An Etymological Dictionary of Astronomy and Astrophysics

A pioneering work in → spectral classification conducted in the 1860s. Secchi divided stars into four main groups based on the visual observation of spectra.

Class I: The white and bluish stars with a continuous spectrum crossed by hydrogen bands, the metallic bands being absent or weak. Examples, → Sirius, → Vega.

Class II: Yellow stars, with spectra in which the hydrogen bands were less prominent and the metallic lines more strong. Examples, Sun, → Capella.

Class III: Red or orange stars, showing bands or flutings. Examples, → Antares, → Betelgeuse.

Class IV: Red stars, showing bands similar to Class III, but with the sharp edge of the flutings toward the other end of the spectrum. Secchi’s scheme was superseded by the photographic → Harvard classification system.

See also: Pietro Angelo Secchi (1818-1878), Italian astronomer and Jesuit priest; → classification.