Hubble’s photonegative plate of M31, with light sources appearing dark against a yellow-gray background. The word “VAR!” Is written at the upper right corner in red wax pencil. The oblong disk of M31 dominates the center of the plate.

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1. Robert McNees (mcnees@mastodon.social)'s status on Friday, 06-Oct-2023 15:37:47 UTC Robert McNees

   VAR!

   In the early morning hours #OTD in 1923, Edwin Hubble took a photo plate of M31 showing a Cepheid variable star.

   Using Henrietta Swan Leavitt’s distance-luminosity relationship, Hubble concluded that M31 is located outside the Milky Way.

   This observation established that there are other galaxies besides our Milky Way, that our little island is not the whole Universe.

   Image: Carnegie Observatories