A star's luminosity class (or Morgan-Keenan luminosity class or MK luminosity class or MK class) is a classification (star class) of the overall size of a star (as are terms like giant star, supergiant, etc.) which is a major contributor to its luminosity. A description of a star might include its spectral class and luminosity class. The general classifications are indicated by Roman numerals:
The system is termed the Morgan-Keenan classification system.