Harlow Shapley's Great Discovery
Since antiquity, the belief still stood that the Sun was the center of the universe, but some astronomers were starting to doubt that assumption. Furthermore, the size of the universe was being contested. Astronomers had roughly deduced the shape of the Milky Way as a disk of stars, but had assumed that the Milky Way was the extent of the universe. In the early 20th century that hypothesis was challenged when some astronomers said that distant clouds of gas were in fact other galaxies. When Harlow Shapley was beginning his career in astronomy, the answers to these questions were still being debated. There was no strong evidence to definitively put the debate to rest.
At the Mount Wilson Observatory, Shapley was happily studying his globular clusters. These are small clumps of stars that orbit the Milky Way outside of the normal plane of stars. Looking at these clusters, Shapley was able to find a certain type of star called a Cepheid variable, which are standard candles used to measure distance. Shapley used these variable stars to determine the distance of several globular clusters and plotted a 3-dimensional map of the Milky Way. When he looked at the position of the Sun in this map, Shapley was finally able to put to rest once and for all the false idea that the Sun was the center of the universe. This new map was 10 times larger than any other previous estimate and showed that the center of the galaxy was 50,000 light years away in the constellation Sagittarius. (Shapley's estimate to the center of the galaxy was incorrect, as he was not aware of the absorption light by intervening dust. The actual distance to the galactic center is about 25,000 ly.) Soon after this discovery, Edwin Hubble would solve the second half of the Great Debate about the size of the cosmos and prove the existence of other galaxies outside the Milky Way.
Shapley would build on his great discovery and go on to be the director of the Harvard College Observatory. This observatory would remain his academic home for the rest of his life, where he discovered dwarf galaxies in Fornax and Sculptor and explored the Magellanic clouds. Before his death in 1972, Shapley went on to preside over the American Astronomical Society, the American Association for the Advancement of Science, and the National Society of Sigma Xi. He was an active lecturer and author and his contributions to astronomy set an example for astronomers for decades.