Astrophysics (Index)About

spectral type

(classification of stars based upon spectral features)

A star's spectral type, indicated by its spectral class with a single-digit appended, is a classification based upon the star's spectral features and the temperatures associated with those features. Ten such types are commonly used, 0 (hottest) through 9 (coolest), except O: O2 is the hottest spectral type. Sometimes fractional types are used, e.g., O9.5, to further distinguish the spectrum. Commonly the spectral type is cited with a following Roman numeral (sometimes appended) and sometimes with a further suffix. For example, the Sun's type is often listed as "G2V" and Barnard's Star as "M4 Ve". The Roman numeral indicates a luminosity class, which indicates the general size of the star (e.g., main sequence versus giant). Prefixes to the spectral type are also used in some cases. Two of them:

With some prefixes, the rest of the spectral type code is specific to that type of star e.g., for white dwarf's spectral types sometimes match the scheme described here, but also has its own white-dwarf-specific types. Suffixes b, ab, a, a+ - after Roman numeral "I" are commonly used to further split luminosity class "I", classifying the star as exceptionally luminous, for supergiants and above. Other luminosity classes are also sometimes given with an a or b suffix, e.g., "A0 Va" for Vega, I presume to indicate more luminosity than usual. Some of the suffixes indicating unusual spectra (termed peculiar), generally attached to the luminosity class:

A colon before the suffix indicates uncertainty, e.g., "G5V:e" for T Tauri. More such suffix codes appear in tables listing stars. I've developed the opinion that codes are sometimes coined for a specific table. Sometimes the table's listed spectral type is later cited with no explanation of the code.

Further reading:

Referenced by pages:
AD Leonis (AD Leo)
Algol (Beta Per)
Alpha Centauri
AU Microscopii (AU Mic)
Barnard's Star
Beta Centauri
binary star
bolometric correction
core collapse supernova (CCSN)
Epsilon Eridani
Epsilon Indi (ε Indi)
51 Pegasi b (51 Peg b)
HR 8799
Kapteyn's Star
Lacaille 9352
Lalande 21185
LHS 1140
Luhman 16
luminosity class
Luyten 726-8
main sequence star (MS)
M-type star (M)
O5 spectral type (O5)
O-type star (O)
red and dead
Ross 154
Ross 248
Scholz's Star
shell star (sh)
superluminous supernova (SLSN)
spectral class
spectral line designation
spectroscopic parallax
standard candle
stellar distance determination
stellar kinematics
stellar luminosity determination
stellar mass determination
stellar temperature determination
Teegarden's Star
T Tauri
TW Hydrae (TW Hya)
variable star
velocity-metallicity relation
weak-line star
WISE 0855-0714
WISE 1506+7027
Wolf 359