A star's spectral class (or spectral type), a categorization based on features of its spectrum, primarily depends upon its surface temperature. The terms early and late (early-type star or late-type star) are used to mean hotter and colder, e.g., "O", "B", and "A" are considered to be early classifications, and a star with an effective temperature of 3600K would be considered an early M-class star. The terminology arose at a time when it was imagined that stars cool over their lifetime.
The classes are useful for relating the spectrum of main sequence stars with their mass, radius, and luminosity
|class||conventional color||surface temp||mass||radius||luminosity||hydrogen spectral lines||abundance|
|O-type star||hottest||"blue"||>33000K||>16 Msun||>6.6Rsun||>30000Lsun||weak||.00003%|
|K-type star||"orange"||3700-5200K||.45-.8Msun||.7-9.6Rsun||.08-.6Lsun||very weak||12.1%|
|M-type star||"red"||2000-3700K||<.45Msun||<.7Rsun||<.08Lsun||very weak||76.56%|
|L-type star||"purple-red"||1300-2000K||?||?||?||extremely weak||n/a|
|T-type star||"brown"||700-1300K||?||?||?||extremely weak||n/a|
|Y-type star||coolest||"dark brown"||<700K||?||?||?||extremely weak||n/a|
Numbers 0-9 are appended to break down classes further.
A well-known mnemonic for the sequence OBAFGKM is "Oh be a fine girl kiss me".