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Starlight - Main Sequence - O - B - A - F - G - K - M - Luminosity Classes - Spectral Class Codes

A star's spectral class code concisely conveys the star's size, temperature, composition and life cycle stage. Some example codes follow:
Sun G2V  Main Sequence star
Achenar B3Vpe Main Sequence star with peculiar spectrum and emission lines
Rigel B8Ia  Supergiant
Sirius A1Vm Main Sequence star with metallic spectrum lines
Antares M1Ib  Red Supergiant
Proxima Cen M5Ve Main Sequence Red Dwarf with emission lines

Until recently, astronomers recorded spectra onto photographic film. Today they are more likely to employ electronic detectors and record the data directly to a computer.

Astronomers classify stars by matching records to spectra in reference catalogs.
Stellar spectral class codes convey three pieces of of information:

Achenar B3Vpe

The initial letter-number pair specifies the star's spectral type (and also indicates its temperature). The spectra shown at left are arranged in order of temperature - hottest at the top. The Sun, a G2 star is slightly hotter than 16 Cyg Prec, a G3 star and slightly cooler than Beta CVn, a G0 star.

Next is the star's luminosity class - the amount of light the star produces. Roman numerals, and perhaps a letter or zero are used to convey this information:
Ia-0 Most luminous stars known
Ia Luminous Supergiants
Ib Luminous Supergiants
II Bright Giants
III Giants
IV Sub Giants
V Main Sequence Stars
VI Sub Dwarfs
VII or DWhite Dwarfs

And thirdly, if the star has special features, letter codes are used to specify them:
eemission lines
mmetallic lines
ppeculiar spectrum
vvariable spectrum
qblue shift in spectral lines

Variable stars oscillate in spectral type - the spectral codes for the two extremes are given.

There is considerable variation in code usage. Any list of spectral types should specify the convention followed.