spectral class

spectral class in astronomy, a classification of the stars by their spectrum and luminosity . In 1885, E. C. Pickering began the first extensive attempt to classify the stars spectroscopically. This work culminated in the publication of the Henry Draper Catalogue (1924), which lists the spectral classes of 255,000 stars. The stars are divided into 7 classes designated by the letters O, B, A, F, G, K, and M; the hottest stars (O and B) are blue-white in color, while the coolest (M) are red. Each of the letter classes has subdivisions indicated by numerals 0 through 9. Thus, a B0 is the hottest B-type star, B5 is halfway between types B and A, and B9 is only slightly hotter than type A. The table entitled Spectral Classes for Main Sequence Stars gives the characteristics of the seven principal types. To the seven main groups, four more groups have since been added. R, N, and S are classes similar to the K and M types but denote somewhat different spectral characteristics; W indicates a Wolf-Rayet star, the hottest type of star that shines with a steady light. According to a system introduced by W. W. Morgan and others, a Roman numeral is added to the spectral class to specify the luminosity, or intrinsic intensity, of a star. A bright supergiant is Ia, a faint supergiant is Ib, a bright giant is II, a normal giant is III, a subgiant is IV, and a normal dwarf or main-sequence star is V. For example, Sirius is classed as A1 V, a main-sequence white star. Betelgeuse, M2 Ia, is a bright red supergiant. See also Hertzsprung-Russell diagram .

luminosity class A classification of stars according to their luminosity, which can vary widely for a given spectral type. Luminosity class indicates, for example, whether a star is a supergiant, a giant, or a dwarf. The luminosity class is assigned by examination of the star's spectrum, looking at luminosity-sensitive lines or line ratios. The luminosity classes generally used are: Ia–0 Extreme supergiants (sometimes called hypergiants)

Ia  Luminous supergiants

Iab  Normal supergiants

Ib  Underluminous supergiants

II   Bright giants

III    Giants

IV    Subgiants

V   Dwarfs (main-sequence stars)

The accompanying diagram shows the luminosity classes on a Hertzsprung–Russell diagram. See also spectral classification.