When did astronomers learn that the Milky Way was not the only galaxy in the universe?

   In 1924 American astronomer Edwin Powell Hubble first proved the existence of other galaxies. He used a very powerful 100-inch (254-centimeter) telescope at Mount Wilson Observatory to discover that a group of stars long thought to be part of the Milky Way was actually a separate galaxy, now known as the Andromeda galaxy. He then discovered many other spiral-shaped galaxies.