- Subdwarf star
A subdwarf star, sometimes denoted by "sd", is
luminosity classVI under the Yerkes spectral classification system. They are defined as stars with luminosity1.5 to 2 magnitudes lower than that of main-sequence stars of the same spectral type. On an Hertzsprung-Russell diagramsubdwarfs appear to lie below the main sequence.
The term "subdwarf" was coined by
Gerard Peter Kuiperin 1939, to refer to a series of stars with anomalous spectra that were previously labeled as "intermediate white dwarfs." [ Ken Croswell, " The Alchemy of the Heavens", (New York: Oxford UP, 1995), 87.]
main sequencestars, cool subdwarfs (of spectral types G to M) produce their energy from hydrogenfusion. The explanation of their underluminosity lies in their low metallicity: these stars are unenriched in elements heavier than helium. The lower metallicity decreases the opacity of their outer layers and decreases the radiation pressure, resulting in a smaller, hotter star for a given mass. [ James Kaler, "Stars and their Spectra", (Cambridge: Cambridge UP, 1989), 122.] This lower opacity also allows them to emit a higher percentage of ultravioletlight for the same spectral typerelative to a Population Istar, a feature known as the ultraviolet excess. [Ibid., 87-92.] Usually members of the Milky Way's halo, they frequently have high space velocities relative to the Sun.
Hot subdwarfs, of spectral types B and O, also termed "extreme
horizontal branchstars" are an entirely different class of objects to cool subdwarfs. These stars represent a late stage in the evolution of some stars, caused when a red giantstar loses its outer hydrogenlayers before the core begins to fuse helium. The reasons why this premature mass loss occurs are unclear, but the interaction of stars in a binary starsystem is thought to be one of the main mechanisms. Single subdwarfs may be the result of a merger of two white dwarfstars. Subdwarf B stars, being more luminous than white dwarfs, are a significant component in the hot star population of old stellar systems, such as globular clusters and elliptical galaxies. [cite journal|url=http://www.ias.ac.in/jaa/junsep2005/index.html|author=Jeffery, C. S.|title=Pulsations in Subdwarf B Stars|journal= Journal of Astrophysics and Astronomy|volume=26|pages=261|year=2005|doi=10.1007/BF02702334]
2MASS J05325346+8246465, a possible halo brown dwarfand the first substellar subdwarf. ["The First Substellar Subdwarf? Discovery of a Metal-Poor L Dwarf with Halo Kinematics", Adam J. Burgasser, et al. 2003 [http://research.amnh.org/~adam/papers/apj592_1186.pdf] ]
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