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]

[edit] Notable subdwarfs

• Kapteyn's star
• Groombridge 1830
• 2MASS J05325346+8246465, a possible halo brown dwarf and the first substellar subdwarf. ^[4]

[edit] References

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.
4. ^ The First Substellar Subdwarf? Discovery of a Metal-Poor L Dwarf with Halo Kinematics, Adam J. Burgasser, et. al. 2003 [1]