Spectral Class (Color-Temperature Class):
Annie Jump Cannon (1912) classified dark lines for 200,000 stars! 
O B A F G K M
                                       <-----------------
                                                   T
         BLUE            RED 
Subclasses: Sun is a G2V star.
A. 1-10 Spectral Subclass.
B.  Luminosity Class I-V (I--supergiant, V--main sequence).
The Evolution of a Star:
Hertzsprung-Russell Diagram (1910): A plot of Luminosity vs Surface Temperature, T (see overlay). A one solar mass star like the sun goes through the stages of:  protostar, main sequence, red giant, planetary nebula, white dwarf.