One of several classes to which a star can be assigned on the basis of certain luminosity indicators in its spectrum. The classes range from I for supergiants to V for main-sequence stars (also known as dwarfs).

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.

distinctions among stars of the same spectral class. (See Spectral types or classes.)

an indication of the type of star --whether it is main sequence , a giant or a dwarf

Stars of the same spectral class may vary widely in luminosity. This difference is indicated by a roman letter after the spectral class; Ia and Ib- bright supergiant and supergiant, II-bright giant, III-giant, IV-subgiant, V-main sequence (dwarfs), VI-subdwarf and VII or wd-white dwarf. The Sun is thus a G2 V star.

A classification of a star according to its luminosity within a given spectral class. Our Sun, a G2V star, has luminosity class V

A category of stars of similar luminosity; determined by the widths of lines in their spectra.