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Harlow Shapley: Measurement of Galaxies of Vast Distances, 1945

Determining Distance

Harlow Shapley was the first to realize that the Milky Way Galaxy was much larger than previously believed. Using the 60-inch telescope at Mount Wilson, he took photographs and observed globular clusters, which were compact spheres composed of many thousands of stars. Shapley used Henrietta Swan Leavitt's method for Cepheid variable stars to determine distances to globular clusters.

In 1912, American astronomer Henrietta Leavitt used the degree of brightness of Cepheid stars—stars whose brightness vary at regular intervals—in the Magellanic Clouds to measure their distance from the Earth. The longer the time a Cepheid star takes to undergo a complete cycle, the higher the star's average brightness or average "absolute magnitude." Leavitt was able to determine the distance from Earth to the nebula by comparing the brightness of the star as seen from Earth with the star's actual brightness, which was estimated using the length of the star's cycle.