clouds of gas that might  someday form into a star and planets.
    In 1929 Edwin Hubble published a landmark paper entitled “A Spiral Nebula as a Stellar System.” 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 “island universes” located incredibly far away. Thus, the universe went from being merely huge to incomprehensibly large.