Leavitt, Henrietta Swan, 1868-1921

Henrietta Swan Leavitt was born in Lancaster, Massachusetts on July 4, 1868. Leavitt attended Oberlin College for two years before transferring to Harvard University's Society for the Collegiate Instruction of Women (later Radcliffe College), where she received a bachelor's degree in 1892. 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, via parallax, the distance to a nearby star of the same type, her discovery became a measuring stick with vastly greater reach.

Leavitt's scientific work at Harvard was frequently interrupted by illness and family obligations. Her early death at the age of 53 from stomach cancer was seen as a tragedy by her colleagues for reasons that went beyond her scientific achievements.