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
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
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.
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
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).
stars are the hottest, with temperatures from about 20,000K up to
more than 100,000K. These stars have few absorption lines, generally
due to helium. These stars burn out in a few million years.
stars have temperatures between about 10,000 and 20,000K. Rigel, in
Orion, and Spica, in Virgo, are B stars. They are noticeably blue.
stars have strong absorption lines of Hydrogen.
Examples are Vega, Altair, and Sirius. Temperatures are about 8000-10,000K.
They appear white.
stars are slightly hotter than the Sun. Absorption lines of metals
appear (to an astronomer, anything that is neither Hydrogen nor Helium is a
metal). Procyon is an F star.
stars have temperatures between 5000 and 6000K. They appear yellow.
Our Sun is a G star, as is Alpha Centauri and Capella.
stars appear orange. Temperatures are 3000-5000K. Arcturus is a
stars are the coolest stars. They are so cool (2000-3000K) that
molecules, including water, carbon monoxide, Vanadium Oxide (used in
sunblock) and Titanium oxide (the base of white paint) are visible.
These stars are noticeably red: examples are Betelgeuse and Antares.
Among the mnemonics used to memorize the spectral sequence are:
Feel free to make up your own (check out
the Great Mnemonic Contest).
- O Be A Fine Girl/Guy, Kiss Me.
- Old Bald And Fat Generals Keep Mistresses.
- O Boy, An F Grade Kills Me.
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
Return to top