Subdwarf star
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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.
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."(1) Often being members of the Milky Way's halo, they frequently have high space velocities relative to the Sun. They also emit a higher percentage of ultraviolet light for the same spectral type relative to a Population I star; this ultraviolet excess is a result of their low metallicity, which allows more of their ultraviolet light to escape.(2) Thus, the relatively low opacity of their outer layers lowers the radiation pressure, resulting in a smaller, hotter star for a given mass.(3)
A notable subdwarf is Kapteyn's star, and another is Groombridge 1830.
1. Ken Croswell, The Alchemy of the Heavens, (New York: Oxford UP, 1995), 87.
2. Ibid., 87-92.
3. James Kaler, Stars and their Spectra, (Cambridge: Cambridge UP, 1989), 122.