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 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 as they age.
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.
The mnemonic for the sequence OBAFGKM is "Oh Be A Fine Girl Kiss Me".