Learn how Edwin Hubble made some of the most important discoveries in modern astronomy in this video from NOVA: Invisible Universe Revealed. In 1923, Hubble observed a Cepheid in what was called the Andromeda "nebula." When he calculated the distance to it, he discovered that the "nebula" was actually a galaxy outside of the Milky Way. At the time, the Milky Way was considered the entire universe, and so the discovery dramatically changed our view of the universe. Hubble went on to measure the relative speed of galaxies, using redshift, and found that the universe was expanding. His observations formed the basis of the Big Bang theory.