In 1999, astronomers were able to calculate the approximate age of the universe - and the number is...   A: about 4,000 years B: about 13 billion years C: about 900 billion years Stars and planets are shaped like spheres because:   A: gravity pulled them into that shape as they formed B: they eroded into round shapes over millions of years C: round shapes have less wind resistance If you went looking for life in outer space, a good place to look would be:   A: Mars, because it may have liquid water below its surface B: Neptune, because it has weather systems like those on Earth C: Uranus, because its blue-green surface looks as though it could sustain life In this picture, almost every blob you see is:   A: a star B: a planet C: a galaxy Men walked on the Moon for the first time in July of 1969. How long will their footprints be visible on the Moon's surface?   A: windstorms have already erased them B: hundreds of thousands of years C: about fifty years For every evening star you can see from Earth, how many more stars in the Milky Way Galaxy remain unseen?   A: about 20 million B: about 3,000 C: about 40 A comet's tail can stretch as long as:   A: about 10 miles B: about 5,000 miles C: hundreds of millions of miles A "day" on the planet Mercury - from sunrise to sunset - lasts about:   A: six Earth hours B: six Earth months C: six Earth years If your spaceship could travel at the speed of light, how long would you take to cross the entire Milky Way?   A: about 10 years B: about 1,000 years C: about 100,000 years When a massive star finally dies, like the star in this picture, what happens?   A: it turns into a white dwarf B: it explodes in a supernova C: it becomes a planet