(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
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:
- p - general "peculiar" indicator, taken to modify the preceding suffix letter code if there is one, otherwise, unspecified.
- e - emission lines present, e.g., "M6.5 Ve" for Wolf 359s.
- [e] - forbidden lines.
- ep - peculiar emission, e.g., "B6 Vep" for Achernar.
- pe - peculiar absorption.
- (e) - I don't, know, e.g., "K5 IV(e)Li" for J1407.
- Li - I don't know, e.g., "K5 IV(e)Li", my guess is lithium absorption lines.
- v - spectrum varies (over time), indicates the preceding suffix letter, if there is one. Note that variable stars have their own separate variable star classification covering the many types of variability.
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.
Referenced by pages:
AD Leonis (AD Leo)
Algol (Beta Per)
AU Microscopii (AU Mic)
Epsilon Indi (ε Indi)
51 Pegasi b (51 Peg b)
main sequence star (MS)
M-type star (M)
O5 spectral type (O5)
O-type star (O)
red and dead
shell star (sh)
superluminous supernova (SLSN)
spectral line designation
stellar distance determination
stellar luminosity determination
stellar mass determination
stellar temperature determination
TW Hydrae (TW Hya)