Spectral Classification from a Theoretical Point of View

Abstract We study the conditions under which a two-dimensional spectral classification according to Teff and ne is possible and when 3- or It-dimensional spectral classification is necessary because the classification criteria are unavoidably influenced by the metal and helium abundances. For late type stars, where molecular absorption becomes important for the continuum a seven-or more- dimensional classification may be necessary. At this point spectral classification may not be useful any more. We find generally that a derivation of the gravitational acceleration from the spectroscopically determined electron density is only possible if the helium abundance is known.