The discovery of the relationship between the period and luminosity of Cepheid variable stars is credited to Henrietta Swan Leavitt.

While working at Harvard College Observatory in the early 1900s, Leavitt studied thousands of variable stars in the Magellanic Clouds. She noticed that the brighter Cepheids had longer periods of variation, a crucial discovery that allowed astronomers to use Cepheids as "standard candles" for measuring distances in the universe.