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Star classification

(Stellar classification)

In astrophysics, stars are classified by their surface temperature, that is associated to specific spectral patterns. An early schema from the 19th century ranked stars from A to P, which is the origin of the currently used spectral classes. After several transformations, today the spectral classification includes 7 main types: O, B, A, F, G, K, M.
A popular mnemonic for remembering this order is "Oh, Be A Fine Girl, Kiss Me".

This is called "Morgan-Keenan spectral classification", even though its form was already by Annie Cannon, also based on the work of other astronomers from the Harvard College Observatory. The classes, listed from hottest to coldest, are:



Star Color


30,000 - 60,000 °K



10,000 - 30,000 °K



 7,500  - 10,000 °K



 6,000  -  7,500 °K

White (yellowish)


 5,000  -  6,000 °K

Yellow (like the Sun)


 3,500  -  5,000 °K



 2,000  -  3,500 °K


Notice that hottest stars are blue, while coldest stars are red. This seems unusual to most people, who associate red with hot and blue with cold. This is because we see fire as yellow, orange or red, but light produced by hotter sources is blue. However, blue sources are hard to find on Earth because it requires a large amount of energy.

Also notice that this is true for light-emitting objects. However, the color of a common object, like a blue shirt or a piece of red paper, is not related to its temperature. Confusion also arises when one considers how artists or photographers may refer to the color of light: usually they describe reds as "warm" colors and blues as "cool".

Kelvin Temperature

K means Kelvin degrees, that can be calculated adding 273 to Celsius degrees. Here are 4 examples of common temperatures in Fahrenheit, Celsius and Kelvin degrees:





Water boils




Room Temperature




Water Freezes




Absolute Zero




However, star temperatures are much higher, so the following table can be useful:

Conditions in different temperatures




1,808 °K

1,535 °C

Melting point of iron

2,013 °K

1,740 °C

Boiling point of lead

3,683 °K

3,410 °C

Melting point of tungsten

3,925 °K

3,652 °C

Sublimation point of carbon

5,780 °K

5,500 °C

Surface temperature of the Sun

5,828 °K

5,555 °C

Boiling point of tungsten

Spectral types

The seven spectral classes were subdivided into tenths (for example B0, B1, B2, B3, ..., B9, A0, A1, A2, A3, ... A9, F0, F1, F2, F3...). The Sun is a G2 star.

  • Class O stars are very hot and luminous, being blue in colour. Naos (in the constellation Puppis) shines with a power close to a million times solar. These stars have prominent ionized and neutral helium lines and only weak hydrogen lines. Class O stars emit most of their radiation in ultra-violet.
  • Class B stars are again very luminous, Rigel (in the great constellation Orion) is a prominent B class blue supergiant. Their spectra have neutral helium and moderate hydrogen lines. As O and B stars are so powerful, they live for a very short time. They do not stray far from the area in which they were formed as they don't have the time. They therefore appear clustered together in the OB associations, which are associated with giant molecular clouds. The Orion OB association is an entire spiral arm of our Galaxy.
  • Class A stars are amongst the more common naked eye stars. Deneb in Cygnus is another very powerful star. Sirius, that appears the brightest star as seen from Earth, is also an A class star, but not nearly as powerful. As with all class A stars, they are white. Many white dwarfs are also A. They have strong hydrogen lines and also ionized metals.
  • Class F stars are still quite powerful and they are average-sized, such as Fomalhaut in Pisces Australis. Their spectra is characterized by the weaker hydrogen lines and ionized metals, their colour is white with a slight tinge of yellow.
  • Class G stars are probably the most well known for the reason that our Sun is of this class. They have even weaker hydrogen lines than F but along with the ionized metals, they have neutral metals.
  • 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 smaller. They have extremely weak hydrogen lines, if they are present at all, and mostly neutral metals.
  • Class M is the most common class by the number of stars. All red dwarfs, such Proxima Centauri, the closest star to our Solar Sysem, go in here, and they are plentiful. M is also host to most giants and some supergiants such as Antares in Scorpio and Betelgeuse in Orion, as well as Mira variable stars. These red giants are old stars. The spectrum of an M star shows lines belonging to molecules and neutral metals but hydrogen is usually absent. Titanium oxide can be strong in M stars.

M stars may be dwarf stars or supergiant stars, and A stars can be white dwarfs or white giants as well. However, not all combinations are possible. For example, F and G stars must be average-sized stars.

This can be understood through the Hertzsprung-Russell diagram, that is very important in astrophysics and relates temperature and spectral classification of stars with their luminosity and size.

While these descriptions of stellar colors are traditional in astronomy, they really describe the light as we see them from Earth, after it has been scattered by the atmosphere. The Sun is not in fact a yellow star, but has the color temperature of a body of 5780 K, that is a white with no trace of yellow which is sometimes used as a definition for standard white.

Spectral type additions

A number of other spectral types have been taken into use for rare types of stars: these are W, L, T, S, and C (that includes R and N).

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