Henrietta Swan Leavitt (1868-1921), American astronomer.
Born born on July 4, 1868 in Massachusetts. She became interested in astronomy during her last year at Radcliffe. Shortly after graduation, she became sick, and although she recovered, she also lost her hearing. In the late 1800s women rarely worked outside of teaching and nursing, and deaf women didn't do either -- but Leavitt managed to get a job at the Harvard College Observatory, under professor Charles Pickering (who believed that women were more efficient workers than men).
At that time, the observatory was trying to determine the magnitude of stars. Leavitt came up with new methods for analysing and categorizing stars, but she is best know for her work on Cepheid variables*. In 1912, she noticed that the pulsations of these stars were regular, and that the rhythm of these pulses were proportional to the intensity of light emitted by the star. By tying speed to brightness, astronomers could now measure distance -- observed brightness decreases with distance, but the speed of the pulses told you how bright the star really was. With this information, astronomers were able to measure the distance of many star clusters from Earth. The first estimate of the size and shape of the galaxy also arose from this observation.
Leavitt discovered about 2,400 variable stars, 1,777 of them in the Magellanic Clouds. She also has a crater on the moon named after her (although she did not discover it -- it was named after her to honour the deaf men and women of astronomy). She died in 1921 of cancer.
* On a random side note, the first Cepheid variable was discovered in 1784 by John Goodricke, who was also deaf (not to mention mute).