Edwin Hubble and Cepheids: Clinching the Case for Galaxies

At the start of the twentieth century, very few astronomers believed external galaxies could be sighted even in the most powerful telescopes. But within two decades there had been a significant shift and a growing number of astronomers accepted a myriad of visible galaxies. It was, however, Edwin Hubble who clinched the case for galaxies in 1923 and 1924 through observations of Cepheid variables in the Andromeda Nebula. In this paper, I examine the growing body of evidence in the 1910s and 1920s in favor of galaxies and why Hubble’s observations were judged decisive.