| Hubble identifies a new galaxy
Photo: Andromeda nebula plate with Hubble's handwritten circle around the variable star he discovered
In 1919 American astronomer Edwin Hubble began work at the Mount Wilson Observatory in California, which boasted the most powerful telescope to date.
He chose to investigate the nebulae, which were believed to be clouds of glowing gas. But with this new, larger telescope, Hubble was able to resolve images of the nebulae that showed individual stars. Around 1924, Hubble was looking at the Andromeda nebula and found to his amazement that one of the stars he'd observed wasn't just any star, but a cepheid variable. In 1912 Henrietta Leavitt had shown that cepheid variables could be used as a virtual "yardstick to the universe" -- knowing the brightness of the star and the time period it took to vary from bright to dim would tell you its distance from Earth. Using this formula Hubble was able to find out how far away from Earth his newly discovered cepheid -- and Andromeda -- was.
He calculated that the distance was about 800,000 light years -- more than eight times the distance to the furthest stars in the Milky Way! This proved that the "nebula" were actually separate systems outside and independent of our galaxy. It was the first evidence of galaxies outside our own, and Hubble soon went on to discover about 23 more galaxies.