Henrietta Leavitt: She Discovered How to Calculate the Distance to Galaxies
Henrietta Swan Leavitt (1868-1921) was born in Lancaster, Massachusetts, and graduated from Radcliffe College in 1892. In 1902 she became a permanent staff member of the Harvard College Observatory. She soon rose "by her scientific ability and intense application" to head the department of photographic stellar photometry.
She spent a great deal of time searching Harvard photographic plates for variable stars in the Magellanic Clouds. Using a laborious process called superposition, in 1904 she discovered 152 variables in the Large Magellanic Cloud (LMC), and 59 in the Small Magellanic Cloud (SMC). The next year she reported 843 new variables in the SMC. These discoveries led Charles Young of Princeton to remark in a letter to HCO director E. C. Pickering, "What a variable-star 'fiend' Miss Leavitt is one can't keep up with the roll of the new discoveries."
Leavitt's greatest discovery came from her study of 1777 variable stars in the Magellanic Clouds. She was able to determine the periods of 25 Cepheid variables in the SMC and in 1912 announced what has since become known as the famous Period-Luminosity relation: "A straight line can be readily drawn among each of the two series of points corresponding to maxima and minima, thus showing that there is a simple relation between the brightness of the variable and their periods." Leavitt also realized that "since the variables are probably nearly the same distance from the earth, their periods are apparently associated with their actual emission of light, as determined by their mass, density, and surface brightness." Today the Period-Luminosity relation is one of the backbones of the "distance ladder" used to calculate the distances of galaxies.
In the course of her work, Leavitt discovered four novae and about 2400 variables - about half of all the variable stars then known to exist. She also studied Algol-type eclipsing binaries and asteroids. She was a member of Phi Beta Kappa, The American Association of University Women, the American Astronomical and Astrophysical Society, the American Association for the Advancement of Science, and an honorary member of the American Association of Variable Star Observers. Unfortunately, she died young of cancer before her work on a new photographic magnitude scale could be completed. Her death was viewed as a "near calamity" by her colleagues. Her important contribution to scientific advancement was internationally acknowledged when, in 1925, a member of the Swedish Academy of Sciences proposed nominating her for the Nobel Prize.
Miss Leavitt inherited, in a somewhat chastened form, the stern virtues of her puritan ancestors. She took life seriously. Her sense of duty, justice and loyalty was strong. For light amusements she appeared to care little. She was a devoted member of her intimate family circle, unselfishly considerate in her friendships, steadfastly loyal to her principles, and deeply conscientious and sincere in her attachment to her religion and church. She had the happy faculty of appreciating all that was worthy and lovable in others, and was possessed of a nature so full of sunshine that, to her, all of life became beautiful and full of meaning.
