Stellar classification is usually based on two classification schemes, based on the star's surface temperature (the Harvard system), and the star's luminosity, the MK system.
In the 1880s at the Harvard College Observatory, original work by the Italian astronomer Angelo Secchi distinguished several main spectral types of stars and labelled them with a letter according to the strength of their hydrogen spectral lines. They were arranged in order of their surface temperature from hot to cool stars : O, B, A, F, G, K, M. A traditional mnemonic being 'Oh Be A Fine Girl/Guy, Kiss Me). There is progressive change in the appearances of the spectra as one moves through the types due to temperature variations and not the stars’ compositions. So for instance, titanium oxide forms in M class stars because the temperature is low enough for it to form. The basic compositions of most stars are very similar.
Additional letters designate less common stars and novas. The types are subdivided with the numbers 0 to 9, with higher numbers for cooler stars. The discovery of brown dwarfs (objects that form like stars but do not shine due to thermonuclear fusion) meant that L, T and Y were added.
Spectral Type | Typical photospheric temperature / K | Colour | Spectral characteristics | Fraction of all sequence stars / % |
W | 50,000+ | Blue | Wolf-Rayet; bright emission lines | |
O | 25,000 - 50,000 | Blue | Ionised helium lines prominent | 0.00003 |
B | 10,000 - 25,000 | Blue - white | Natural helium and hydrogen | 0.12 |
A | 7,400 - 11,000 | White | Hydrogen dominant | 0.61 |
F | 6,000 - 7,600 | Yellow - white | Weaker hydrogen; calcium lines strong | 3.0 |
G | 5,000 - 6,000 | Yellow | Metal lines prominent | 7.6 |
K | 3,500 - 5,000 | Orange | Metal lines prominent; some molecular bands | 12 |
M | 2,000 - 3,000 | Orange - red | Titanium oxide bands dominant | 76 |
L | 1,500 - 2,000 | Red | Brown dwarfs; complex spectra, including metal hydrides and metals | - |
T | 800 - 1,500 | Deep red | Brown dwarfs prominent methane absorption; complex spectra including water | - |
Y | <800 | Deep red | - | - |
Cool stars are Class R, Class N (often called C- type or carbon stars, < 3000k) and Class S, which resemble Class M stars but have spectral bands of zirconium oxide prominent instead of titanium oxide. C-type stars are aged and have sooty envelopes, which is why their spectra show the presence of carbon, as well as CN and CH.
T type stars and L type stars are cooler because they do not shine as a result of normal fusion reactions in their core. The hottest of them produce energy by fusion of deuterium nuclei, but this produces much less energy than normal hydrogen fusion.
W-type stars (Wolf-Rayet stars) show emission lines and they are almost certainly the result of mass transfer processes in binary systems.
The MK or Yerkes system was devised by American astronomers and is based on two sets of parameters:
a revised version of the Harvard O-M scale, and
a luminosity scale of grades
The luminosity scale is based on the width of the spectral lines, which increase as the gas pressure increases. This pressure broadening is due to the perturbation of atomic energy levels by other nearby species. The largest stars have lower surface densities and pressures, so their lines are broader than smaller stars.
Luminosity class | Star type |
I | Supergiants |
II | Bright giants |
III | Giants |
IV | Subgiants |
V | Main sequence |
VI | Subdwarfs |
VII | White dwarfs |
Further specifications are used, such as Ia for bright supergiants such as Rigel (designated B8 1a). Our Sun is a yellow dwarf star of around 5800K designated G2 V.
The spectral sequence is a temperature sequence and according to Stefan’s law the amount of energy emitted per unit area of a star’s surface relates to the fourth power of its temperature. So a star’s spectral type determines its photospheric temperature and so two stars in the same spectral type have similar photospheric temperatures, although lower photospheric pressures in larger stars mean they mimic stars of a slightly different type and astronomers have to make appropriate corrections. So if two stars of the same spectral type differ in brightness, they must have different surface areas: the brighter star must have a greater surface area than the dimmer one.
So if one star is 100 times brighter than another of the same spectral class, it must have 100 times the photospheric surface area.
Surface area of a sphere A = 4𝛑r, where r is the sphere’s radius.
So the brighter star has 10 times the radius of the dimmer one. In this way the luminosity types relate to the physical sizes of the stars.
Class I Supergiants
radii between 20 and 500 times as large as the Sun
stars at the blue end of the spectral sequence are said to be of early spectral type and those at the red end of late spectral type. In this class stellar radii increase going from early to late spectral type.
Examples:
- Rigel (𝛃 Orionis): spectral type B8; blue supergiant of about 20 solar radii
- Betelgeuse (𝛂 Orionis): spectral type M2; orange-red supergiant of about 500 solar radii. Its lower photospheric temperature makes its brightness only about a fifth of Rigel’s, but it is still 10,000 times that of the Sun.
Class II Bright giants
about 1000 to 10 000 times the luminosity of the Sun, with radii 10 to 100 times that of the Sun.
Example: - 𝛜 Canis Majoris is a blue example.
Class III Giants
30 to 1000 times the solar luminosity and 5 to 50 times the solar radius. Mainly spectral types G, K and M. Most giants are yellow or red stars such as Arcturus (spectral type K2). Blue giants are much brighter than their red counterparts, such as Bellatrix (𝛄 Orionis) which has 4000 times solar luminosity.
Class IV Subgiants
luminosities 5 to 5000 times that of the Sun with radii around 5 times that of the Sun. Spectral types K to B.
Class V Main sequence stars
most stars are of this class
spectral type O main sequence stars have luminosities about 300,000 times and radii about 20 times that of the Sun. Both the radius and brightness decrease with later spectral types
A type M main sequence star has a brightness around 1% and a radius one third that of the Sun
Example - the Sun: spectral type G2.
Class VI Subdwarfs
mostly spectral types later than F
they are slightly smaller and slightly less bright than main sequence stars.
Class VII dwarfs
very high density and radius typically 1/100th that of the Sun
Usually <1% luminosity of the Sun.
The luminosity class of a star is often given with its spectral type, for instance our Sun is G2V.
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