The Morgan-Keenan classification is an extension of the Harvard classification of stellar spectra (see spectral type) to include luminosity classes and a more precise observational description of each type, based on temperature, luminosity, and chemical abundance. It was introduced by the American astronomers William Morgan, Philip Keenan (1908–2000), and Edith Kellman (1911–2007) in 1943 and is also known as the MKK system.