Astrophysics (index)about

spectral class

(spectral type, stellar class, stellar type, star class, star type, stellar classification, spectral classification)
(classification of spectrums of stars (O, B, A, etc.))

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:

classconventional colorsurface tempmassradiusluminosityhydrogen spectral linesabundance
O-type starhottest"blue">33000K>16 Msun>6.6Rsun>30000Lsunweak.00003%
B-type star"blue-white"10000-33000K2.1-16Msun1.8-6.6Rsun25-30000Lsunmedium.13%
A-type star"white"7500-10000K1.4-2.1Msun1.4-1.8Rsun5-25Lsunstrong.6%
F-type star"yellow-white"6000-7500K1.04-1.4Msun1.15-1.4Rsun1.5-5Lsunmedium3%
G-type star"yellow"5200-6000K.8-1.04Msun.96-1.15Rsun.6-1.5Lsunweak7.6%
K-type star"orange"3700-5200K.45-.8Msun.7-9.6Rsun.08-.6Lsunvery weak12.1%
M-type star"red"2000-3700K<.45Msun<.7Rsun<.08Lsunvery weak76.56%
L-type star"purple-red"1300-2000K???extremely weakn/a
T-type star"brown"700-1300K???extremely weakn/a
Y-type starcoolest"dark brown"<700K???extremely weakn/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".


Referenced by:
absorption line
A-type star (A)
AD Leonis (AD Leo)
Algol (Beta Per)
Alpha Centauri
astronomical quantities
AU Microscopii (AU Mic)
Balmer jump
Barnard's Star
B-type star (B)
Be star
Beta Centauri
binary star
blue horizontal branch (BHB)
bolometric correction
brown dwarf (BD)
evaporating gas globule (EGG)
Epsilon Eridani
Epsilon Indi (ε Indi)
escape fraction
F-type star (F)
51 Pegasi b (51 Peg b)
G-type star (G)
Gould's Belt
neutral atomic hydrogen (HI)
ionized hydrogen (HII)
HII region (HII)
HR 8799
ionizing radiation
Kapteyn's Star
K-type star (K)
Lacaille 9352
Lalande 21185
L-type star (L)
LHS 1140
Luhman 16
luminosity class
Luyten 726-8
main sequence star (V)
M-type star (M)
M dwarf
Morgan classification
O5 spectral class (O5)
OB association
OB star
O-type star (O)
planet demographics
post main sequence star
quenched galaxy
radio star
red giant
Ross 154
Ross 248
Scholz's Star
spectral signature
spectroscopic parallax
spiral arm
spiral galaxy
standard candle
star count
star formation (SF)
stellar association
stellar cluster (SC)
stellar demographics
stellar distance determination
stellar luminosity determination
stellar mass determination
stellar population
stellar temperature determination
T-type star (T)
T Tauri
TW Hydrae (TW Hya)
variable star
velocity-metallicity relation
weak-line star
WISE 0855-0714
WISE 1506+7027
Wolf 359
X-ray source (RS)
Y-type star (Y)