luminosity class

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 type 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 - subdwarf (or sd).

Additional classification:

• 0 (zero) (or Ia⁺ or IA-0) - extreme luminous supergiant, aka, hypergiant.
• Ia - luminous supergiant.
• Ib - less luminous supergiant.
• Iab - intermediate between Ia and Ib.
• D - white dwarf (or VII).

The system is termed the Morgan-Keenan classification system.

An analogous classification for galaxies is the van den Bergh galaxy classification.