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. In astronomy, stellar classification is a classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequently refined in terms of other characteristics. ... In astronomy, stellar classification is a classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequently refined in terms of other characteristics. ... The Pleiades star cluster A star is a massive body of plasma in outer space that is currently producing or has produced energy through nuclear fusion. ... Luminosity has different meanings in several different fields of science. ... In astronomy, absolute magnitude is the apparent magnitude, m, an object would have if it were at a standard luminosity distance away from us. ... In astronomy, stellar classification is a classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequenly refined in terms of other characteristics. ... In astronomy, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium. ... The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a Colour-Magnitude (CM) diagram) shows the relationship between absolute magnitude, luminosity, classification, and surface temperature of stars. ... Hertzsprung-Russell diagram The main sequence of the Hertzsprung-Russell diagram is the curve where the majority of stars are located in this diagram. ... Stars can be grouped into two general types called Population I and Population II. The criteria for classification include space velocity, location in the galaxy, age, chemical composition, and differences in distribution on the Hertzsprung-Russell diagram. ...

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) Gerard Peter Kuiper, born Gerrit Pieter Kuiper (December 7, 1905 – December 23, 1973) was a Dutch-American astronomer. ... A white dwarf is an astronomical object which is produced when a low to medium mass star dies. ... Halo around the sun at the South Pole (NOAA) Halos are optical phenomena that appear near or around the Sun or Moon, and sometimes near other strong light sources such as street lights. ... The Sun is the star at the center of Earths solar system. ... Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than soft X-rays. ... In astronomy, stellar classification is a classification of stars based initially on photospheric temperature and its associated spectral characteristics, and subsequenly refined in terms of other characteristics. ... A substance or object that is opaque is neither transparent nor translucent. ... Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. ...

A notable subdwarf is Kapteyn's star, and another is Groombridge 1830. Kapteyns Star (also known as GJ 191, HD 33793 or CD -45 1841) is a class M0 subdwarf discovered by Jacobus Kapteyn in 1897. ... Groombridge 1830 is a star. ...

Sources

1. Ken Croswell, The Alchemy of the Heavens, (New York: Oxford UP, 1995), 87. Ken Croswell is an astronomer and author living in Berkeley, California. ... The Alchemy of the Heavens is a book by Ken Croswell. ...

2. Ibid., 87-92.

3. James Kaler, Stars and their Spectra, (Cambridge: Cambridge UP, 1989), 122.