Spectral Type

As noted, from numbers to a series of numbers and letters came the spectral type classification of Annie Jump Cannon. The basic Spectral Types are O – B – A – F – G – K – M . Cannon decided to drop several letters and numbers, thus the seemingly unorganized system we employ today. As can be seen in the graphic, the Spectral Type is indicative of the star’s color, thus the star’s photosphere temperature.

This diagram shows the colors of stars as shown by their photosphere color and identified by their Spectral Types. Blue is O, light blue is B, lightest blue is A, lightest yellow is F, light yellow is G the sun, orange is K, and red is M.
Image courtesy of Mike Reynolds, Ph.D. of Florida State College at Jacksonville.