A system of classifying stars according to the characteristics of their spectra, introduced at Harvard College Observatory in 1890 by E. C. Pickering, which culminated in the *Henry Draper Catalogue. At first, stars were ordered according to the strength of their hydrogen absorption lines (the Balmer series), from A (strongest) to P. Eventually several letters were merged or dropped, and the remaining spectral types were rearranged into the sequence O, B, A, F, G, K, M, in order of decreasing surface temperature. Early in the 20th century, spectral types R and N were applied to carbon-rich versions of types G, K, and M (now known as carbon stars), and type S (M stars with heavy-metal lines) was subsequently added (see s star). The Harvard system has since been superseded by the Morgan–Keenan classification system. See also spectral classification.