Perhaps more than any other person, Edwin Hubble (1889-1953) expanded our view of the universe. At the dawn of the 20th century, most astronomers thought that the Milky Way Galaxy was the universe, and it measured only a few thousand light-years across. In the 1910s, Harlow Shapley showed that the galaxy actually stretches about 100,000 light-years, and Henrietta Leavitt determined that the Large and Small Magellanic Clouds (two companion galaxies to our own, visible from the Southern Hemisphere) lay slightly outside the Milky Way’s border. But one big question that remained was the nature of the fuzzy patches of light known as nebulae.
In 1923 and 1924, Hubble used the largest telescope in the world—the 100-inch Hooker Telescope at Mt. Wilson—to examine the Andromeda Nebula. The Oxford-trained lawyer-turned-astronomer detected for the first time stars similar to those in our own galaxy. By comparing how bright the stars appeared with how much light they actually gave off, he estimated the distance to the nebula as nearly a million light-years, clearly making it a huge galaxy in its own right.
Hubble went on to find the distances to many other galaxies, eventually pushing the frontiers of the universe out to hundreds of millions of light-years. He then compared the distances to the speeds with which the galaxies were racing away, and deduced that the farther away the galaxy, the faster it moved. This relation, known as Hubble’s Law, was observational proof that the universe was expanding. Appropriately, the famed Hubble telescope was named in his honor.