5 October

1923: On 5 October 1923, the US astronomer Edwin Hubble identified the first Cepheid variable star in the Andromeda galaxy. This proved that the galaxy was not part of the Milky Way.

A Cepheid variable star one of a class of variable stars whose periods (i.e., the time for one cycle) of variation are proportional to their luminosity. Because of this correlation (discovered by Henrietta Swan Leavitt in 1912), Cepheid variables can be used as a standard to determine the distance to their host globular clusters or galaxies. Since the period-luminosity relation can be calibrated with great precision using the nearest Cepheid stars, the distances found with this method are among the most accurate available.