History of astronomy
Two great discoveries changed our picture of the Universe. Both were made by Edwin Hubble in the 1920s. Before then, it was thought that our galaxy, the Milky Way, was the whole universe. Hubble first showed that the Andromeda galaxy was a separate galaxy and similar. Many more galaxies were soon discovered. Later in the 1920s he showed that all distant galaxies were moving away from each other in a general expansion of the universe. He did this by discovering the red shift of spectroscopic light from distant galaxies. That does not apply to galaxies that are clustered together, such as Andromeda which, with our galaxy, is part of the Local Group in the Virgo Supercluster.