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 factor in its luminosity. A star's spectral type is often cited with its luminosity class as a suffix. A general class is indicated by a Roman numeral:

• I - supergiant.
• II - bright giant.
• III - normal giant star.
• IV - subgiant.
• V - main sequence star (dwarf star).
• VI - subdwarf (or sd).

As an example, the Sun's spectral type (G2) is commonly given with a "V" suffix indicating it is in the main sequence, as "G2 V" or "G2V" . Additional more-specific classes:

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

Sometimes a, b, or ab are used for other class numbers besides I.

An analogous classification (luminosity class) for galaxies is the van den Bergh galaxy classification.