spectral type

A star's spectral type, indicated by its spectral class letter-code with a single-digit appended (e.g., "G2"), is a classification based upon the star's spectral features, the classes and specific types organized by the temperatures associated with the associated set of features. Ten such types for each class are commonly used, 0 (hottest) through 9 (coolest), except class O in which O2 is the hottest spectral type in common use. The difference between adjacent types (e.g., M5 and M6) is that particular spectral features indicated by the class are more or less prominent, reflecting some difference in temperature. Fractional types are sometimes used, e.g., O9.5, to further distinguish the spectrum. Commonly the spectral type is cited with a following Roman numeral (optionally 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:

• sd - subdwarf, e.g., "sdM1" for Kapteyn's Star.
• D - white dwarf, e.g., "DA2" for Sirius B.

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, or a⁺ after the luminosity class "I" indicate the star as exceptionally luminous, used for supergiants and above. Other luminosity classes may also be given an a or b suffix, e.g., "A0 Va" for Vega, I presume to indicate more luminosity than usual. Other suffixes indicate unusual spectra (termed peculiar), generally appended 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.
• m - absorption lines indicating a specific pattern of peculiar metallicity, commonly used for "A" stars, i.e., Am stars.
• n - broad lines ("n" for "nebulous").
• s - narrow lines ("s" for "sharp").
• [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 - A chemical symbol (such as Li for lithium) indicates presence lines of that element, but I'm not sure whether emission or absorption is presumed. Example: "K5 IV(e)Li".
• 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 specific tables or lists of stars, and that sometimes such a "coined spectral type code" is later cited for the star with no explanation of the code.