Today, we know that Copernicus was right: the stars are very far from Earth. In fact, stars are so distant that a new unit of length—the lightyear— was created to measure their distance. A light-year is a unit of length equal to the distance that light travels through space in 1 year. Because the speed of light through space is about 300,000 km/s, light travels approximately 9.46 trillion kilometers in one year.
Even after astronomers figured out that stars were far from Earth, the nature of the universe was hard to understand. Some astronomers thought that our galaxy, the Milky Way, included every object in space. In the early 1920's, Edwin Hubble made one of the most important discoveries in astronomy. He discovered that the Andromeda galaxy, which is the closest major galaxy to our own, was past the edge of the Milky Way. This fact confirmed the belief of many astronomers that the universe is larger than our galaxy.

Why was Edwin Hubble's discovery important?