Spectral Classification of Stars

Stars are large balls of gas. The force of gravity compresses the central core of the star, raising it to temperatures over 10 million Kelvins. At these temperatures and densities, nuclear fusion can start. Four Hydrogen atoms combine to form one Helium atom, releasing energy. This is the source of starlight.

More massive stars have hotter cores, and fuse their hydrogen faster. They are more luminous, and hotter, than the Sun. The Sun has sufficient Hydrogen to survive for about 10 billion years; more massive stars burn out more quickly.

Astronomers classify stars according to their spectra. Because to first approximation, stars are black bodies, and the hotter stars are blue and the cooler stars are red (the visble surface of a star is called the photosphere). In addition, the pattern of absorption lines due to elements in their atmospheres changes markedly with temperature. Stars are given spectral types. The spectral sequence of stars runs OBAFGKM.

Stellar spectra
NOTE: The colors in the figure above are solely for the purpose of making it easier to distinquish between the spectra. That O stars appear blue, and M stars appear red, is a consequence of the strength of the brightness as a function of the wavelength. O stars appear blue because they put out more light at short (blue) wavelengths athan they do at long (red) wavelengths. All photons at a given wavelength have the same energy. A "true color" diagram would be shaded blue at low wavelengths (4000 Angstroms) and red at long wavelengths (7000 Angstroms).

Among the mnemonics used to memorize the spectral sequence are:

Feel free to make up your own (check out the Great Mnemonic Contest).

Most stars are called main sequence stars or dwarfs. These are stars which are burning Hydrogen stably. O dwarfs are about 50 times as massive as the Sun, and a million times as bright; M dwarfs may be as small as a tenth the mass of the sun and one ten-thousandth as bright. Stars which have used up their core hydrogen and are beginning to burn out expand into subgiants, giants, and super giants. Later they may become white dwarfs. These categories are called luminosity classes. Note that there is no such thing as a normal star.

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