Leavitt, Henrietta Swan (1868–1921)

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Leavitt, Henrietta Swan (1868–1921)

American astronomer who established a standard by which to chart the magnitude of the stars and discovered the period-luminosity of Cepheid variable stars during her 26-year career. Born on July 4, 1868, in Lancaster, Massachusetts; died on December 12, 1921, in Cambridge, Massachusetts; daughter of Reverend George Roswell Leavitt and Henrietta S. (Kendrick) Leavitt; educated at Cambridge public schools and Oberlin College; graduated from the Society for the Collegiate Instruction of Women (later Radcliffe College), 1892; never married; no children.

One of seven children of Congregationalist minister Reverend George Roswell Leavitt and Henrietta Kendrick Leavitt , Henrietta Swan Leavitt was born on July 4, 1868, and grew up primarily in Cambridge, Massachusetts. Although she was almost completely deaf, she did not allow that fact to interfere with her education and career. When she was 17, her family moved to Cleveland, Ohio, where she attended Oberlin College from 1886 to 1888, originally studying music. She transferred to the Society for the Collegiate Instruction of Women (also known as the "Harvard Annex," later Radcliffe College) in 1888; a senior-year course in astronomy so interested her that she enrolled in another after graduating in 1892. After spending some time traveling, Leavitt volunteered at the Harvard Observatory in 1895.

At first working as an assistant in the examination of variable stars (stars which become brighter or dimmer in a regular or irregular pattern over a period of time), Leavitt was made a permanent member of the observatory staff, which also included Annie Jump Cannon and Williamina Paton Fleming , in 1902. She was soon promoted by Edward C. Pickering, the head of the observatory and an important advocate for women in astronomy, to chief of the photographic photometry department. The use of photographs in astronomy, rather than sketches made at the telescope, had begun in the late 19th century, as photographic technology itself progressed; by the early years of the 20th century, it was recognized that photographic plates can detect wave lengths the human eye cannot, and thus they became invaluable to the science. Photometry is the measurement of the intensity of light, which in astronomy is used to assess the magnitude of stars. In 1907, Pickering instituted a major program to overhaul previous (naked eye) calculations of star magnitudes, beginning with the creation of a basic sequence of magnitudes, photographically determined, against which other stars could be measured. Leavitt was named to devise this basic sequence.

The result of her work, a standard of brightness known as the "North Pole Sequence," was achieved through her observation of a group of stars near the North Pole. Leavitt started by observing and determining the magnitudes of 46 stars, then used those magnitudes to analyze a much larger portion of the surrounding sky, eventually determining brightness down to the 21st magnitude. Her initial research involved studying over 300 photographic plates taken from 13 different telescopes; the standards were published in 1912 and 1917, and before her death she had fully sequenced 108 areas of the sky. In 1913, the International Committee on Photographic Magnitudes adopted Leavitt's system as the standard for their project, known as the Astrographic Map of the Sky, to catalog the position of the stars; her system was also used by J.C. Kapteyn in his statistical studies of star distribution.

Her most important work, however, took place in 1912, with her discovery of the period-luminosity relation of Cepheid variable stars. Cepheids are a type of variable star with a rigidly regular pattern of brightness and dimness (pulsation); Leavitt discovered that the length of the pulsation of a star is an indication of its luminosity. This knowledge was later expanded by astronomers such as Edwin Hubble (who would discover Cepheid variables outside this galaxy in 1923) and used to measure the distances between the Earth and distant stars and galaxies.

In 26 years of research, Leavitt discovered four novas (a faint star which suddenly increases in brightness by 10,000 or more times) and about 2,400 variable stars, which made up more than half of those known until 1930. Of the stars she discovered, 1,700 were found while studying photographs of the Magellanic Clouds (known now to be nearby galaxies of stars). She was also the first to notice that the fainter stars in a sequence were generally redder than the brighter stars, which brought up the question of whether they were genuinely redder or only appeared so because of particles in the atmosphere (a distinction now determined through photo-electrical technology). Henrietta Leavitt worked at the Harvard Observatory until her death from cancer on December 12, 1921, at the age of 52.