The Spectral Types of Stars
The modern spectral classification system is so successful that it has hardly been changed since 1943. It is based on just two physical properties that imprint themselves on the spectrum of a star's light: the star's surface temperature and atmospheric pressure. These reveal an abundance of information that paints the star's portrait and tells its life story.
The temperature sets the star's color and determines its surface brightness: how much light comes from each square meter of its surface. The atmospheric pressure depends on the star's surface gravity and therefore, roughly, on its size — telling whether it is a giant, dwarf, or something in between. The size and surface brightness in turn yield the star's luminosity (its total light output, or absolute magnitude) and often its evolutionary status (young, middle-aged, or nearing death). The luminosity (when compared to the star's apparent brightness in our sky) also gives a good idea of the star's distance. Appended to the basic spectral type may be letters for chemical peculiarities, an extended atmosphere, unusual surface activity, fast rotation, or other special characteristics.
Every starwatcher needs to have a feel for spectral types. Here are the most important things to know.
