science · 19 February 1924 · 100 years ago
In 1924, Edwin Hubble wrote a letter to Harlow Shapley, which he concluded by saying, “...the distance [to the Andromeda nebula] comes out something over 300,000 parsecs.” Hubble discussed in the letter his measurement of the magnitudes of the Cepheid variable stars in the Andromeda nebula he had found and confirmed. He used their measured characteristics to calculate their distance, definitely about a million light years from our Solar System. This was the evidence that Andromeda was a separate galaxy, far beyond the Milky Way. This was the first proof of an “island universe.” After collecting more data, Hubble sent a paper read on 1 Jan 1925 to a meeting of the American Astronomical Society. Meanwhile, Shapley remained unconvinced, as when he debated Heber Curtis on 26 Apr 1920.
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