Leavitt establishes Cepheid period–luminosity relation
Leavitt establishes Cepheid period–luminosity relation
Overview
In 1908, analysing photographic plates of the Magellanic Clouds at Harvard, Henrietta Swan Leavitt found that the brighter Cepheid variables pulse with longer periods — a strict period–luminosity relation. Because all stars in a cloud lie at nearly the same distance, the relation turned Cepheids into standard candles measurable across millions of light-years. Within two decades it enabled Hubble to prove galaxies lie far beyond the Milky Way and to measure cosmic expansion.