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:

• I - supergiant.
• II - Bright Giant.
• III - normal giant star.
• IV - Subgiant.
• V - Main Sequence Star (Dwarf Star).
• VI - subdwarfs.

Additional classification:

• Ia-O - Extreme Luminous Supergiants.
• Ia - Luminous Supergiants.
• Ib - less luminous supergiants.
• D - White Dwarves.