Which American astronomer discovered that Cepheid variable stars could measure distances across the universe?

Henrietta Swan Leavitt discovered the period–luminosity relation that lets astronomers use Cepheid variable stars to measure cosmic distances.

Working at Harvard College Observatory, Leavitt examined photographic plates and catalogued variable stars in the Small Magellanic Cloud. In 1912, she showed that brighter Cepheids have longer pulsation periods. Because the stars in that cloud were assumed to be at roughly the same distance, their apparent brightness could be compared reliably.

The relation provided a standard candle: once a Cepheid’s period reveals its true luminosity, astronomers can compare that luminosity with its observed brightness to estimate distance. Edwin Hubble later used Cepheids to show that Andromeda lies beyond the Milky Way.

Leavitt did not receive the Nobel Prize, and her discovery was often overshadowed by later users of the method. Modern distance measurements still build on the principle she established.