Annie Jump Cannon, passionate astronomer and suffragette, was born in 1863 in Dover, Delaware. A lifetime in astronomy began there when her mother taught her the constellations as they observed the night sky together.
As a young woman at Wellesley College in Massachusetts, Cannon studied physics and astronomy with the college's first professor of physics, Sarah Whiting. Whiting had made Wellesley one of the first institutions to teach the new technique of applying spectroscopy to starlight.
Spectroscopy analyzes a light spectrum. If you've seen a rainbow in the sky or through a prism, you've seen a spectrum. Ordinary light is a combination of all the other colors and a prism breaks it up again.
At its simplest, a spectroscope is only a prism with lenses. If we put one on a telescope, it spreads a star's light into a spectrum. But we also get a series of narrow lines crossing the spectrum. These lines tell us a lot, including the chemical elements that are present and the temperature of the gas. It's made possible by our knowing from laboratory studies that each element, when heated, produces a unique set of spectral lines.
Spectroscopy was useful to Cannon when she enrolled in Radcliffe College (now part of Harvard University) to do a master's degree in astronomy. She chose Radcliffe for its access to Harvard Observatory, and within months of her arrival Cannon was hired as a computer at the observatory. In the pre-electronic age people had to do the work of turning complicated star data into a useful form.
A decade before Cannon's arrival, the widow of pioneering astronomical photographer Henry Draper had given the observatory a large sum of money. The donation was so that its director Edward Pickering could complete her husband's work, a star catalog with the stars grouped by spectra.
When Cannon began working on the catalog, she found the existing spectral classification unsatisfactory. She revised it into the system which is essentially the one used today with the stars grouped by temperature. It's commonly known as the OBAFGKM system, "O" stars being the hottest and "M" the coolest. Famously, it's remembered by the mnemonic "Oh! Be a fine girl (guy) – kiss me."
Classifying faint stars from photographs of their spectra was not an easy job, but Cannon had the knack. She seemed to recognize them as one might an old friend, and was able to work quickly. Over her career she classified nearly a quarter of a million stars for the Henry Draper Catalog. She also oversaw the publication of most of it after Edward Pickering's death. With subsequent additions, it is still an important reference work.
Annie Cannon was remarkable. As a deaf woman she had a double handicap, but she didn't let that stop her astronomical work. She was also a tireless advocate of women's rights, supporting the suffragettes and opportunities for women in science. She had an international reputation and among her many honors, was the first honorary doctorate awarded to a woman by Oxford University.
Despite this, it wasn't until 1938, when she was 74, that the university gave her a permanent position. Most people retire before that age, but it was only her death in 1941 that ended her work.