Henrietta Swan Leavitt

Henrietta Swan Leavitt was born in Lancaster, Massachusetts, in 1868, the daughter of a Congregationalist minister. She studied at Radcliffe College (then the Society for the Collegiate Instruction of Women) and took an astronomy course in her final year that set the direction of her life. After a period of travel and a serious illness that left her increasingly deaf, she joined the Harvard College Observatory in 1895, initially as a volunteer and later as a paid staff member earning thirty cents an hour. She was one of the 'Harvard Computers,' the team of women hired by director Edward Pickering to measure and catalog the brightness of stars on the observatory's vast collection of photographic glass plates.

Leavitt was assigned the study of variable stars, and she was prodigiously productive — she discovered over 2,400 of them, about half of all those known in her lifetime. Her decisive work concerned Cepheid variables in the Small Magellanic Cloud. Because all the stars in that cloud lie at essentially the same distance from Earth, their apparent brightnesses are directly proportional to their true luminosities. In 1908, and definitively in a 1912 paper, Leavitt reported a tight relationship: the longer a Cepheid's pulsation period, the more luminous the star. The period-luminosity relation meant that timing a Cepheid's flicker revealed its intrinsic brightness, and comparing that to its apparent brightness gave its distance.

This was the breakthrough the science of cosmic distances needed. A Cepheid is a 'standard candle' — an object whose true output can be known independently of its distance — and standard candles are the rungs of the distance ladder. Ejnar Hertzsprung and Harlow Shapley used Leavitt's relation to calibrate it in absolute terms and to map the Milky Way; Edwin Hubble used it to measure the distance to Andromeda and then to build the velocity-distance relation that revealed cosmic expansion. Every one of those results rests on Leavitt's law.

Leavitt worked under the constraints imposed on women in science at the time: she was assigned data reduction rather than her own research program, and Pickering directed much of her effort. She died of cancer in 1921 at the age of 53. In 1925 the Swedish mathematician Gosta Mittag-Leffler, unaware of her death, wrote to nominate her for the Nobel Prize; the prize cannot be awarded posthumously. Her period-luminosity relation is now often called the Leavitt law, a belated correction to the historical record.