Working at the Harvard College Observatory at the turn of the 20th Century, Henrietta Leavitt developed the information about Cepheid stars that would make Edwin Hubble’s great discoveries possible. Noticing that these stars varied in brightness at consistent rate that they could be used to measure stellar distances. Applying this yardstick herself, Leavitt showed in 1912 that such variable stars had to be far outside our home galaxy.
She had become especially expert in the characteristics of these variable stars, whose luminosity dims and flares up in cycles, and eventually cataloged about twenty four hundred of them. Most important for Edwin Hubble’s research, she was very able to time the cycles of the Cepheids, By showing that the length of their cycles of luminosity varies in close relation to their brightness, Leavitt provided astronomers with a hitherto unknown celestial yardstick, a method of measuring the distance of stellar objects. If a nearby Cepheid with a certain cycle has a certain degree of luminosity, a remote Cepheid with the same cycle should have the same luminosity. The degree of its relative dimness, from our point of view on Earth, shows how far away it is.
As you can imagine, in this age when very few women were given opportunities in science, these women of the Harvard College Observatory were restricted to classifying celestial phenomena in photographic plates taken by male astronomers. It’s remarkable to notice that in this group of women were to seminal and original thinkers, such as Henrietta Leavitt and Antonia C. Maury. The latter classified stars by the width of the lines they produced on a spectrograph. All of these helped others to nail down the exact dimensions of the Milky Way.