Henrietta Swan Leavitt

Born on 4 July 1868 in Lancaster, Massachusetts, Henrietta Swan Leavitt was an astronomer whose discovery concerning a class of pulsating star revolutionized the field. In 1895, three years after graduating from the Society for the Collegiate Instruction of Women (which later became Radcliffe College) in Cambridge, Massachusetts, Leavitt became a volunteer assistant at the Harvard Observatory, where she would spend her entire career. She joined Williamina Fleming, Annie Jump Cannon, and other women researchers at the observatory in a project to obtain and analyze images and spectra of hundreds of thousands of stars. Leavitt’s specialty was the study of objects whose brightness changes over time. She discovered multiple novae and about 2400 variable stars. In 1912 she determined that there is a relationship between the luminosity and pulsation period of a class of variable star known as a Cepheid variable. Knowing that relationship allowed astronomers to deduce the distances to the variable stars and the clusters or galaxies to which they belonged. Astronomer Edwin Hubble used Cepheids to ascertain the distance to Andromeda and to conclude that the universe is expanding . Leavitt didn’t get to witness the impact of her discoveries, as she died of cancer in 1921 at age 53. Cepheid variables continue to be used to determine stellar distances, particularly in studies to determine the Hubble constant and the evolution of the universe’s expansion rate. (Photo credit: AIP Emilio Segrè Visual Archives, Physics Today Collection)

Date in History: 4 July 1868