Photo plate taken by Hubble on October 6, 1923. It shows M31 in negative - black on a light yellowish background. Several annotations are present, including a crossed out “N” with “VAR!” written next to it in red grease pencil.

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1. Robert McNees (mcnees@mastodon.social)'s status on Sunday, 01-Jan-2023 16:51:08 UTC Robert McNees

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   In the early morning hours of October 6, 1923, Edwin Hubble took a photo plate of M31 showing a Cepheid variable star. He originally mistook it for a nova - you can see where he has crossed out an “N” on the plate and excitedly replaced it with “VAR!”

   Applying Henrietta Swan Leavitt’s period-luminosity relationship to subsequent observations, Hubble concluded that the distance to M31 was greater than reliable size estimates for the Milky Way.

   Image: Carnegie Observatories