively charged, and so they repel each other.
But if they can get close enough, the nuclear force (which makes them
attract each other) overcomes the electric force.