In 1925, the astronomer Henry Norris Russell read a paper at the 33rd meeting of the American Astronomical Society. The paper, written by Edwin Hubble, a staff astronomer at the Mt. Wilson observatory, detailed observations of Cepheid variable stars in the Andromeda Nebula. These observations and the analysis of them showed that the spiral was a million light years outside the Milky Way Galaxy, thus establishing it as an island universe once and for all. The Great Debate was settled and the size of the universe was expanded to a scale unimaginable just a decade earlier.
Edwin Hubble
The photographic plate Hubble first discovered a variable star in the Andromeda Nebula
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