Stellar success of a lowly computer in Pickering's harem

George Johnson is the author of a brilliant biography of Murray Gell-Mann, the physicist and Nobel laureate who developed the Standard Model of particle physics, who posited the existence of quarks and who is surely one of the best-known scientists of the modern era. By contrast, the subject of Johnson's latest biography is one of the less well-known figures in the history of science. Henrietta Leavitt changed our understanding of the universe, yet her name is hardly known beyond the small community of astronomers who study a peculiar type of star known as a Cepheid variable.

As the name suggests, Cepheid variable stars fade and flare periodically, but with such subtlety that they went unnoticed until 1784, when the teenage astronomer John Goodricke detected that the star Delta Cephei varied in brightness every five days. Spotting this faint pulsing was an extraordinary achievement, because Goodricke had to take into account the impact of moonshine and weather on his perception of the star's brightness.

One theory is that he had exceptional visual acuity, developed to compensate for his profound deafness. He made a host of discoveries, for which the Royal Society made him a fellow and awarded him its prestigious Copley Medal.

Leavitt took up the study of Cepheid variables a century later. Like Goodricke, she was profoundly deaf, and perhaps this played a part in her ability to make observations that went beyond those of her colleagues.

However, whereas Goodricke stared up into space, Leavitt stared down at photographs. The photographs had been taken by senior astronomers at Harvard College Observatory, and Leavitt had been employed there as a lowly "computer", a term used to describe anyone given the dull job of processing data, such as measuring the position, brightness and colour of stars.

In fact, the observatory director Edward Pickering employed a gang of women as computers, who were often referred to as "Pickering's harem". They were paid a minimal salary for what was mundane work but were bright and found time during the drudgery to draw significant conclusions from their measurements. Leavitt had access to the data, she scrutinised it in incredible detail, and she went further than any of her supervisors could ever have imagined.

While discovering more than 2,000 variable stars, Leavitt realised that the rate at which Cepheids varied indicated their brightness. This was one of the most important breakthroughs in the history of astronomy because it enabled astronomers to compare a star's actual brightness with its apparent brightness and thus work out how far away the star was. The "variable-star fiend" (as Leavitt was called by Charles Young of Princeton University) had provided astronomers for the first time with a yardstick for measuring cosmic distances. There is a direct link between Leavitt's research and the development of 20th-century cosmology, from the discovery of galaxies to the Big Bang theory of the universe.

The story of Leavitt and the Harvard computers has appealed to me for several years, so much so that I included it in a lecture at Harvard Bookstore last year. This was just a couple of days after Lawrence Summers, then president of Harvard University, made his notorious comments suggesting that women might have less "innate ability" than men in science and mathematics. If he had been aware of Leavitt's story, he might have realised that he was on the wrong track.

Leavitt was just one of the women in Pickering's harem who made important breakthroughs. There was also Annie Jump Cannon, who developed the modern system for classifying stars, and Williamina Fleming, who focused on discovering novae. Johnson describes Leavitt's research and the breakthroughs of her colleagues in vivid detail while also providing a remarkable insight into women in science at the start of the 20th century.

Miss Leavitt's Stars is a short, excellent account of Leavitt's extraordinary life and achievements, reminding us of one of astronomy's forgotten heroes. Perhaps Johnson will consider writing a similar biography of Goodricke, who died at the tragically early age of 21. Goodricke never knew of his election to the Royal Society in London four days before his death because he was in Yorkshire at the time. It seems he died from pneumonia caught as a result of spending too many winter nights staring into space.

Simon Singh is the author of Big Bang, a history of cosmology.