The Harvard Observatory Program to Classify Stellar Spectra

Edward Pickering & Williamina Fleming

As director of the Harvard College Observatory Edward C. Pickering (1846-1919) undertook the oversight for completion of the Henry Draper Catalogue. Because it was the goal of this project to classify a sufficient number of stars so that it would be years before anyone felt the need to repeat such an undertaking, it was Pickering's goal to classify at least 100,000 stars for the Henry Draper (HD) Catalogue. By World War I, objective prism photography was used with telescopes at Harvard and Peru to obtain spectra of more than 250,000 stars.

Pickering was unhappy with the work performed by his male employees and declared that his maid could do a better job than they did. In 1881, Pickering did hire his maid, Williamina Fleming (1857-1911), to do some mathematical calculations at the Harvard Observatory. Fleming was soon promoted to work directly with the spectral classification project, although she was paid half of what the men were paid. Nevertheless, Fleming developed a relatively simple classification scheme, with 22 different classes, and in 1890 published the classifications for 10,000 stars as the Draper Catalogue of Stellar Spectra. Fleming sorted stars by decreasing Hydrogen absorption-line strength. Stars with the strongest hydrogen lines in their spectrum were designated as spectral type "A", followed by types B, C, D, etc. for stars with weaker hydrogen lines in their spectrum. Unfortunately, few of the spectral lines due to elements other than hydrogen fit into this sequence.