Astronomers began categorizing stars into groups with similar spectral qualities. Early classification systems proved inadequate, however, due to either their simplicity or their complexity. The first Harvard classification system, developed by Edward Pickering and Williamina Fleming, was a vast improvement. Their system placed the stars into 16 alphabetical classes based on the decreasing strengths of the Hydrogen absorption lines. This system proved to be inadequate as well, however, as it did not take into account any of the other absorption lines present in the spectra.

In 1910 Annie Jump Cannon, working under the direction of Pickering, modified the Harvard classification system taking all of the absorption lines into account. Her contribution, to combine or omit over half of the classes, had the unintended result of arranging the stars in order of their color (Fig. 5). Cannon’s scheme, which employed the letters O B A F G K M, is known as the Henry Draper Classification System, and with only slight modifications is still in use today [3].

Figure 5
These images of the stars are roughly the color one would see for a star of the typical temperature of that spectral type. Temperature ranges given in Kelvin. Diagram created from images included in the online astronomy course notes created by R.A. Pogge.