Edwin Hubble announced #OTD in 1925 that Andromeda and other spiral nebulae were in fact separate galaxies outside the Milky Way, in a paper read to an AAS meeting by H.N. Russell.

The Universe was more than just our little island of stars, far greater in extent than what many astronomers had allowed themselves to imagine.

Hubble built on observations by Vesto Slipher, collaborated with Milton Humason, and relied on Henrietta Swan Leavitt’s work on Cepheid variables to reach this conclusion.