On October 5, 1923, Edwin Hubble identified the first Cepheid variable in M31, the Andromeda Galaxy. This proved that the Andromeda nebula was not part of the Milky Way and paved the way for measuring the expansion of the Universe. Cepheid variables are luminous yellow stars which pulsate in brightness with periods of several days. The first was discovered by Edward Pigott on September 10, 1784 and by John Goodricke a few months later.