A classification scheme which groups stars according to the width of their spectral lines. For a group of stars with the same temperature, luminosity class differentiates between supergiants, giants, main-sequence stars, and subdwarfs.
A shorthand description of a star based on spectral line widths denoted by the Roman numerals I toV. For a given spectral type, the luminosity of the star decreases from I through V. Our Sun is a G2 V star. The Roman numeral V denotes the luminosity class, which is the main sequence, while G2 refers to a spectral class. Thus, the luminosity of a B8 I supergiant star is far greater than that of a B8 V main sequence star, yet the surface temperatures are equal. The prominent hydrogen absorption-line patterns are the same for both stars, but the width of the absorption lines differ. The B8 V star lines are wider than the B8 I lines, indicating a vast difference in luminosity.