Secchi classification

…his survey of stellar spectra, Secchi concluded that stars could be arranged in four classes according to the type of spectra they display. These divisions were later expanded into the Harvard classification system, which is based on a simple temperature sequence. Secchi proved that prominences seen during a solar eclipse…

Angelo Secchi distinguished four main spectral types of stars. At the Harvard College Observatory in the 1880s, during the compilation of the Henry Draper Catalogue of stars, more types were distinguished and were designated by letter in alphabetic sequence according to the strength of their hydrogen spectral lines. Most of…