Henrietta Swan Leavitt

Henrietta Swan Leavitt (1868-1921) was an American astronomer who worked as a human computer at the Harvard College Observatory, cataloging variable stars from photographic plates. Her most important discovery, published in 1908 and refined in 1912, was the period-luminosity relationship in Cepheid variable stars: the longer such a star's pulsation period, the greater its true brightness. This relationship gave astronomers the first reliable way to measure distances to far-off star clusters and galaxies, since a Cepheid's period reveals its actual luminosity, which can then be compared to its observed brightness to calculate distance. Edwin Hubble later used Leavitt's period-luminosity law to establish that the Andromeda Nebula lay far outside the Milky Way, a discovery that proved the universe contains many separate galaxies. Leavitt's method remains a cornerstone of the astronomical distance scale.