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subdwarf(sub-dworf ) A star that is smaller and about 1.5–2 magnitudes fainter than normal dwarf (main-sequence) stars of the same spectral type. The subdwarfs are mainly old (halo population II) objects that lie just beneath the main sequence on the Hertzsprung–Russell diagram. Their anomalous position is due to their low metal content, which affects their apparent temperature and hence their spectral type. They are usually denoted by the prefix sd , as in sdK4, or given the luminosity class VI.
a dwarf star in the halo of the Galaxy. By comparison with stars of the same mass in the main sequence of the Hertzsprung-Russell diagram, subdwarfs have a higher surface temperature and a lower metal content in their atmospheres. Subdwarfs do not obey the mass-luminosity law that holds for most stars.
The use of the term “subdwarf” is due to the circumstance that for a long time the stars of this type were believed to form in the Hertzsprung-Russell diagram a separate sequence parallel to the main sequence and lying 1.5–2 stellar magnitudes below it. In actuality, subdwarfs are for the most part main sequence stars. The displacement of subdwarfs below the main sequence was a result of a failure to correctly allow for the blanketing effect in subdwarf spectra. Subdwarfs are believed to be old stars.