A method of classifying stars which is based upon the appearance of the absorption lines in their spectra. Various classification schemes have been proposed, but the one most generally accepted evolved from work done at the Harvard Observatory. It is based on the appearance of the photographic region of a star's spectrum, from green to ultra-violet. Each class is denoted by a capital letter (O, B, A, F, G, K, and M), and are intended to represent stages of stellar evolution. Each class is subdivided by placing a numeral after the capital leter (e.g., B8 and K2). Over 90% of all stars fall into four classes: A, F, G, and K.