Hubble Proves Andromeda is an External Galaxy

On December 30, 1924, Edwin Hubble announced that the Andromeda Nebula (M31) was a separate galaxy beyond the Milky Way. Using the 100-inch Hooker telescope at Mount Wilson Observatory during 1923-1924, Hubble identified Cepheid variable stars in Andromeda. He applied Henrietta Leavitt's period-luminosity relationship to calculate Andromeda's distance at approximately 900,000 light-years away (later refined to 2.5 million light-years). This discovery demonstrated that the Milky Way was one of multiple galaxies and expanded the known cosmos by orders of magnitude.