Astronomer Edwin Hubble

During the past 100 years, astronomers have discovered quasars, pulsars, black holes and planets orbiting distant suns. But all these pale next to the discoveries Edwin Hubble made in a few remarkable years in the 1920s. At the time, most of his colleagues believed the Milky Way galaxy, a swirling collection of stars a few hundred thousand light-years across, made up the entire cosmos. But peering deep into space from the chilly summit of Mount Wilson, in Southern California, Hubble realized that the Milky Way is just one of millions of galaxies that dot an incomparably larger setting.

Hubble went...