1901

Pickering’s “Harvard Computers”

Annie Jump Cannon (1863–1941), Edward Charles Pickering (1846–1919)

Astronomers, like most other scientists, like to be able to group the objects they study into convenient classes or groups, making it easier to compare and contrast their properties and histories. Coming up with a suitable scheme to classify the stars was particularly important; thousands of them are visible to the naked eye and millions are accessible from the telescope.

The early star catalogs of Hipparchus, Ptolemy, and al-Sūfī recorded the relative magnitudes of the brighter stars and sometimes their relative colors, from bluer to redder. In the 1860s, the Italian astronomer Father Angelo Secchi acquired spectroscopic data for thousands of stars, developing the first stellar classification scheme and dividing the stars into five main classes based on their spectral patterns.

Many astronomers worked on refining and extending Secchi’s scheme to classify millions more stars, including Harvard College Observatory director Edward Charles Pickering. Pickering had access to excellent telescopes for his project, and, like other observatory directors of the time, he hired human “computers” to help sift through and analyze the enormous data set (thousands of photographic plates) that was being collected.

Portrait of Annie Jump Cannon from 1922.

Many of these computers were women, hired for little or no pay to perform what many of their male employers regarded as the rather menial and tedious work of measuring stellar spectral lines. Some of these women became quite skilled in stellar spectroscopy and went on to make important contributions to the field. Among the standouts was Annie Jump Cannon, who was able to use her knowledge of the strengths of absorption lines to reorganize and simplify what had been becoming overly complex and competing schemes. Cannon’s 1901 class names—OBAFGKM, from bluer and weaker to redder and stronger lines—are still used by astronomers today. Later, her classes would be shown to be directly correlated with stellar temperature and stellar evolution.

Schematic representation of the so-called Harvard classification scheme developed by Annie Jump Cannon in 1901, grouping the stars by weakest lines (O) to strongest (M) based on their spectra.