 Spectral classification

Spectral classification groups stars by the patterns of absorption lines, emission lines and molecular bands in their spectra. The Harvard spectral classification orders the familiar O, B, A, F, G, K and M types mainly by effective temperature. Changes in ionization and excitation make a given element's lines strongest over only part of that sequence; weak hydrogen lines do not directly imply little hydrogen.

Surface gravity also affects line shapes and ratios, supplying a separate luminosity class. Together the temperature type and luminosity class help distinguish a cool red giant from a cool main-sequence star. Composition, stellar activity, rotation and unresolved companions complicate the inference, so classification is an observational guide to a physical model rather than a complete measurement of the stellar structure.