A subdwarf, sometimes denoted by `sd`, is a star with luminosity class VI under the Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 magnitudes lower than that of main-sequence stars of the same spectral type. On an Hertzsprung–Russell diagram subdwarfs appear to lie below the main sequence. The term `......
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http://en.wikipedia.org/wiki/Subdwarf
A star that is smaller and less luminous by one to two magnitudes (2.5–6 times fainter) than a normal dwarf (main sequence) star of the same spectral type. Subdwarfs are mainly old, galactic halo population II objects of types F, G, and K. They are denoted by the prefix 'sd', as in sdK7, and a...
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http://www.daviddarling.info/encyclopedia/S/subdwarf.html
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