Characteristics
O Stars
Hottest Stars: T>30,000 K; Strong He+ lines; no H lines
B Stars
T=15,000 - 30,000 K; Strong neutral He lines; very weak H lines
A Stars
T=10,000 - 7500 K; Strongest H lines, Weak Ca+ lines.
F Stars
T=7500-6000 K; H weaker, Ca+ stronger, weak metals
G Stars
T=6000-5000 K; Strong Ca+, Fe+, other metals, weak
H
K Stars
T=5000-3500 K; Strong metal lines, weak CH & CN
M Stars
Coolest Stars: T<3500 K; strong molecule bands (especially TiO),
no H lines (or only very weak).