11.20: 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.