Cannon, Annie Jump (1863–1941)

US astronomer. She carried out revolutionary work on the classification of stars by examining their spectra. Her system, still used today, has spectra arranged according to temperature into categories labelled O, B, A, F, G, K, M, R, N, and S. O-type stars are the hottest, with surface temperatures ranging from 25,000 to 50,000 K (from 24,700°C/44,450°F to 49,700°C/89,500°F).

Studying photographs of the spectra of stars, Cannon discovered 300 new variable stars. In 1901 she published a catalogue of the spectra of more than 1,000 stars, using her new classification system. She went on to classify the spectra of over 300,000 stars. Most of this work was published in a ten-volume set which was completed 1924. It described almost all stars with magnitudes greater than nine. Her later work included classification of the spectra of even fainter stars.

Cannon was born in Dover, Delaware, and studied at Wellesley College, Boston, Massachusetts, and Radcliffe College, Cambridge, Massachusetts. She spent her career at the Harvard College Observatory, Cambridge, Massachusetts, as assistant 1896–1911, curator of astronomical photographs 1911–38, and astronomer and curator 1938–40.

A system had been established 1890 for classifying stellar spectra into categories labelled alphabetically A–Q. Cannon reformed this system. Stars in the O, B, A group are white or bluish, those in the F, G group yellow, those in the K group orange, and those in the M, R, N, S group red. Our Sun is yellow so its spectrum places it in the G group.