Henrietta Leavitt Biography

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 left her severely deaf. After some traveling, Leavitt volunteered as a research assistant at Harvard College Observatory in 1895, and was appointed to the permanent staff in 1902 by the astronomer Edward Pickering at a salary of 30 cents an hour. Leavitt worked at Harvard from 1902 until her death. While Pickering gave Leavitt little chance to do theoretical work, she became chief of the photographic photometry department at the observatory.

Establishes Scale of Star Brightnesses

In 1907 Pickering asked Leavitt to establish a "north polar sequence" of star brightnesses to serve as a standard for the entire sky. This standard was desirable because the photographic process in astronomy was complex--the brightness of star images on film was not proportional to their actual brightness, and each telescope gave different results for different color stars. Once determined, brightnesses could be estimated by comparing one star with another, rather than referring to photographic images. Leavitt used 299 plates from 13 telescopes, and compared stars ranging from the fourth to the twenty-first magnitude in brightness (each increasing unit of magnitude corresponds to a reduction in brightness by a factor of 2.512 on a logarithmic scale). The results were published in the Annals of Harvard College Observatory. In 1913 Leavitt's system was adopted by the International Committee on Photographic Magnitudes. She made this work a lifelong project, and established brightness sequences for 108 areas in the sky. When Pickering developed 48 "Harvard standard regions" in the sky, Leavitt derived secondary brightness standards for them. These were used as international standards until superseded by improved methods.

Discovers Period-Luminosity Relation

Leavitt discovered 2,400 new variable stars, about half those known at the time. Most notably, she studied photographs of the Magellanic Clouds (the Milky Way's two companion galaxies) taken at Harvard's observatory in Arequipo, Peru. Of the 1,800 variable stars Leavitt detected on the Magellanic Cloud pictures, some were Cepheid variables, whose change in brightness is extremely regular. (Cepheids were named after the first star of this type to be discovered, Delta Cephei.) In 1908 Leavitt found that the brighter the Cepheid was overall, the longer it took to change its magnitude. Leavitt reasoned that since the Cepheids in the Magellanic Clouds were nearly all the same distance from Earth, their periods were related to their light output: the longer the period of pulsation, the brighter the star. By 1912 Leavitt had proven that Cepheids' apparent brightness increased linearly with the logarithm of their periods. That year, she published a table of the length of 25 Cepheid periods--ranging from 1.253 days to 127 days, with an average of 5 days--and their apparent brightness.

Before this period-luminosity relation had been established, cosmic distances could be determined only out to about 100 light-years. With it, however, distances of out to 10 million light-years could be calculated. The intrinsic brightness of a Cepheid could now be gotten directly from a measure of its period, and this allowed the distance to be calculated. Astronomer Ejnar Hertzsprung adapted the period-luminosity relation so it could determine the actual distance of stars from Earth. Hertzsprung and astronomer Harlow Shapley found that the Magellanic Clouds were in the range of 100,000 light-years from Earth--an astonishing and unexpectedly high value. Leavitt was not aware as she worked that the Magellanic Clouds lay outside our own Galaxy.

Leavitt died of cancer December 21, 1921, in Cambridge. During her lifetime she had little opportunity to give free rein to her intellect, but her talents did not go unnoticed. According to Women of Science, her colleague Margaret Harwood described her as possessing the best mind at the observatory, and contemporary astronomer Cecilia Payne-Gaposchkin said Leavitt was the most brilliant woman at Harvard.