Henrietta Leavitt's most famous discovery was the "period-luminosity" relation for variable stars (those changing in brightness), an important method of obtaining distances to far-off galaxies. She also identified 2,400 new variable stars and established brightness scales that helped other astronomers with their own observations.

Henrietta Swan Leavitt was born in Lancaster, Massachusetts, on July 4, 1868, to Henrietta Swan Kendrick and George Roswell Leavitt, a Congregationalist minister who had a parish in Cambridge. After attending public school in Cambridge, Leavitt moved with her family to Cleveland, Ohio, where she attended Oberlin College from 1885 to 1888. She switched, however, to the Society for the Collegiate Instruction of Women (now Radcliffe College) in 1892. During her senior year, she took an astronomy course, which fired her interest in the subject. After receiving an A.B. from Radcliffe in 1892, Leavitt took another astronomy course, then spent a number of years home because of an illness that...