Classification of stellar spectra by features controlled mainly by temperature and also by surface gravity and composition. The O-B-A-F-G-K-M temperature sequence does not mean a corresponding sequence in bulk hydrogen abundance.
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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