3. a. For a total solar eclipse to occur, the apparent size of the Moon must be equal to or larger than that of the Sun. The Sun is physically larger than the Moon, but because it is farther away, it appears to be about the same size in the sky. If the Moon were closer, it would appear bigger and easily eclipse the Sun. Answers B, C, and D would all result in the Moon appearing smaller than the Sun, so only "annular" eclipses (in which a ring of full sunlight is visible around the Moon) could occur.

4. True. Stars greatly outshine planets, so taking a direct image of an exoplanet is extremely difficult— although it has indeed been accomplished in a number of cases.

5. False. While the Sun will run out of hydrogen fuel in its core in about 5 billion years, it isn't massive enough to undergo a spectacular supernova explosion.

6. True. While black holes can not be seen directly, their presence can be inferred by the gravitational influence that they have on a companion star. The Milky Way galaxy is also known to have at its center a supermassive black hole, one with a mass of more than 4 million Suns!

Advanced 1. b. Astronomers initially expected that the gravitational pull of the matter in the universe would be slowing down its expansion. In 1998, however, they measured that the universe's rate of expansion was increasing! While astronomers do not understand what is causing this accelerated expansion, they have labeled it "Dark Energy."

Energy." 2. d. About 5 percent of the universe is made up of normal particles. Fully 23 percent of the universe is composed of "Dark Matter," which acts gravitationally like normal matter but does not emit light. The remaining 72 percent or so of the universe is the even more mysterious "Dark Energy."

3. c. Because the Sun illuminates the Moon, from your point of view, the Moon will always appear full. When Earth experiences a total lunar eclipse, the Moon will disappear behind Earth. This alignment occurs only about twice a year because the Moon's orbit is tilted.

4. False. When Mars and Earth are closest together, it will take a message about 3 minutes to travel one way. At greatest separation, it would take about 22 minutes!

5. True. The solar system orbits the supermassive black hole at the center of the Milky Way, completing one orbit every 200 million years!

6. True. Galaxies commonly undergo collisions. During this merger, the solar system will remain basically unaffected, but the night sky will change: The new galaxy will have more stars and a different shape. ■

Bethany E. Cobb is an associate professor of honors and physics at George Washington University.