Clouds in the Heavens.

In the early 1920s Vesto M. Slipher, an astronomer working at the Lowell Observatory near Flagstaff, Arizona, was examining spiral-shaped nebulae in the night sky. According to contemporary scientific opinion these nebulae were cloudy patches of light caused by gases, but Slipher came to the conclusion that they were entire, separate galaxies like the Milky Way.