Subdwarf

A subdwarf star, sometimes denoted by "sd", is 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; this is due to subdwarfs having lower metallicity than other main sequence stars. On an Hertzsprung-Russell diagram subdwarfs appear to lie below the main sequence. Subdwarfs are mostly Population II stars.

A notable subdwarf is Kapteyn's star.