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Harlow Shapley: Measurement of Galaxies of Vast Distances, 1945

Cepheids

At Mount Wilson, Shapley studied Cepheid variable stars, using them as indicators of the distances of globular clusters—compact, spherical collections of stars. Within a year, Shapley realized that the Cepheid variable stars are not eclipsing binaries, but rather single pulsating stars. In other words, the Cepheids are single stars that periodically expand and contract with an accompanying change in luminosity. Their distances could then be determined by measuring their apparent magnitudes, using Henrietta S. Leavitt's 1912 period-luminosity relationship to obtain their average brightness or average "absolute magnitude." And, since pulsating Cepheid stars frequently occur in globular clusters, Shapley could use the former to determine distances to the latter. His greatest contribution to science came of this work; in 1918, Shapley developed a new picture of the shape and size of the galaxy.