Earth & Cosmos / Space & Astronomy

Cepheid distance yardstick

Henrietta Leavitt found in 1912 that a Cepheid star's pulsation period reveals its true brightness, giving a yardstick across space.

Studying photographic plates of variable stars in the Small Magellanic Cloud at Harvard, Henrietta Swan Leavitt found that brighter Cepheids pulse more slowly, a period-luminosity relation published in 1912. Once calibrated, the period of any Cepheid gives its distance. Edwin Hubble used it in 1923-24 to show that Andromeda lies far outside the Milky Way, and then to measure the expansion of the universe.