Leavitt discovers a correlation between Cepheids' period and luminosity
Henrietta Leavitt joined the Harvard College Observatory as a volunteer in 1895. She was appointed to the permanent staff in 1902, and eventually became chief of the photometry department. She worked there for the rest of her life.
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 in the Magellenic Clouds -- the Milky Way's two companion galaxies. By intense observation and mathematical calculation, Leavitt realized that with cepheid variable stars (which change brightness with great regularity), there is a direct correlation between a star's magnitude (degree of brightness) and the length of time it is most luminous. The brighter the star is overall, the longer the period of luminosity. Since the cepheids in the Magellanic Clouds were all about the same distance from Earth, Leavitt concluded that the period, or time it took to complete one cycle of dimming and brightening, was related to the star's magnitude, not distance. Yet magnitude itself allowed you to calculate distance.
Leavitt published her findings in 1912 -- in a chart of 25 cepheid periods and their apparent brightness. Using this, astronomers only needed to know the period of a cepheid variable to figure out how bright, and therefore how far away it was. Until then, methods for measuring distances in space only worked within about 100 light years. With Leavitt's findings, distances of cepheids could be determined up to 10 million light years. This became the "yardstick to the universe" used by Edwin Hubble and others to make discoveries that changed our view of our galaxy and the universe.