In the early 20^th century, an astronomer named Edwin Hubble (picture below) discovered that the “Andromeda Nebula” is actually over 2 million light years away—many times farther than the farthest distances we had measured before. He realized that many of the objects astronomers called nebulas were not clouds of gas, but collections of millions or billions of stars—what we now call galaxies. Our view of the universe changed again—we now knew that the universe was much larger than our own galaxy. Today, we know that the universe contains about a hundred billion galaxies—about the same number of galaxies as there are stars in the Milky Way Galaxy.

Edwin Hubble used the 100-inch reflecting telescope at the Mount Wilson Observatory in California to show that some distant specks of light seen through telescopes are actually other galaxies. He also measured these distances to hundreds of galaxies, and discovered that the universe is expanding.

After discovering that there are galaxies outside our own, Edwin Hubble went on to measure the distance to hundreds of other galaxies. His data would eventually show us how the universe is changing, and even give us clues as to how the universe formed.

David Bethel
http://ck12.org/flexr/assemble/?fid=732 (CC BY-SA)