A star's spectral class (or spectral type, stellar class, stellar type, stellar classification, or spectral classification), a category 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, and in context, the spectral-class label (e.g., G-class) may specifically mean a main sequence star of that class. However, the classes apply to any star: the class is a reflection of its spectral energy distribution, which is the result of its surface temperature even if it is a giant star, in which case its mass, radius, and luminosity will be radically different than main sequence stars of the same class.
Spectral class classifications, and the associated characteristics of main sequence stars in the class:
|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. Some M stars, most L stars, and all T and Y stars are insufficiently massive to have a main sequence, i.e., they are brown dwarfs.
A well-known mnemonic for the sequence OBAFGKM is "Oh be a fine girl kiss me".