Cepheid Variables Discovered in Andromeda Galaxy

[M31 Cepheid discovery image] In 1923, Edwin Hubble was examining photographic plates of the Andromeda Nebula M31, taken with the 100-inch telescope in order to find novae -- stars that would suddenly increase in brightness (because of cataclysmic explosions). On this place from the night of October 5-6, 1923, Hubble located three novae, each marked with an "N." One of these novae, however, turned out to be a Cepheid variable, a star that changes predictably in brightness, and the "N" was crossed out and the star was relabeled "VAR!" This Cepheid, and others subsequently discovered in the Andromeda Nebula, enabled Hubble to prove that the Nebula was not a star cluster within our own Milky Way, but a galaxy more than a million light years away.

From: Mount Wilson Observatory History page

  • This image was featured as Astronomy Picture of the Day April 6 1996
  • Image also available as positive (JPG) [or PNG]

    Hartmut Frommert
    Christine Kronberg

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    Last Modification: July 2, 1999