Which astronomer discovered the period–luminosity relation for Cepheid variable stars in 1908?

Henrietta Swan Leavitt discovered the period–luminosity relation for Cepheid variable stars in 1908.

Working at the Harvard College Observatory, Leavitt studied variable stars in the Small Magellanic Cloud. She noticed that brighter Cepheids had longer periods of variation. Because the stars in that distant system were approximately at the same distance from Earth, their apparent brightnesses could be compared to reveal a relationship between pulsation period and intrinsic luminosity.

This relation made Cepheids valuable “standard candles.” Once astronomers measured a Cepheid’s period, they could estimate its true brightness and then compare it with how bright it appeared to calculate distance. Edwin Hubble later used Cepheid variables to establish distances to galaxies beyond the Milky Way.

Leavitt’s discovery was initially published in 1908 and refined in a 1912 Harvard Circular. She did not receive a Nobel Prize; she died in 1921, before the wider importance of her work was fully recognized.