Around 1924, Hubble was looking at the Andromeda nebula and found to his amazement that one of the stars he'd observed wasn't just any star, but a cepheid variable. In 1912 Henrietta Leavitt had ...

Leavitt discovered 2,400 variable stars, about half of the known total in her day. Through these discoveries came her most important contribution to the field: the study of cepheid variable stars ...