A system of classifying stellar spectra developed at Yerkes Observatory by W. W. Morgan, Philip Childs Keenan (1908–2000), and Edith Marie Kellman (1911–2007), and published in 1943; also known as the MK (or MKK) classification or the Yerkes system. The Morgan–Keenan system retained the sequence of stellar spectral types O, B, A, F, G, K, M introduced in the Harvard classification, but with a more precise observational definition of each type. To these spectral types were added a range of luminosity classes which indicate whether the star is a supergiant, giant, dwarf, or some intermediate class. Unusual stars which do not fit this system, such as the carbon stars (types R and N in the Harvard system), the S stars, white dwarfs (see D star), and Wolf–Rayet stars, have their own individual classification schemes. Recently the scheme has been extended at the cool end with the introduction of types L and T for brown dwarfs. See also Spectral Classification.
Subjects: Astronomy and Astrophysics.