A subdwarf star, sometimes denoted by "sd",'s luminosity class VI under the Yerkes spectral classification system. They're 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 "subdwarf" was coined by Gerard Peter Kuiper in 1939, to refer to a series of stars with anomalous spectra that were previously labeled as "intermediate white dwarfs.… (More on Subdwarf)