A type of spectral classification system. The Yerkes Spectral Classification system is a more precise systom than the Harvard Spectral Classification as the Yerkes method takes luminosity into account. It is also known as the MKK system after it's developers: William Morgan, Philip Keenan and Edith Kellerman.

Within the system six luminosity classes are defined on the basis of standard stars and luminosity criteria.

The Six classes are:
Ia: most luminous supergiantsb
Ib: less luminous supergiants
II: luminous giants
III: normal giants
IV: subgiants
V: main sequence stars