In the early 20th century, Shapley was studying globular clusters, which are dense collections of stars. He noticed that RR Lyrae stars were consistently present in these clusters, and he realized that they could be used as "standard candles." This means that their intrinsic brightness is known, allowing astronomers to calculate their distance based on their apparent brightness.
Shapley's groundbreaking research using RR Lyrae stars led him to conclude that the Sun was not at the center of the Milky Way, as previously thought. Instead, he determined that the Milky Way was much larger than previously believed, with the Sun located well within one of its spiral arms. His work revolutionized our understanding of the Milky Way and played a vital role in establishing the foundations of modern cosmology.
