Cepheid Variables Discovered in Andromeda Galaxy

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.

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