Henrietta Swan Leavitt

Henrietta Swan Leavitt (; July 4, 1868 – December 12, 1921) was an American astronomer. Her discovery of how to effectively measure vast astronomical distances led to a shift in the understanding of the scale and nature of the universe. A graduate of Radcliffe College, she worked at the Harvard College Observatory as a human computer, tasked with measuring photographic plates to catalog the positions and brightness of stars. This work led her to discover the relation between the luminosity and the period of Cepheid variables. Leavitt's discovery provided astronomers with the first standard candle with which to measure the distance to other galaxies. Before Leavitt discovered the period-luminosity relationship for Cepheid variables (sometimes referred to as Leavitt's Law), the only techniques available to astronomers for measuring the distance to a star were based on stellar parallax. Such techniques can only be used for measuring distances out to several hundred light years. Leavitt's great insight was that while no one knew the distance to the Small Magellanic Cloud, all its stars must be roughly the same distance from Earth. Therefore, a relationship she discovered in it, between the period of certain variable stars (Cepheids) and their apparent brightness, reflected a relationship in their absolute brightness. Once calibrated by measuring the distance to a nearby star of the same type via parallax, her discovery became a measuring stick with vastly greater reach. After Leavitt's death, Edwin Hubble found Cepheids in several nebulae, including the Andromeda Nebula, and, using Leavitt's Law, calculated that their distance was far too great to be part of the Milky Way and were separate galaxies in their own right. This settled astronomy's Great Debate over the size of the universe. Hubble later used Leavitt's Law, together with galactic redshifts, to establish that the universe is expanding (see Hubble's law).

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