Henrietta Swan Leavitt

Henrietta Swan Leavitt was an American astronomer whose discovery of the period–luminosity relation for Cepheid variable stars made them tools for measuring astronomical distances.

Cepheid variable: A pulsating star whose brightness varies in a regular cycle, with a period linked to its intrinsic luminosity. Leavitt established that longer-period Cepheids are intrinsically brighter.
Small Magellanic Cloud: A dwarf galaxy orbiting the Milky Way, visible from the Southern Hemisphere. Leavitt compared its Cepheids, which were approximately at a common distance from Earth.
Harvard College Observatory: An astronomical research institution in Cambridge, Massachusetts, founded in 1839. Leavitt carried out her photographic stellar research at this observatory.
Williamina Fleming: A Scottish-American astronomer who developed stellar classification methods at Harvard College Observatory. Fleming was a senior colleague in the observatory’s photographic research program.
Period–luminosity relation: The relationship between a Cepheid’s pulsation period and its intrinsic luminosity. This is the relation Leavitt found among Cepheids in the Small Magellanic Cloud.
Variable star: A star whose apparent brightness changes over time, intrinsically or because of changing viewing conditions. Cepheids belong to this broader class of changing stars.
Harvard Computers: Women employed by Harvard College Observatory to perform astronomical calculations and analyze data. Leavitt was among the women whose observational work advanced the observatory’s research.
Henrietta Leavitt’s 1912 paper: A short Harvard College Observatory publication reporting the relation between Cepheid periods and brightness in the Small Magellanic Cloud. Leavitt reported her finding in this 1912 publication.
Cosmic distance ladder: A sequence of astronomical methods that calibrate distances from nearby stars to the most distant galaxies. Cepheids became a rung connecting stellar measurements to extragalactic distances.
Apparent magnitude: A logarithmic measure of how bright a celestial object appears from Earth. Leavitt measured changing apparent brightness to identify Cepheid periods.

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