Stellar temperatures can be classified by using Wien's displacement law, but this poses difficulties for distant stars. Stellar spectroscopy offers a way to classify stars according to their absorption lines; particular absorption lines can be observed only for a certain range of temperatures because only in that range are the involved atomic energy levels populated. An early scheme (from the 19th century) ranked stars from A to Q, which is the origin of the currently used spectral classes.
Secchi Spectral Classification
During the 1860s and 1870s, pioneering stellar spectroscopist Father Angelo Secchi created the Secchi Spectral Classification in order to classify observed spectra. The classifications group stars into classes as follows:
- Class I: white and blue stars with broad heavy hydrogen lines, such as Vega and Altair. This includes the modern class A and early class F.
- Class I, Orion subtype: a subtype of class I with narrow lines in place of wide bands, such as Rigel and γ Orionis. In modern terms, this corresponds to early B-type stars.
- Class II: yellow stars - hydrogen less strong, but evident metallic lines, such as Arcturus and Capella. This includes the modern classes G and K as well as late class F.
- Class III: orange to red stars with complex band spectra, such as Betelgeuse and Antares. This corresponds to the modern class M.
- Class IV: red stars with significant carbon bands and lines (carbon stars.)
- Class V: emission-line stars, such as γ Cassiopeiae and β Lyrae.
In the late 1890s, this classification began to be superseded by the Harvard classification.
Harvard Spectral Classification
The Harvard classification system is a one-dimensional classification scheme. Physically, the classes indicate the temperature of the star's atmosphere.
A popular mnemonic for remembering the order of spectral classification is "Oh Be AFine Girl/Guy, Kiss Me", although there are many variants of this mnemonic.
The Hertzsprung-Russell diagram relates stellar classification with absolute magnitude, luminosity, and surface temperature and provides a useful tool for analysis of stars.
Spectral Class O
Spectral Classification O stars are very hot and very luminous, being bluish in colour; in fact, most of their output is in the ultraviolet range. These are the rarest of all main sequence stars, constituting as few as 1 in 3,000,000 in the solar neighbourhood. These stars have dominant lines of absorption and sometimes emission for He II lines, prominent ionised (Si IV, O III, N III, and C-III) and neutral helium lines, strengthening from O5 to O9, and prominent hydrogen Balmer lines, although not as strong as in later types. Because they are so huge, class O stars burn through their hydrogen fuel very quickly and are the first stars to leave the main sequence.
Examples: Zeta Orionis, Zeta Puppis, Lambda Orionis, Delta Orionis
Spectral Class B
Spectral Classification B stars are extremely luminous and blue. Their spectra have neutral helium, which is most prominent in the B2 subclass, and moderate hydrogen lines. Ionised metal lines include Mg II and Si II. As O and B stars are so powerful, they only live for a very short time, and thus they do not stray far from the area in which they were formed. They constitute about 1 in 800 main-sequence stars in the solar neighbourhood.
Examples: Rigel, Spica, the brighter Pleiades
Spectral Class A
Spectral Classification A stars are amongst the more common naked eye stars and are white or bluish-white. They have strong hydrogen lines, at a maximum by A0, and also lines of ionised metals (Fe II, Mg II, Si II) at a maximum at A5. The presence of Ca II lines is notably strengthening by this point. They comprise about 1 in 160 of the main sequence stars in the solar neighbourhood.
Examples: Vega, Sirius, Deneb
Spectral Class F
Class F stars have strengthening H and K lines of Ca II. Neutral metals (Fe I, Cr I) beginning to gain on ionised metal lines by late F. Their spectra are characterised by the weaker hydrogen lines and ionised metals. Their colour is white. These represent about 1 in 33 of the main sequence stars in the solar neighbourhood.
Examples: Arrakis, Canopus, Procyon
Spectral Class G
Class G stars are probably the best known, if only for the reason that our Sun is of this class. Most notable are the H and K lines of Ca II, which are most prominent at G2. They have even weaker hydrogen lines than F, but along with the ionised metals, they have neutral metals. There is a prominent spike in the G band of CH molecules. G stars represent about 1 in 13 of the main sequence stars in the solar neighbourhood.
Examples: Sun, Alpha Centauri A, Capella, Tau Ceti
Spectral Class K
Class K are orange stars which are slightly cooler than our Sun. Some K stars are giants and supergiants, such as Arcturus, while others, like Alpha Centauri B, are main sequence stars. They have extremely weak hydrogen lines if they are present at all, and mostly neutral metals (Mn I, Fe I, Si I).
Examples: Alpha Centauri B, Epsilon Eridani, Arcturus, Aldebaran
Spectral Class M
Class M is by far the most common class. About 76% of the main-sequence stars in the solar neighbourhood are red dwarfs. M is also hosting to most giants and some supergiants such as Antares and Betelgeuse, as well as Mira variables. The late-M group holds hotter brown dwarfs that are above the L spectrum.
Examples: Betelgeuse, Proxima Centauri, Barnard's star, Gliese 581