Hubble's famous "Var!" plate. This photographic plate shows a fuzzy black disc at the center -- the photo negative of M31 - with a small dot on the outskirts labeled "Var!"

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1. Robert McNees (mcnees@mastodon.social)'s status on Sunday, 20-Nov-2022 20:04:05 UTC Robert McNees

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   In 1923, Hubble took a photographic plate of the spiral nebula M31. It showed what he at first thought was a nova. Over subsequent observations he realized it was actually a Cepheid variable star. He crossed out “N” and excitedly wrote “VAR!”

   Using Henrietta Swan-Leavitt's relationship between the period and brightness of Cepheids, Hubble established that M31 was situated outside the Milky Way. Spiral nebula were in fact distinct galaxies.

   IMAGE: Carnegie Observatories