In 1923 the American astronomer Edwin Hubble studied the outer regions of what appeared to be a nebula in the Andromeda constellation. With the high-powered 100-inch (2.5 m) telescope at Mount Wilson, he was able to see that the "nebulous" part of the body is composed of stars, some of which were bright, variable stars called Cepheids. Hubble realized that for these intrinsically bright stars to appear so dim, they must be extremely far away from Earth. He went on to explain that all the other galaxies in the universe are millions of light-years from our galaxy. His research helped astronomers begin to understand the immense size of the universe.
The Discovery
Hubble used his knowledge about variable stars to plot the distances to some of the nearer galaxies. In 1929 he announced the results of this experiment. He graphed this data with velocity on one axis and distance on the other. Then he plotted the location of each galaxy on the graph. When he did this he was surprised to find out that all the points on the graph made a diagonal straight line. Hubble found that the galaxies are moving faster the farther away they are. This astonishing discovery became known as Hubble's Law.