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spectral types(spectral classes) The different groups into which stars may be classified according to the characteristics of their spectra, principally the absorption lines and bands resulting from the presence of particular atoms and molecules in the star. Spectral lines of the various elements and compounds have widely different strengths in stars of different temperatures: this is the basis of the current classification. In the Secchi classification made in the 1860s by the Italian Angelo Secchi, stars were divided into four groups based on the visual observation of spectra. The subsequent development of photographic spectroscopy allowed a much more precise division. The Harvard classification was introduced by astronomers at Harvard Observatory in the 1890s and used in subsequent volumes of the Henry Draper Catalog. It was developed into its present form in the 1920s by E.C. Pickering, Annie J. Cannon, and others.
The original scheme was based on the strength of the hydrogen Balmer absorption lines in stellar spectra (see hydrogen spectrum), the order being alphabetical from A to P: A stars had the strongest hydrogen lines. Some letters were later dropped and the ordering rearranged to correspond to a sequence of decreasing surface temperature. The majority of stars can be divided into seven spectral types – O, B, A, F, G, K, M – the order often remembered by the mnemonic ‘Oh Be A Fine Girl (or Guy) Kiss Me’. The principal spectral features of these classes are shown on the graph (above) and listed in Table (facing) and also at separate entries: O stars, B stars, etc. Since temperature and color are directly linked the Harvard classification is also a sequence of colors ranging from hot blue O stars to cool red M stars. Additional classes are the carbon stars (C stars) and the S stars. The spectral types are further subdivided into 10 subclasses denoted by digits 0 to 9 placed after the spectral type letter, as with A5 – midway between A0 and F0. These subdivisions are based on a variety of complex empirical criteria, mainly the ratios of certain sets of lines in a spectrum, and may be further subdivided, as with O9.5. Other spectral characteristics, such as the presence of emission lines, are indicated by an additional small letter placed after the spectral type, as with M5e.
It was realized in the 1890s that stars of a particular spectral type could have widely differing luminosities and several luminosity classifications were developed. The currently used MK system was first published in 1941 by W.W. Morgan and P.C. Keenan of Yerkes Observatory: stars of a given spectral type are further classified into one of six luminosity classes, denoted by Roman numerals placed after the spectral type, as with G2 V, and indicating whether the star is a supergiant, giant, subgiant, or main-sequence star; subdwarfs and white dwarfs are also sometimes considered as luminosity classes. All these classes occupy distinct positions on the Hertzsprung–Russell diagram. Stars of a similar temperature but different luminosities must differ in surface area and consequently in surface gravity and atmospheric density. These differences produce spectral effects that are used to differentiate between the luminosity classes. A major effect is the pressure broadening of spectral lines, which increases as the atmospheric density and pressure increase (and radius decreases): spectral lines of a bright supergiant are much narrower than those of a main-sequence star of the same spectral type. The spectra of a set of standard stars are used in assigning spectral type and luminosity class to a star.