Henrietta Swan Leavitt

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Henrietta Swan Leavitt (July 4 1868 – December 12 1921) was an American astronomer.

At Harvard College Observatory, she studied Cepheid variable stars, whose brightness varies in a regular periodic fashion.

She discovered and catalogued variable stars in the Magellanic Clouds, and from this catalog, she discovered in 1912 that Cepheids of greater intrinsic luminosity had longer periods, with a fairly close and predictable relation between the two.

One year later, Ejnar Hertzsprung determined the distance of a few Cepheids, and with this calibration the distance to any Cepheid could now be known. When Cepheids were detected in other galaxies such as the Andromeda galaxy, the distance to those galaxies could then be determined.