Astronomy

Astronomers define the universe as all of space and everything in it, such as planets, stars, galaxies, and other matter. Most of the matter in the universe is actually dark matter, so called because it does not interact with light and is therefore difficult to measure. The universe also contains cosmic phenomena called black holes, or regions in space with gravity so intense that nothing—not even light—can escape.

How did it all start? The leading theory, called the Big Bang Theory, is that the universe began in an instant, billions of years ago, with an explosion or a big bang. After that explosion, the universe cooled and expanded, inflating over time to become the cosmos that we know today.

Because the distances between objects in space are so large, astronomers use a special unit of measurement, called a light-year, to evaluate them. A light-year is the distance that light travels in space in one Earth year, about 9.5 trillion kilometers. The sun is approximately 150 million kilometers, or 8 light-minutes, away from Earth, whereas the North Star, also called Polaris, is 2 billion kilometers or 320 light-years away.