American astronomer Harlow Shapley (1885–1972) earned his doctoral degree at Princeton University in 1913, working with Henry Norris Russell (1877–1957), who was famous for the Hertzsprung-Russell diagram. Shapley and Russell studied eclipsing binary stars, systems of two stars orbiting around one another in such a way that one star would regularly block the other star from our view. Later, while working at the Mount Wilson Observatory in Pasadena, California, he studied other kinds of variable stars, including RR Lyrae and Cepheid variable stars, which could be used as “standard candles” to measure distances. With them, he measured the distances to many of the globular star clusters that orbit around the Milky Way. By mapping out the positions of the globular clusters, Shapley showed that the disk of our Milky Way galaxy was some 100,000 light-years across—much larger than had been previously thought—and that our Sun and solar system were off to one side of the Milky Way, rather than at its center.

In 1921 Shapley became the director of the Harvard College Observatory. There, he began to study variable stars in the Large and Small Magellanic Clouds. In 1924 he used those variable stars as standard candles to show that the Small Magellanic Cloud was at least two hundred thousand light-years away from Earth, and thus must be a small galaxy of its own, rather than part of the Milky Way.