Using the technique of spectroscopy, stars can be classified by their colour (or temperature) into a series of letters which denote their spectral type. The hottest stars are denoted by the letter O, with the sequence progressing through B, A, F, G, K to the coolest M stars (see Figure 1). Properties and examples of each spectral type are listed below. Each spectral type is split further by the numbers 0 - 9 so that a B0 star is bluer (and therefore hotter) than a B9 star, which in turn, is slightly bluer than an A0 star.
- O-type stars have surface temperatures between 30,000 and 40,000 K. Using Wien's Law, we see that these stars have a peak wavelength of emission in the ultraviolet part of the electromagnetic spectrum. On average, 1 in every 3 million stars is an O-type star. The eastern (from Europe, the furthest left) star in Orion's Belt, Alnitak, is classified as O9.5 spectral type.
- B-type stars have surface temperatures between 10,000 and 30,000 K. On average, around 1 in 800 stars are B-type stars. Rigel, the brightest and bluest star in Orion is of spectral type B8.
- A-type stars have surface temperatures between 7,500 and 10,000 K. On average, around 1 in 160 stars are A-type stars. Vega, the brightest star in Lyra is of spectral type A0. When spectra are taken, it is A-type stars that display the strongest hydrogen lines, however this is more an indication of the star's temperature than of the abundance of hydrogen (which is generally 70 - 80 % of a star's total mass in all main sequence stars).
- F-type stars have surface temperatures between 6,000 and 7,500 K. On average 1 in 30 stars are F-type stars. Procyon, the brightest star in Canis Major is of spectral type F5.
- G-type stars have surface temperatures between 5,200 and 6,000 K. On average 1 in 12 stars are G-type stars. Our Sun is of spectral type G2.
- K-type stars have surface temperatures between 3,700 and 5,200 K. On average 1 in 8 stars are K-type stars. Pollux, the lower of the two bright stars in Gemini is of spectral type K0.
- M-type stars have surface temperatures between 2,400 and 3,700 K. They are the reddest and coolest of the common stellar spectral types. On average 3 in every 4 stars are M-type stars. Betelgeuse, the red star in Orion is of spectral type M1.
Find out more about the technique of spectroscopy.