Beyond the Milky Way
Astronomer Harlow Shapley, who also worked at Mount Wilson and was a rival to Hubble, had already made a name for himself by measuring the size of the Milky Way. Shapley used Leavitt's method that relied on Cepheid brightness to establish the distance of the galaxy, and he, like a majority of astronomers at that time, thought the Milky Way made up the entire universe.
Hubble made great strides in his study of nebulae with Mount Wilson's Hooker telescope at his disposal, and worked many long nights with janitor-turned-night-assistant Milton Lasell Humason to try to prove Shapley wrong. He aimed the telescope at nebulae thought to be outside the Milky Way galaxy, and searched for Cepheid stars in them. In October 1923, he discovered a Cepheid, or variable star, in the Andromeda Nebula. Hubble identified 12 Cepheid stars in this Andromeda Galaxy within a year, and was able to conclude that Andromeda was approximately 900,000 light years away from the Earth. Since the Milky Way's diameter was known to be only a few hundred thousand light years across as established by Shapley, Hubble's measurements demonstrated that Andromeda was significantly outside the limits of Earth's galaxy. By 1924, Hubble proposed that these and other far away nebulae were indeed other galaxies just like the Milky Way, and this theory became known as the "island universe."
1924 was an eventful year for Edwin Hubble on a personal level as well. He married Grace Burke in Pasadena on February 26.
Letter from CSA Secretary to Dr. Edwin Hubble, Inquiring as to how name should appear on medal and certificate, 3/17/1939 (810k)
Letter from Secretary and Director to Mr. Hubble, Informing that Hubble may make public announcement of Franklin Award, 3/20/1939 (285k)