Spectral Classification of Stars - Background, Description Of The Spectral Classes

Although the composition of most stars is very similar, there are systematic variations in stellar spectra based on their temperatures. A typical star has a spectrum consisting of a continuous range of colors overlaid with dark lines. The positions, strengths, and shapes of these lines are determined by the temperature, density, gravitational fields, velocity, and other properties of the star. In order to be able to study stars systematically, it is useful to classify stars with others that have similar properties. This is the basis for the classification scheme used by astronomers. Stars are classified according to the patterns and relative strengths of their dark spectral lines, which are indicators of both their temperature and their intrinsic luminosity, or brightness. Although roughly 10% of stars do not fit into the classification scheme, it provides a convenient way to understand the systematics of stellar formation and evolution.