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Spectral Types

Stars are divided into groups called spectral types which are based on the strength of the hydrogen absorption lines. The A-type stars have the strongest (darkest) hydrogen lines, B-type next strongest, F-type next, etc. Originally there was the whole alphabet of types, based on hydrogen line strengths, but then astronomers discovered that the line strengths depended on the temperature. Also, the figure above shows that more than just the hydrogen lines must be used.

After some rearranging and merging of some classes, the class sequence is now OBAFGKM ("Oh be a fine girl, kiss me.") when ordered by temperature. The O-type stars are the hottest stars and the M-type stars are the coolest. Each class is subdivided into 10 intervals, e.g., G2 or F5, with 0 hotter than 1, 1 hotter than 2, etc. About 90% of the stars are called main sequence stars. The other 10% are either red giants, supergiants, white dwarfs, proto-stars, neutron stars, or black holes. The characteristics of these types of stars will be explored in the following chapters. The table below gives some basic characteristics of the different spectral classes of main sequence stars. Notice the trends in the table: as the temperature of the main sequence star increases, the mass and size increase. Also, because of the relation between luminosity and the size and temperature of a star, hotter main sequence stars are more luminous than cooler main sequence stars. However, there are limits to how hot a star will be, or how massive and large it can be. Understanding why the constraints exist is the key to understanding how stars work.

Main Sequence Star Properties
Color Class solar masses solar diameters Temperature Prominent Lines
bluest O 20 - 100 12 - 25 40,000 ionized helium
bluish B 4 - 20 4 - 12 18,000 neutral helium, neutral hydrogen
blue-white A 2 - 4 1.5 - 4 10,000 neutral hydrogen
white F 1.05 - 2 1.1 - 1.5 7,000 neutral hydrogen, ionized calcium
yellow-white G 0.8 - 1.05 0.85 - 1.1 5,500 neutral hydrogen, strongest ionized calcium
orange K 0.5 - 0.8 0.6 - 0.85 4,000 neutral metals (calcium, iron), ionized calcium
red M 0.08 - 0.5 0.1 - 0.6 3,000 molecules and neutral metals

Red giants can get up to about 50 times the size of the Sun. Supergiants are between 20 times the size of the Sun for the BO supergiants and 1000 times the size of the Sun for the M0 supergiants. Despite the tremendous size of some stars, even the largest supergiant is only 1/7000 light years across. Since stars are several light years from each other, they do not collide with each other (even the fat ones!).