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Stellar Classification
Stars are classified based on their spectral characteristics. EM radiation from the star is split through a prism or a piece of equipment called a diffraction grating. This produces a spectrum of light containing spectral lines - each line indicates a particular molecule or chemical element. The strengths of different lines vary mostly due to the temperature of the star’s photosphere, but in some cases the line strengths vary due to the abundance of a particular element or molecule.
Most stars are classified using the letters O B A F G K M - O being the hottest, M being the coolest. Each letter is subdivided using arabic numerals - 0 being the hottest, 9 being the coolest. Fractional numbers are also allowed - for example, O9.7
A luminosity class is then added, often using roman numerals -
Main sequence stars - V
Subdwarfs - VI (or the prefix sd)
Subgiants - IV
Giants - III
Bright giants - II
Less luminous supergiants - Ib
More luminous supergiants - Ia
Hypergiants - 0 or Ia+
Class M
M-type stars have an orange or red colour and are by far the most common - around 76% of nearby stars are type M. A class M star’s spectrum contains lines from oxide molecules and neutral metals. Although most class M stars are very small main-sequence stars, many large supergiants are class M, including the largest known star, Stephenson 2-18.
Examples include:
Betelgeuse - M1Ia
Proxima Centauri - M5.5V
Class K
K-type stars are orange and slightly cooler than the sun. They have spectra containing lines of neutral metals. About 12% of nearby stars are type K. K-type main-sequence stars are of interest in searches for alien life because they have very long lifetimes, giving planets time to develop. Unlike M-type main-sequence stars, which live longer, class K stars are unlikely to produce violent solar flares which could be damaging to life.
Examples include:
Arcturus - K1.5 III
ε eridani - K2V
61 cygni A - K5V
Class G
The spectra of G-type stars have lines of Ca II. These stars are yellow or yellow-white. Around 7.5% of nearby stars are type G. G-type supergiants are very rare and unstable - most supergiants are blue(O, B) or red(K, M). Many G-type stars have planets, including the Sun.
Examples include:
β Canum Venaticorum - G0V
Sun - G2V
Class F
F-type stars have spectra with lines of ionised metals. These stars are white or yellow-white.
Around 3.03% of nearby stars are type F. F-type stars have surface temperatures of between 6000 and 7600 K.
Examples include:
78 Ursae Majoris - F2V
Procyon A - F5V or F5IV
Class A
A-type stars have spectra with lines of ionised metals and strong hydrogen lines. These stars are white or blue-white. Around 0.625% of nearby stars are type A. Many of the brightest stars in the sky are A-type stars.
Examples include:
Altair - A7V
Sirius A - A1V
Vega - A0V
Class B
B-type stars have spectra with neutral helium lines and moderate hydrogen lines. These stars are blue in colour. O and B type stars are very energetic, so they have very short lives.
Around 0.125% of nearby stars are type B
Examples include:
Regulus A - B8IV
Rigel - B8Ia
Class O
O-type stars have prominent neutral helium lines. O-type stars are blue. Around 0.00003% of nearby stars are type O. These stars have temperatures in excess of 30000 K.
Examples include:
Mintaka A- O9.5 II
S Monocerotis - O7V
Info from:
https://en.wikipedia.org/wiki/Stellar_classification
By Lawrence Henry (Y9)
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