Who discovered the Cepheid period-luminosity relation that Hubble used for distance scaling?

Henrietta Swan Leavitt discovered the Cepheid period-luminosity relation that Hubble used for distance scaling.

While working at Harvard College Observatory, Leavitt examined photographic plates of variable stars in the Small Magellanic Cloud. In 1908 she noted that brighter Cepheids tended to have longer periods; her 1912 analysis established the relationship more clearly. Because the stars in that cloud were approximately at the same distance, differences in apparent brightness revealed differences in intrinsic luminosity.

That pattern became a calibrated “standard candle.” Once astronomers knew a Cepheid’s pulsation period, they could estimate its true brightness, compare it with its observed brightness, and infer distance. Hubble used Cepheids in Andromeda and other nebulae in the 1920s to show that they lay far beyond the Milky Way.

Leavitt’s discovery is often called Leavitt’s law. A common mix-up is crediting Hubble with the relation itself: Hubble applied it to extragalactic astronomy, but Leavitt found the underlying stellar rule. She died in 1921, before the full significance of her work was widely recognized.