Spectral type

Stars are usually classified according to their spectrum. The spectral type roughly reveals the star's temperature as can be seen in the table below.

The table describes Harvard spectral types and their corresponding effective surface temperature ranges, absolute V magnitudes, B-V colours, and spectral descriptions. Magnitudes and colours are given for main sequence stars; giants and supergiants are brighter and bluer than the main sequence stars. Each Harvard type is further divided into 10 subtypes, which are marked with numbers 0, 1 ... 9 (in the order from the hottest to the coolest) after the lettered type. (A mnemonic for beginners: "Oh, Be A Fine Girl, Kiss Me!" - The mnemonic was invented by Henry Norris Russell to aid students of astronomy to remember the sequence. This was done before the discovery of carbon, C, stars.)

Below the main spectral classification, there is a MKK or Yerkes class by Morgan, Keenan, and Kellman from Yerkes Observatory and further below some additional prefixes/suffixes. These classes are usually written in this order (Harvard class, Yerkes class, suffix); e.g. the full spectral type of the Sun is G2V.

*Spectral Type*	*T_eff*	*M_V (main sequence)*	*B-V (main sequence)*	*Description (of the spectrum)*
O	> 25000 K	< -4.1	< -0.31	Relatively few absorption lines. Lines of ionized He, doubly ionized N, triply ionized Si. H lines weak.
B	11000 - 25000 K	0.7 - -4.1	0.0 - -0.31	Lines of neutral He, singly ionized O and Mg. H lines stronger than in O stars.
A	7500 - 11000 K	2.6 - 0.7	0.27 - 0.0	Strong H lines. Lines of singly ionized Mg, Si, Fe, Ti, Ca etc. and some neutral metals.
F	6000 - 7500 K	4.4 - 2.6	0.58 - 0.27	H lines weaker and neutral metal lines stronger than in A stars. Lines of singly ionized Ca, Fe, Cr.
G	5000 - 6000 K	5.9 - 4.4	0.89 - 0.58	Lines of ionized Ca most conspicuous features. Many lines of ionized and neutral metals. CH bands.
K	3500 - 5000 K	9.0 - 5.9	1.45 - 0.89	Neutral metal lines predominate. CH bands present.
M	< 3500 K	> 9.0	> 1.45	Strong lines of neutral metals and molecular bands of TiO.
C	2000 - 5400 K			CARBON STARS. Strong bands of molecular carbon, CN, CH, or other carbon compounds; no TiO.
*Spectral Type*	*T_eff*	*M_V (main sequence)*	*B-V (main sequence)*	*Description (of the spectrum)*

			*MKK / Yerkes class*	*Description*
			Ia	luminous supergiants
			Ib	less luminous supergiants
			II	bright giants
			III	normal giants
			IV	sub giants
			V	dwarfs (main sequence stars)
		rarely used:	VI	subdwarfs
		rarely used:	VII	white dwarfs
		*Additional info:*	*prefix / suffix*	*Description*
			c	sharp lines
			d	dwarf (main sequence star)
			D	white dwarf
			e	emission (hydrogen emission in O stars)
			em	emission in metal lines
			ep	peculiar emission
			eq	emission with shorter wavelength absorption
			d	dwarf (main sequence star)
			D	white dwarf
			e	emission (hydrogen emission in O stars)
			em	emission in metal lines
			ep	peculiar emission
			eq	emission with shorter wavelength absorption
			d	dwarf (main sequence star)
			D	white dwarf
			e	emission (hydrogen emission in O stars)
			em	emission in metal lines
			ep	peculiar emission
			eq	emission with shorter wavelength absorption
			f	emission by helium and neon in O stars
			g	giant
			k	interstellar lines
			m	strong metallic lines
			n	diffuse lines
			nn	very diffuse lines
			p	peculiar spectrum
			s	sharp lines
			sd	subdwarf
			wd	white dwarf
			wk	weak lines