Q. Why is a day 24 hours long?
A. Because that is how long it takes for the Earth to spin around once so that the same side faces the Sun again.

Q. When there is a full moon (the moon is fully illuminated)
A. the Moon is farther from the Sun than the Earth is.

Q. The main reason it is hotter in the summer than in the winter is:
A. the Sun is up longer and is higher in the sky.

Q. The Earth spins (rotates/revolves) around an axis that is tilted compared to the axis about which it orbits the Sun. Which of the following would be true if the Earth's spin was {\bf not} tilted? (that is, which of the following would be true if the axis the Earth spins around was the same as the axis about which it orbits its sun).
A. The temperature would not change much during the year in both the northern and southern hemispheres.

Q. Which of the following would make your weight half what it is now?
A. Being on a planet the same size as Earth but half the mass.

Q. When you see a star, light
A. is emitted by the star and enters your eye.

Q. The center of our Milky Way Galaxy is about 25 million light years away. If a star explodes there, how long will it be before we see this happen?
A. 25 million years.

Q. A photon that has a wavelength of 600 nanometers
A. is less energetic than a photon with a wavelength of 400 nanometers.

Q. As a glowing body gets cooler, it...
A. emits fewer photons, and these are typically less energetic.

Q. If the Earth were to suddenly start spinning twice as fast as it is now
A. The Sun would rise every 12 hours.

Q. Which answer shows the correct pattern from closest object to the Earth to farther from the Earth?
A. Space Station and other objects orbiting the Earth - Jupiter - stars other than the Sun.

Q. If you are playing pool and hit the cue ball, which of the following describes the situation based on Newton's Laws?
A. The ball moves in the direction the force was applied, and your hand feels an equal force in the opposite direction.

Q. Now you are still hitting the ball with the same force, but you would like the ball to accelerate more slowly (move slower). Which of the following should you do?
A. Increase the mass of the ball.

Q. What led to the acceptance of the idea that the Earth revolves around the Sun over the previous view that the Earth was the center of the Universe?
A. That theory described the observations better and explained the motions of all objects with one consistent model.

Q. As viewed from the Earth, some planets appear to double back in their orbits for a while (retrograde motion). This is due to:
A. the fact that the planets are in orbit about the Sun rather than about the Earth.

Q. A group of hydrogen atoms emitting photons is moving towards you. Which of the following correctly describes how you will observe these photons?
A. The observed photons will be shifted to shorter wavelengths compared to the wavelengths emitted by a stationary (not moving) group of hydrogen atoms.

Q. Now imagine that the same group of hydrogen atoms is moving even faster (larger velocity) towards you. The wavelengths you observe will be
A. shifted even more compared to their original values.

Q. If you dropped the following objects from the top of a ladder on the Moon, which one would hit first?
A. all of these objects would hit at the same time.

Q. Which of the following is the main advantage of a larger telescope (bigger mirror)?
A. To be able to collect more light to see fainter objects.

Q. Which of the following is NOT an important reason for having telescopes in space?
A. To place the telescope closer to the stars.

Q. If there are two identical stars, but one is three times farther away than the other one.
A. The closer one will appear to be nine times brighter than the more distant one.

Q. Why are there no X-ray telescopes on the surface of the Earth?
A. X-ray photons are not able to pass through the atmosphere.

Q. Which of the following statements is most correct?
A. The Sun is about 200 times as massive as Jupiter.

Q. The force that prevents the Earth from flying out of the Solar System is
A. gravity

Q. If Venus were moved two times closer to the Sun than it is now, the gravitational force that the Sun exerts on Venus would become
A. 4 times as large.

Q. If you have two glowing bodies, how can you tell which one is cooler (this generally works for stars)?
A. The cooler one has a redder color.

Q. Which of the following photons passes through space the fastest?
A. They all travel at the same speed.

Q. The gravitational force between two objects is
A. larger between objects of larger masses, and decreases as the square of the distance between the objects.

Q. Why does an atom only absorb some wavelengths of light and not all of them?
A. It only absorbs wavelengths of light with energies that correspond to specific differences between energies of allowed electron orbits.

Q. Why does the Earth have the heaviest elements in its center and lighter ones on its surface, while asteroids typically have the same composition everywhere?
A. The Earth was once hot and liquid so heavier elements could sink to the center, and asteroids never were that hot.

Q. The Sun and the giant planets are composed mainly of the same two elements. These are
A. hydrogen and helium.

Q. Which of the following is an important technique for determing the age of the Earth and Solar System?
A. Age dating rocks using radioactive atoms with experimentally determined half-lives.

Q. Which of the following planets is a terrestial planet with a higher density and without a lot of lighter elements?
A. Mars

Q. What is the approximate measured age of the solar system and of the Earth?
A. 4.5 billion years (4,500,000,000 years)

Q. Who made observations of the phases of Venus and the moons of Jupiter that provided strong evidence that the Earth and the planets orbited around the Sun?
A. Galileo

Q. What planet do you live on?
A. Earth

Q. Which of the following is true?
A. The planets all orbit the Sun in the same direction, and with the exception of Pluto, they all do so in the same plane like a flat disk.

Q. All of the planets together make up how much of the total mass of the Solar System?
A. Less than $1\%$

Q. Which of the following is NOT a good reason why the major impact model is the best explanation for the formation of the moon?
A. We have directly observed moons forming from major impacts on other planets.

Q. The terrestrial planets (Mercury, Venus, Earth, and Mars)
A. are more dense than the giant planets.

Q. The Moon has many more impact craters on its surface than the Earth does. What does this tell us?
A. That the Moon's surface is old, and that the Earth's is young, because the Moon stopped having major geological activity a long time ago, while the Earth still does.

Q. If Venus was to suddenly cool off, so that its surface temperature would be similar to the Earth's, which one of the following would be most correct
A. Venus would not have any water because its hydrogen has escaped its atmosphere into space.

Q . Which one below has the correct order starting with planet with the highest surface pressure, going to the planet with the lowest surface pressure?
A. Venus (highest) - Earth - Mars (lowest)

Q. What is the primary source of Jupiter's internal energy?
A. Gravitational energy from the accretion and settling of material onto Jupiter.

Q. Which of the following best describes what would be required to transform Mars' atmosphere into something closer to the Earth's?
A. Heat it up to release the frozen water and carbon dioxide ice to start a greenhouse effect, which will warm up the surface, keeping some of the water and carbon dioxide in the atmosphere.

Q. What would happen eventually to Mars' atmosphere if you did manage to make it more like Earth's?
A. Over time it will start to cool off and lose some its atmosphere, because Mars doesn't get as much solar heating, and is less massive than the Earth and therefore has less gravity to hold on to its atmosphere.

Q. Which of the following is *NOT* true of Pluto?
A. It originated from the solar system from another star and was brought into our solar system by the gravity of the Sun and Jupiter.

Q. What did we see happen on Jupiter in 1994?
A. The comet Shoemaker-Levy crashed into it, releasing large amounts of energy and making dark spots at the impact sites.

Q. If a solar system was forming around a star that was cooler than the Sun, the most likely place to find a habitable region with an Earth-like atmosphere would be
A. a planet closer to the star than the Earth is from the Sun.

Q. Which of the following gases is most responsible for the greenhouse effect on planets in the inner solar system?
A. Carbon dioxide, CO$_2$

Q. Of the large inner moons of Jupiter, Io is the closest to the planet itself. It is also very active, with significant volcanism. What is the source of energy for Io's activity?
A. Tidal heating from gravitational interaction with Jupiter.

Q. Uranus revolves (spins) on its side (perpindicular to its orbit). As a result, Uranus
A. has very long seasons, since it needs to orbit around the Sun to go through all of the seasons.

Q. Which of the following is *NOT* true about comets?
A. There are approximately 10,000 of them in total, and the typical mass of each one is a little less than the mass of the Earth's Moon.

Q. The inner and outer Galilean moons of Jupiter differ, in that
A. the inner moons have more geological activity.

Q. Which of the following is *NOT* a method (either current or planned for the future) for detecting planets around stars other than the Sun?
A. Measuring the changes in the colors of a stars caused by the strong weather patterns on giant planets.

Q. Which is the best description of the process of formation of the planets?
A. The initial gas condensed into small bodies called plantesimals, which then coalesced through collisions into larger and larger bodies.

Q. Why doesn't the material in Saturn's inner rings clump together into just a few large moons?
A. Because they are very close to Saturn, so the gravitational tidal forces tear them apart.

Q. What is actually measured in the Doppler technique that indicates the presence of a massive planet?
A. The systematic shifting to shorter and longer wavelengths of light emitted by the star.

Q. With the transit technique for finding extrasolar planets, why do many stars have to be searched?
A. Few planets will have orbits that line up to block a little light from their star from reaching us.

Q. What causes a protostar to collapse?
A. When its gets dense enough so gravity pulls it together.

Q. Why does the Earth have less Hydrogen than the Sun and the giant planets?
A. When the planets were forming, the inner solar system was too hot for light elements and compounds to condense onto protoplanets.

Q. Why is it highly unlikely we will be able to travel to any other solar system
A. The great distance to even the most nearby star.

Q. What is the best explanation for why many of the planets found by the doppler (wobble) technique are massive and close to their Sun?
A. Because close in, massive planets have the strongest gravitational effect on their star, and thus create the biggest doppler wobble.

Q. Why are the heaviest elements at the center of the terrestial planets?
A. The terrestial planets once hot and molten, and the heaviest elements sank to the center through a process called differentiation.

Q. Why does the SETI program looking for extraterrestrial life use the radio region of the electromagnetic spectrum in their search?
A. Radio waves can pass through our atmosphere, and most solar-like stars are not strong radio emitters.

Q. How does the fusion of hydrogen into helium produce the energy that powers the Sun?
A. The helium has a little bit less mass than the four hydrogens that get fused together to make it, and this difference comes out as energy according to $E = m c^{2}$.

Q. Why does some material in the proto-solar nebular end up in a disk around the central star instead of all falling into the center?
A. The conservation of angular momentum from the initial cloud out of which the protostar forms.

Q. Why does the fusion of Hydrogen into Helium happen in the Sun, but we can not easily do it on Earth?
A. Because it requires very high temperatures and densities to bring the hydrogen nuclei together, since their positive charges repel each other.

Q. With all of the energy produced in the Sun and the high temperature and pressure this creates, why doesn't the Sun just explode?
A. Gravity acts to pull the mass together and this balances the pressure pushing out.

Q. Which of the following statements is false?
A. The sun is one of the hottest known stars.

Q. Stars on the main sequence produce their energy by
A. fusion of Hydrogen into Helium.

Q. Where did the iron and other heavy elements in the universe come from?
A. They were produced by nuclear reactions that occur inside massive stars.

Q. The order of the stages of evolution a star like our Sun goes through is
A. main sequence, red giant, white dwarf.

Q. The most massive stars
A. have the shortest lives.

Q. If you see a star that is cool and has a large diameter, what kind of star is it?
A. A giant or supergiant star.

Q. When a star settles down to a stable existence as a main-sequence star, what characteristic determines where on the main sequence in an H-R diagram the star will fall?
A. its mass.

Q. What happens to a star on the main sequence?
A. It stays there until its core is converted from hydrogen to helium, then it evolves to become a red giant.

Q. Why are parallax (triangulation) measurements limited to only nearby stars?
A. The shift in the apparent position of a star in the sky at different times of year is too small to measure for distant stars.

Q. Which types of stars evolve so they have iron cores and may explode as supernovae?
A. Very massive stars.

Q. Cepheid variable stars are important tools for measuring cosmic distances. This is because their average luminosity (intrinsic brightness) is directly related to
A. the period with which they vary in brightness.

Q. Why are the less massive stars (like the Sun) NOT able to produce elements heavier than carbon and oxygen?
A. Because the star's center cannot get hot and dense enough for the nuclear reactions that build heavier nuclei.

Q. If you see a cluster of stars that has many bright, blue stars, you can infer which of the following
A. That it is a young star cluster

Q. Which of the following stars is the most dense (density is mass divided by volume)?
A. A neutron star.

Q. If you observe a star on the main sequence and find that it has almost no elements heavier than hydrogen and helium (even much less than the Sun), which of the following would be accurate
A. It is probably a very old star.

Q. This same star with almost no elements heavier than hydrogen and helium (even much less than the Sun) is also probably
A. located in the halo of our Galaxy.

Q. Which of the following best describes the formation of a spiral galaxy?
A. It cools by radiation, it contracts by gravity, and it spins up because of angular momentum conservation.

Q. Why is it important to determine the distances to stars to understand their basic properties?
A. We need to know the distance in order to determine a star's intrinsic brightness, so we can tell if it is a highly luminous star that is far away, or a lower luminosity star that is closer.

Q. Where is the Sun located within our own Galaxy?
A. About thirty thousand light years from the center, in the disk of the Galaxy.

Q. What type of galaxy is our Milky Way galaxy?
A. A spiral galaxy.

Q. What is the source of the very high luminosities (intrinsic brightness) of quasars and QSOs?
A. Radiation from material falling onto a black hole.

Q. How did Harlow Shapley figure out that the Sun was not at the center of our Milky Way Galaxy?
A. Most of the globular clusters are on one side of the sky, which should not happen if the Sun was in the center of the Galaxy.

Q. Which component of our Galaxy contains the largest amount of mass?
A. Dark matter.

Q. If you see a galaxy that is a round ball of stars with no stars currently forming, it is
A. an elliptical galaxy.

Q. Which of the following best describes how a galaxy without dark matter would differ from those with dark matter?
A. The motions of stars and gas in the galaxy without dark matter would be smaller in the outer regions beyond where most of the galaxy light is.

Q. Why do we know "nebulae" are galaxies of their own well outside of our Milky Way Galaxy?
A. We have observed Cepheid variable stars and determined distances that place them well beyond our own Galaxy.

Q. What effect does the black hole at the center of our Milky Way Galaxy have on the Sun?
A. None.

Q. What do the orbits of the stars in an elliptical galaxy look like?
A. The stars move in almost completely random directions, often with orbits that are not perfectly circular.

Q. What is the evidence that there is a massive black hole at the center of our Galaxy?
A. We observe that stars very close to the Galactic center orbit (move) very quickly, indicating that there is a massive, dense object at the center.

Q. What will happen to the Sun if/when our Milky Way Galaxy collides with our nearest neighboring galaxy, the Andromeda Galaxy?
A. The orbit of the Sun in the resulting galaxy will be all jumbled up.

Q. Approximately how many stars are there in a typical galaxy like our Milky Way galaxy?
A. Several billion stars

Q. Which of the following is the best model for how elliptical galaxies form?
A. When spiral galaxies collide, many stars form, and their rotating disks get disrupted by the collision and turned into a more random ball of stars.

Q. What is a good working definition of the radius of a black hole?
A. The radius where the escape velocity equals the speed of light so no light can escape.

Q. Your nice but not very bright roommate offers to drive you to the exam, but takes a wrong turn, and accidentally drives $inside$ the Schwarzschild radius (event horizon) of a black hole. You use your cell phone to try to send a radio SOS message back to AAA (who prudently have located their office outside the Schwarzschild radius). What will be the fate of that message?
A. The message will never emerge from the Schwarzschild radius.