In 1896 she began work under E. C. Pickering at the Harvard College Observatory, where she undertook the classification of stellar spectra. She refined W. P. Fleming’s alphabetical sequence (A, B, C, etc.), dropping some categories and rearranging to yield a continuous series of spectral types: from hot O and B stars, through A, F, and G, to cool K and M stars. Cannon’s classifications of the spectra of over 225 000 stars were published in the Henry Draper Catalogue.