Henrietta Swan Leavitt

Henrietta Swan Leavitt was an American astronomer who discovered the relationship between the period and luminosity of Cepheid variable stars, a finding that later became one of the most important tools for measuring distances in the universe. Leavitt worked at the Harvard College Observatory as one of a group of women employed to examine photographic plates of the sky, a role sometimes informally called a computer, where in 1908 and more fully in a 1912 paper she showed that among a class of pulsating stars called Cepheid variables found in the Small Magellanic Cloud, the brighter stars in the group consistently took longer to complete their cycle of brightening and dimming, establishing a precise mathematical relationship between the two properties. Because this period-luminosity relationship allowed a star's true brightness to be inferred just from timing how it varies, astronomers could then compare that true brightness to its apparent brightness as seen from Earth to calculate its distance, a technique that Edwin Hubble later relied on directly to establish that many observed nebulae were in fact entire separate galaxies far outside the Milky Way. Leavitt's discovery is now recognized as one of the foundational tools of modern observational cosmology, though she received relatively little public recognition for it during her own lifetime.