Annie Jump Cannon: Star Classifier

Astronomer Annie Jump Cannon (1863-1941) classified hundreds of thousands of stars and created the system we use to understand stars today.

Annie Jump Cannon Library of Congress

Annie Jump Cannon
Library of Congress

Annie Jump Cannon was the eldest child of shipbuilder and Senator Wilson Cannon and his second wife Mary Jump. Cannon’s interest in astronomy began when her mother taught her the constellations. She went onto study physics and astronomy at Wellesley College, one of the top schools for women at the time. There she learned spectroscopy, which became the foundation for her work classifying stars. Cannon had become almost completely deaf, likely due to a case of scarlet fever. Nonetheless, she graduated with a degree in physics in 1884 and returned home to Dover, Delaware.

But Cannon was restless with the limited career options available to women so she learned photography. She even visited Europe in 1892 to photograph the solar eclipse. In 1894, after she reached out to her former professors, she returned to Wellesley as an assistant in the physics department and became a “special student” of astronomy at Radcliffe Women’s College at Harvard. This status gave her access to the Harvard College Observatory. By 1907 she had received her master’s degree from Wellesley.

In 1896 Cannon was hired to work with Edward Charles Pickering, director of Harvard College Observatory. She and other women who worked there were referred to as “computers” because they measured the brightness, positions, and spectra of hundreds of thousands of stars imaged on glass plates. Pickering was especially interested in obtaining and classifying visible-light spectra of thousands of stars.

Nettie Farrar began analyzing different star spectra and, when she left a few months later, Antonia Maury and Williamina P. S. Fleming continued the project. Fleming devised a 13-class system based on the presence or absence of specific lines in a stellar spectrum, such as the hydrogen lines. But she didn’t take into consideration what a spectrum may reveal about the star’s physical properties. Maury examined plates showing spectra with many more lines than the ones Fleming studied, so she could see more subtle differences between stars in the same group.  She then came up with a 22-group classification system.

Annie Cannon joined the project in 1911, focusing on stars in the southern sky, and she classified stars at an incredible rate. In one minute, she could examine three stars and classify them by their spectral patterns. Using a magnifying loupe (the type of magnifying glass more often used by jewelers), she could distinguish stars down to 9th magnitude, about 16 times fainter than the human eye can see.

Between 1911 and 1915, she classified 5,000 stars a month. To help with the process, she developed her own scheme, which resulted in the famous OBAFGKM classification that is still used today. Remembered by the mnemonic phrase, “Oh! Be A Fine Girl/Guy—Kiss me!”, the classification was initially based on the appearance and strength of certain spectral lines.  The organization was later understood to reflect stellar temperatures: O stars are hot and blue while M stars are cool and red stars.

Although she finished classifying 225,300 stellar spectra by 1915, her work wasn’t published until 1918 because every star had to be properly identified and verified with other catalogues. The ninth volume of The Henry Draper Catalog finally became available in 1924.

Cannon’s hard work earned her numerous academic honors, several prizes, and many “firsts”—including the first woman recipient of an honorary doctorate from Oxford and the first woman elected an officer of the American Astronomical Society. The American Association of Astronomy annually presents the Annie Jump Cannon Award to women astronomers who have made outstanding contributions in astronomy.

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