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A star's luminosity class (or Morgan-Keenan luminosity class or MK luminosity class or MK class) is a classification (star class) of a star's luminosity, the electromagnetic radiation (EMR) it emits. It is basically a number, cited as a Roman Numeral, with V representing the luminosity of a main sequence star with the star's spectral type, smaller numbers indicating brighter than that, and larger numbers indicating dimmer. This class is associated with the size of the star because given its spectral type, the amount of EMR it emits depends upon its size. Classes:
A star's spectral type is often cited with its luminosity class as a suffix. 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:
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.