ANNIE JUMP CANNON
The monumental task of classifying the spectra of stars was under-taken in the early 1900s by Annie jump Cannon: (1863-1941) at the Harvard College Observatory, Massachusetts. Cannon worked as a "computer," which meant someone who tirelessly examined hundreds of thousands of photographic plates, of stellar spectra.
The data was eventually compiled into the Henry Draper Catalog, which contains the spectral classification of 225,300 stars.
Cannon refined the patterns seen in the endless array of spectra into a single sequence. Her elegant classification system arranged the stars by color, rather than by the strength of hydrogen lines, as the first classification scheme did. Cannon's sequence went from the hottest, blue stars of type O, through the Sun-like G stars, ___ and ended with the cool, red stars. Almost a century later, her system is still in use.