In 1785, the musician turned astronomer, William Herschel pioneered the technique of star counts in an attempt to deduce the structure of the Milky Way. Herschel pointed his telescope in various directions in the sky and counted the number of stars he could see in a standard size field of view. If he saw more stars in a given direction, Herschel assumed that the Milky Way extended farther in that direction. Herschel correctly concluded that the Milky Way was a disk shape, but he mistakenly concluded that we were at the center of that disk. The star counting technique he used was misleading. It didn’t work because the galaxy is so vast that the interstellar dust—dust between the stars—blocks out light from the more distant stars, while we are at the center of the small part of the galaxy mapable by the star counting method, the galaxy extends beyond the region we can map. We must find beacons enough brighter than individual stars to be seen from the edges of the galaxy.

In 1917 Harlow Shapley mapped the extent of the galaxy by counting globular clusters rather than stars. Globular clusters are collections of roughly 100,000 stars. They can be seen from the distant reaches of the Milky Way. Just as Copernicus before him concluded that Earth is not the center of the solar system, Shapley proved that the solar system was not at the center of the galaxy. Since Shapley’s time, astronomers have refined his technique and discovered new ways to deduce the size, structure, and contents of the Milky Way.