Please do NOT turn this in. It is for your own use as you study the material since exam three. Solutions may be found in the online notes for Dec. 12 at www.astro.umd.edu/~ecasper
1 . Which statement about accretion disks is NOT true?
- The primary factor determining whether a white dwarf has an accretion disk is the white dwarf's mass.
- The gas in the inner parts of the disk travels faster than the gas in the outer parts of the disk.
- Accretion disks are made primarily of hydrogen and helium gas.
- The gas in the inner parts of the disk is hotter than the gas in the outer parts of the disk.
2 . Which of the following is NOT true about differences between novae and supernovae?
A. Novae occur only in binary star systems, while supernovae can occur both among single stars and among binary star systems.
B. Supernovae eject gas into space but novae do not.
C. Novae are much less luminous than supernovae.
D. The same star can undergo novae explosions more than once, but can undergo only a single supernova.
3 . Which statement about pulsars is NOT thought to be true?
A. Pulsars are kept from collapsing by neutron degeneracy pressure.
B. Pulsars can form only in close binary systems.
C. A pulsar must have a very strong magnetic field and rotate quite rapidly.
D. All pulsars are neutron stars, but not all neutron stars are pulsars.
4 . What is the basic definition of a black hole?
A. A black hole is a compact mass that emits no visible light.
B. A black hole is a dead star that has faded from view.
C. A black hole is any object made from dark matter.
D. A black hole is an object with gravity so strong that not even light can escape.
5 . What do we mean by the event horizon of a black hole?
A. It is the place where X rays are emitted from black holes.
B. It is the very center of the black hole.
C. It is the distance from the black hole at which stable orbits are possible.
D. It is the point inside of which neither light nor anything else can escape the black hole.
6 . Which of the following statements about black holes is NOT true?
A. If you fell into a black hole, you would experience time to be running normally as you plunged rapidly across the event horizon.
B. A spaceship passing near a 10 solar mass black hole is much more likely to be destroyed than a spaceship passing at the same distance from the center of a 10 solar mass main-sequence star.
C. If you watch someone else fall into a black hole, you will never see them cross the event horizon. However, they will fade from view as the light they emit (or reflect) becomes more and more redshifted.
D. Although we are not 100% certain that black holes exist, we have strong observational evidence in favor of their existence.
7 . All the following types of objects are found almost exclusively in the disk (rather than the halo) of the Milky Way except ______.
A. globular clusters
B. high-mass, red supergiant stars
C. x-ray binaries
D. young stars
8 . Which of the following statements about how halo stars compare to our Sun is NOT true?
A. Most stars in the halo contain a much lower percentage of heavy elements than the Sun
B. Most stars in the halo are less luminous than the Sun
C. Most stars in the halo have cooler surface temperatures than the Sun.
D. Most stars in the halo are in a more advanced stage of life than the Sun, such as in red giant stage.
9 . Which of the following statements is NOT true of the object known as Sgr A* in the center of our Galaxy?
A. Its accretion disk is so large that it shines brightly in visible light.
B. It is a source of bright radio emission.
C. It is a source of X-ray emission that we have observed with telescopes in space.
D. It is thought to harbor a black hole of more than 2 million solar masses.
10 . In a photo like the Hubble Deep Field (Figure 15.1 in your text), we see galaxies in many different stages of their lives. In general, which galaxies are seen in the earliest (youngest) stages of their lives?
A. The galaxies that are the reddest in color
B. The galaxies that have the most hot, young O and B stars
C. The galaxies that are farthest away
D. The galaxies that are nearest to us
11 . Suppose that we suddenly discovered that all these years we'd been wrong about the distance from Earth to the Sun, and it is actually 10% greater than we'd thought. How would that affect our estimate of the distance to the Andromeda Galaxy?
A. It would mean the distance to the Andromeda Galaxy is also 10% greater than we thought.
B. It would mean the distance to the Andromeda Galaxy is 10% less than we thought.
C. It would not have any effect on our estimate of the distance to the Andromeda Galaxy.
D. It would mean that all the objects we've assumed are standard candles really are not good standard candles, and therefore that we have no idea of the true distance to the Andromeda Galaxy.
12 . Which of the following phenomena are NOT thought to be results of collisions or other interactions between galaxies?
A. The fact that elliptical galaxies are more common in clusters of galaxies than outside clusters
B. Starbursts
C. The fact that spiral galaxies have both disk and halo components
D. The presence of very large, central dominant galaxies in clusters of galaxies
13 . Most active galactic nuclei are found at large distances from us, with relatively few nearby. What does this imply?
A. Active galactic nuclei can form only at large distances from the Milky Way.
B. Supermassive black holes existed only when the universe was young, and no longer exist today.
C. The jets seen in many active galactic nuclei must cause them to move far away from us.
D. Active galactic nuclei tend to become less active as they age.
14 . Which of the following does NOT support the theory that active galactic nuclei are powered by accretion disks around massive black holes?
A. Spectral lines from the galactic center indicate that clouds of gas are orbiting a central object at very high speed.
B. The total amount of radiation coming from the galactic center is, in some cases, comparable to the amount of radiation put out by 10 billion or more ordinary stars.
C. Observed radiation from the galactic center varies significantly in brightness in times as short as a few days.
D. Infrared observations show that many stars are forming near the centers of galaxies with active galactic nuclei
15 . Why do we call dark matter "dark"?
A. It blocks out the light of stars in a galaxy.
B. It contains large amounts of dark-colored dust.
C. It emits no radiation that we have been able to detect.
D. It is dark brown or dark red in color.
16 . The flat rotation curves of spiral galaxies tell us that they contain a lot of dark matter. Do they tell us anything about where the dark matter is located within the galaxy?
A. No, we cannot determine anything about the location of dark matter from the rotation curve.
B. Yes, they tell us that the mass is concentrated near the center of the galaxy.
C. Yes, they tell us that the dark matter is spread throughout the galaxy, with most located at large distances from the galactic center; that is, out in the halo that surrounds the disk.
D. Yes, they tell us that dark matter is spread uniformly throughout the galactic disk.
17 . Why does the temperature of the gas between galaxies in galaxy clusters tell us about the mass of the cluster?
A. The temperature tells us the average speeds of the gas particles, which are held in the cluster by gravity, so we can use these speeds to determine the cluster mass.
B. The question is nonsense --- gas temperature cannot possibly tell us anything about mass.
C. Temperature is always directly related to mass, which is why massive objects are always hotter than less massive objects.
D. The temperature of the gas tells us the gas density, so we can use the density to determine the cluster's mass.
18 . How does gravitational lensing tell us about the mass of a galaxy cluster?
A. The lensing broadens spectral lines, and we can use the broadening to "weigh" the cluster.
B. Newton's universal law of gravitation predicts how mass can distort light, so we can apply Newton's law to determine the mass of the cluster.
C. Using Einstein's general theory of relativity, we can calculate the cluster's mass from the precise way in which it distorts the light of galaxies behind it.
D. The lensing allows us to determine the orbital speeds of galaxies in the cluster, so that we can determine the mass of the cluster from the orbital velocity law.
19 . If WIMPs really exist and make up most of the dark matter in galaxies, which of the following is NOT one of their characteristics?
A. They travel at speeds close to the speed of light.
B. They are subatomic particles.
C. They can neither emit nor absorb light.
D. They tend to orbit at large distances from the galactic center.
20 . Which of the following statements best describes the current state of understanding regarding the apparent acceleration of the expansion of the universe?
A. We have moderately strong evidence that the acceleration is real, but essentially no idea what is causing it.
B. The acceleration was something a few astronomers thought they saw, but we no longer think it is real and assume that the original claims were based on errors in interpreting the data.
C. We are quite confident that we understand the cause of the acceleration: the cause is dark energy.
D. The acceleration is very important in the cosmos today, but we think the acceleration will eventually slow down.
21 . How do we determine the conditions that existed in the very early universe?
A. By looking all the way to the cosmological horizon, we can see the actual conditions that prevailed all the way back to the first instant of the Big Bang.
B. From the current expansion rate we can work backward to estimate temperature and densities at various times in the early universe.
C. The conditions in the very early universe must have been much like those found in stars today, so we learn about them by studying stars.
D. We can only guess at the conditions, since we have no way to calculate or observe what they were.
22 . When we say that the electroweak and strong forces "freeze out" at 10--35 seconds after the Big Bang, we mean that ______.
A. prior to this time, the electroweak and strong forces were indistinguishable from each other, but after this time they behaved differently from each other
B. following this time, neither the strong nor electroweak forces are ever important in the universe again
C. these forces are important only at temperatures below the freezing point of water --- a temperature that the universe reached at an age of about 10--35 seconds
D. freezing out was a term coined by particle physicists who think that the Big Bang theory is really cool
23 . According to the Big Bang theory, how many forces --- and which ones --- operated in the universe during the GUT era?
A. 1 force that represented the unification of all four forces that operate today
B. 2: the strong force and the electroweak force
C. 2: gravity and a single force that later became the strong, weak, and electromagnetic forces
D. 3: gravity, the strong force, and the electroweak force
24 . What was the significance of the end of the era of nucleosynthesis, when the universe was about 3 minutes old?
A. It marks the time at which the first stars formed.
B. The basic chemical composition of the universe had been determined.
C. The proportions of dark matter and luminous matter had been determined.
D. It marks the time at which the expansion of the universe had settled down to its current rate.
25 . According to the Big Bang theory, why do we live in a universe that is made of matter rather than antimatter?
A. The fact that we live in a universe made of matter is not surprising, because antimatter has never been shown to exist for real.
B. Einstein's famous equation E = mc2 tells us that energy can turn into matter, but does not tell us that it can turn into antimatter.
C. During the first 0.001 second after the Big Bang, particles and antiparticles were made in almost but not perfectly equal numbers. Everything annihilated except the very slight excess of matter particles.
D. GUT theories predict that under the conditions that prevailed in the early universe, the normal laws of physics would have been suspended so that only matter particles were created, and no particles of antimatter.