In the early twentieth century, it was not yet known whether so-called “spiral nebulae” were inside our Milky Way galaxy or outside of it. In 1924, American astronomer Edwin Powell Hubble (1889–1953) began a study of spiral nebulae, using the one hundred-inch Hooker Telescope at Mount Wilson Observatory in California. Over many months, Hubble identified hundreds of Cepheid variable stars in the largest spiral nebula toward the constellation Andromeda. Using the period-luminosity relation of Cepheid variables, he showed that the Andromeda spiral nebula is at least about one million light-years away—a far larger distance than the size of the Milky Way. Also, at that distance Andromeda would have to be many thousands of light-years across to be visible. Thus, Hubble proved that the Andromeda spiral nebula is, in fact, the Andromeda galaxy, and that the universe contains not just one, but many galaxies that are millions of light-years apart.