Annie Jump Cannon

Annie Jump Cannon analyzes a photographic plate of the night sky.
Credit: Harvard-Smithsonian Center for Astrophysics

We have  Annie Jump Cannon to thank for the stellar spectral classification scheme in use today. Cannon was born in December 1863 in Delaware and studied physics and astronomy at Wellesley College. She later went on to work at the Harvard Observatory as one of the Harvard "computers" (see post on Williamina Fleming). It is here where she spent many years analyzing photographic plates to classify the spectra of stars. Using new data, she simplified Fleming's earlier work on stellar classification and came up with a scheme composed of just seven letters: O, B, A, F, G, K, M. Cannon even came up with a mnemonic to remember the ordering - "Oh, Be A Fine Guy/Girl, Kiss Me!" This classification was initially based on the appearance and strength of certain spectral lines but today is recognized as a reflection of stellar temperatures: O stars are the hottest and bluest while M stars are the coolest and reddest. This is one of the identifying features of the Hertzsprung-Russell diagram today (see below plot).

The Hertzsprung-Russell (H-R) diagram displays where a star is at along its evolutionary journey. Note Cannon's spectral classes on the x-axis.

Credit: ESO

Cannon classified more than 225,000 stellar spectra, and her work was published in nine volumes of the  Draper Catalogue of Stellar Spectra between 1918 to 1924. It is thought that she became so good at classifying stars that she could look at any stellar spectra and classify it in just three seconds. Wow!

Cannon's hard work earned her numerous awards and honors. Among these were honorary degrees from the University of Delaware, Oglethorpe University, and Mount Holyoke College. She also became the first woman to receive an honorary doctorate from the University of Oxford as well as the first woman to hold an officer position in the American Astronomical Society. Each year, this organization also presents the Annie Jump Cannon Award to a female astronomer who has made outstanding contributions to the field early in her career. The first recipient of the award (in 1934) also became the first woman to be promoted to full professor of astronomy at Harvard University. Her name was Cecilia Payne-Gaposchkin, and she will be the subject of a future post.