The Computers of Harvard
Henrietta Swan Leavitt was born in Lancaster, Massachusetts, in 1868, the daughter of a Congregational minister, and studied at the institution that became Radcliffe College. An illness contracted after graduation left her severely deaf. In 1895 she joined the Harvard College Observatory as a volunteer assistant and was eventually placed on the payroll at thirty cents an hour, one of a group of women hired by the director Edward Pickering to process the observatory's enormous archive of glass photographic plates.
They were called computers, and the term was literal. Williamina Fleming, Annie Jump Cannon, Antonia Maury and Leavitt among others measured, classified and catalogued hundreds of thousands of stellar images, work that produced the spectral classification scheme still in use. The women were paid roughly half a male assistant's wage and were not permitted to direct the telescopes whose output they analysed. Their names appear in the observatory's annals more often as labour than as authorship. Pickering could employ several of them for the price of one male assistant.
The Clouds of Magellan
Leavitt was assigned the search for variable stars, and she was extraordinarily good at it, eventually identifying more than two thousand. Her attention settled on the Small Magellanic Cloud, where she catalogued over seventeen hundred variables on plates taken from Harvard's southern station in Peru. Because everything in the Cloud lies at approximately the same distance from us, differences in apparent brightness there reflect real differences in output rather than the accident of proximity, and that made a comparison possible that no other field allowed.
In 1908 she noted that the brighter variables appeared to have longer cycles. In 1912 she published the relation properly, based on twenty-five Cepheid variables, showing a straight-line link between the logarithm of a star's period and its brightness. The paper was issued over Pickering's signature with a line stating that it had been prepared by Miss Leavitt. Measure how long such a star takes to pulse, and you know how luminous it truly is; compare that with how bright it looks, and you have its distance.
The Ladder She Never Climbed
Ejnar Hertzsprung and Harlow Shapley set about calibrating the scale against nearby stars, converting her relation into absolute distances. Shapley used it to map the Milky Way and dislodge the Sun from its centre. In 1923 and 1924 Edwin Hubble identified Cepheids in the Andromeda nebula, applied Leavitt's law, and showed that the object lay far outside our own galaxy, which settled the question of whether the universe contained others. The expansion of the universe was read off the same rungs a few years later.
Leavitt was repeatedly reassigned to routine photometric standards, illness interrupted her work, and she never developed the discovery further herself. She died of cancer in December 1921, aged fifty-three. In 1925 the Swedish mathematician Gosta Mittag-Leffler wrote to Harvard proposing to nominate her for a Nobel Prize, and had to be told she had been dead four years. The prize is not awarded posthumously, and the distance ladder of modern cosmology still rests on her measurements. Her name is attached to the relation only through the insistence of later astronomers.
