If you go out into the countryside on a clear summer night, far from city lights and look up, you will see a band of light arching across the vault of the sky. In Ancient Greek and Roman legend, this was a milky river that flowed across the sky. They called it the Milky Way, inadvertently christening the galaxy in which we live.

Seen through binoculars or a telescope, the true nature of the Milky Way is apparent; it is composed of billions of stars, so numerous and distant that they all blur together when observed with the naked eye. Our galaxy is a flat spiral, but since we are inside looking out, we do not see a spiral structure. The band of light we see curving across the sky is one of the galaxy's spiral arms.

As late as 1918, it was believed that...