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.
A stellar spectral category based on gravity-sensitive line widths and ratios, distinguishing supergiants, giants and dwarfs at comparable spectral temperature. Examples are I for supergiants, III for giants and V for main-sequence dwarfs. It is inferred spectroscopically and is not a direct substitute for measuring absolute luminosity in every individual star.
The stellar temperature sequence O, B, A, F, G, K, M, with numerical subclasses approximately from hotter 0 to cooler 9 within each letter class. Spectral features follow temperature-dependent excitation, ionization and molecule survival, not a corresponding change in the bulk hydrogen abundance. A separate luminosity class describes gravity-sensitive spectral differences.
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