Harvard star classification

Star Classification

The Harvard System of star classification.

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A system to classify stars into groups according to their temperature.

Type	Spectrum	Surface Temperature	Examples	Notes
W	Many bright lines: divided into WN (nitrogen sequence) & WC (carbon sequence)	Up to 80 000ºC	Rare: About 150 have been found in our Galaxy, and 50 in the large megellanic Cloud. 16 cases are known as planetary Nebula in which the central star is a Wolf-Raayet; one of these is NGC 7009, 'The Saturn Nebula' in Aquarius. Zeta Puppis 05.8	Known as Wolf-Rayet stars. Have expanding shells, moving outwards at up to 3000 km/s. All very remote and appear faint despite their considerable luminosity.
O	Both bright and dark lines.	40 000ºC to 35000ºC	Gamma Velorum WC7	Represents a transition between W and B stars, Though this does not imply any evolutionary sequence.
B	Bluish-white (BO) to white (B9). No emission lines, but dominant absorption lines of hydrogen and (particularly) helium.	Over 25 000ºC for BO. 12 000ºC for B9.	Rigel B8 (i.e. 8/10 of way from BO to AO ) Spica B1.	Rigel is particularly luminous with peculiarities in its spectrum.
A	White stars. Spectra dominated by Hydrogen lines.	10 000ºC to 8000ºC	Sirius, Vega, Altair.
F	Yellowish hue. Calcium very conspicuous, with less prominent hydrogen lines.	7500ºC to 6000ºC	Procyon, Polaris.	Yellow hue so elusive that to the naked eye most F-stars will be regarded as white.
G	Yellowish, weaker hydrogen lines, numerous conspicuous metallic lines.	5500ºC to 4200ºC (giants) 6000ºC to 5000ºC (dwarfs)	Capella (giant) Sun (dwarf)	Beginning the division into dwarf or main sequence stars and the giants.
K	Orange: weak hydrogen lines, strong metallic lines.	4000ºC to 3000ºC (giant) 5000ºC to 4000ºC (dwalf)	Arcturus, Aldebaran, Pollux (giant), Epsilon Eridani, Tau Ceti (dwalf).	K stars are more numerous than any other type.
M	Orange-Red: Very complicated spectra with maney bands due to molecules.	3400ºC (giant) 3000ºC (dwalf)	Mira Ceti (variable), Betelgeux, Antares (giant), Proxima Centauri (dwalf)	Strong differences between giants and main sequence dwalf. Proxima Centauri closest star beyond the solar system.
R	Reddish	2600ºC	V Arietis, T Lyra.	Remote, and appear faint.
N	Reddish: Strong Carbon lines	2500ºC	R Leporis, V Aquita.	Remote with maney variable examples.
S	Reddish: prominent bands of titanium oxide and zirconium oxide.	2600ºC	Chi Cygni, R Cygni.	Examples given are long period variables.
C	Types R and N are often combined as type C
Q				Used for Nova.
P				Used for gaseous nebula

The Harvard system was introduced by E.C. Pickering in 1890, and was extended by two famous woman astronomers, Miss A.Cannon and Mrs W. Fleming. It has been further modified into what is now called the MKK (after Morgan, Keenan and Kellerman of the U.S.A.) or Yerkes system.
Most stars lie in the sequence from B to M. The Very hot W stars are very rare. O stars are also rare. Types R,N and S are all giants.
Originally it had been planned to make the sequence alphabetical, beginning at A, representing the spectra with the strongest hydrogen lines. When it was realized that the spectra represented star temperature major modifications had to be made and the result is alphabetic chaos.