An Etymological Dictionary of Astronomy and Astrophysics

A system that assigns a → spectral type to a star according to characteristics of its spectrum. The earliest attempt to divide stars on the basis of their spectra was the → Secchi classification in the 1860s. This scheme paved the way for the → Harvard classification that led to the current → Morgan-Keenan classification of spectral types. In the Harvard system stars were originally thought to follow an evolutionary sequence from the “early” O and B types to the “late” K and M types. Although this is now known to be wrong, the terms
→ early-type star and → late-type star are still in use. In the Morgan-Keenan system stars are classified as type O, B, A, F, G, K, or M in order of decreasing → effective temperature, and each type further subdivided into subclasses from 0 (hottest, except for → O-type stars) to 9 (coolest).
They are also accompanied by a → luminosity class.
In the late 1990s, spectral types L and T were added to the sequence to accommodate the coolest stars and → brown dwarfs (with class Y reserved for the coolest brown dwarfs of all, as yet unobserved).

See also: → spectral; → classification.