Luminosity Classes

The gas pressure in the photospheric layer of a star can also affect the way absorption features appear. If the pressure is low (as would be the case in the atmosphere of a supergiant star due to the large size giving it a low surface gravity), the atoms absorb at the specific wavelengths set by their internal atomic energy levels. But if the pressure is higher (as in a giant star), the higher pressure means that the individual atoms are packed more closely together. The electronic properties of atoms will affect each neighbor's internal energy levels and the atoms will be able to absorb at slightly smaller and slightly greater wavelengths—the absorption features will appear broader. In a small star (like the Sun), the gas pressure must be higher to balance against gravity. The atoms are even more closely packed together, neighboring atom's electronic properties are even more affected, and the absorption features will appear still wider.

A second classification of stellar spectra therefore may be done on the basis of the appearance of the absorption lines, with the narrowest line spectra designated I (subdivided into Ia and Ib) through types II, III, and IV, to V, the broadest line spectra. For stars of a given temperature, narrow lines correlate with low pressure atmospheres, large stellar radii, and hence a high luminosity: Type Ia, therefore, are the brightest supergiant stars, and type Ib are the fainter supergiants. Type III stars are still large stars of intermediate brightness, and are termed giants. Type V stars are like the Sun, dwarf or Main Sequence stars, which are fainter than the larger stars. Because this line‐width classification correlates with luminosity, the designation Ia to V is termed a luminosity class.

The full spectral classification of a star therefore includes both the spectral type (OBAFGKM) as well as its luminosity class (I‐V); for example, we would classify the Sun as a G2 V star.