clouds of gas that might someday form into a star and planets.
In 1929 Edwin Hubble published a landmark paper entitled ΓÇ£A Spiral Nebula as a Stellar System.ΓÇ¥ Hubble had found stars known as Cepheid variables in Andromeda, just like those in the Galactic Center. A simple calculation showed that the Cepheids in Andromeda lay outside the Milky Way. This meant the fuzzy mass of Andromeda was not a gas cloud but contained billions of stars. The gas clouds were com- pletely separate ΓÇ£island universesΓÇ¥ located incredibly far away. Thus, the universe went from being merely huge to incomprehensibly large.
