Glossary term: Luminosity Class

Description: The intrinsic brightness of a star depends on its temperature and physical size. Hotter stars are brighter, as are larger stars. Spectral type categorizes stars based on the appearance of their spectral lines. This roughly follows changes in temperature. The luminosity class categorizes stars based on how bright they are compared to other stars of their spectral type. Roman numerals are used to denote luminosity class: Stars on the main sequence burning hydrogen are classified as dwarfs (V). As an example, the class of the Sun is G2 V. Above these lie (in order of increasing size and brightness) subgiants (IV), giants (III), bright giants (II), and supergiants (Ib for less luminous and Ia for the most luminous). All of these are types of evolved stars that have finished burning hydrogen in their core. Metal poor subdwarfs are hydrogen burning stars with a low metal content that lie below the main sequence. Finally white dwarfs (stellar remnants) lie further still below the main sequence.

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