A large celestial body, usually wholly gaseous, massive enough to initiate (or to have once initiated) nuclear reactions in its central region.
Stars are classified in various ways, with these basic classifications going all the way back to late Industrial and Atomic Age Old Earth. The Harvard system uses a temperature sequence based on spectral characteristics. The sequence is O B A, F G K M, with decimal subdivisions, so that G5 is half way between G0 and K0. The Morgan, Keenan, and Kellman system classifies stars by luminosity as well as temperature. They also gave numerical definitions of the Harvard spectral types and designated bright stars to be the standards. Another system assigned stars to two broad categories depending on composition, speed and location, as well as origin: Population I and Population II, with a rare Population III added. More advanced Interplanetary Age astronomy techniques and AI interest added further stellar taxonomies, many of which were inscrutable or seemingly irrelevant to baseline humans. Even today many of the old classifications are used by SI:<1 sophonts.