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(sub-dworf ) A star that is smaller and about 1.5–2 magnitudes fainter than normal dwarf (main-sequence) stars of the same spectral type. The subdwarfs are mainly old (halo population II) objects that lie just beneath the main sequence on the Hertzsprung–Russell diagram. Their anomalous position is due to their low metal content, which affects their apparent temperature and hence their spectral type. They are usually denoted by the prefix sd , as in sdK4, or given the luminosity class VI.



a dwarf star in the halo of the Galaxy. By comparison with stars of the same mass in the main sequence of the Hertzsprung-Russell diagram, subdwarfs have a higher surface temperature and a lower metal content in their atmospheres. Subdwarfs do not obey the mass-luminosity law that holds for most stars.

The use of the term “subdwarf” is due to the circumstance that for a long time the stars of this type were believed to form in the Hertzsprung-Russell diagram a separate sequence parallel to the main sequence and lying 1.5–2 stellar magnitudes below it. In actuality, subdwarfs are for the most part main sequence stars. The displacement of subdwarfs below the main sequence was a result of a failure to correctly allow for the blanketing effect in subdwarf spectra. Subdwarfs are believed to be old stars.

References in periodicals archive ?
The primary goal of the EXOTIME project is to use the timing method to detect exoplanets around evolved pulsating stars such as subdwarf B (sdB) stars and white dwarfs.
Hot subdwarf stars and related objects; proceedings.
Topics examined include the H1 distribution of the Milky Way; progenitors of core-collapse supernovae; gravitational waves from merging compact binaries; physical properties and environments of nearby galaxies; hot subdwarf stars; high-contrast observations in optical and infrared astronomy; magnetic reconnection in astrophysical and laboratory plasmas; magnetic fields of nondegenerate stars; star-formation histories, abundances, and kinematics of dwarf galaxies in the local group; complex organic interstellar molecules; the chemical composition of the sun; teraelectronvolt astronomy; and the study of gamma-ray bursts in the era of the Swift satellite.
Ultracool subdwarfs were first recognized as a unique class of stars in 2003, and are distinguished by their low temperatures ("ultracool") and low concentrations of elements other than hydrogen and helium ("subdwarf").
Hot subdwarf stars represent several stages in the very late evolution of low-mass stars.
There are some key differences between the sun and V391 Pegasi, which is now a B-type subdwarf.
Napiwotzki (eds), Proceedings of Hot Subdwarf Stars and Related Objects, ASP Conference Proceedings, Vol.
The stellar classes discussed in detail at the conference include R Coronae Borealis stars, extreme helium stars, Wolf-Rayet central stars of planetary nebulae, white dwarfs, and helium-rich subdwarf O and B stars.
Hot Subdwarfs in Bamberg, in Hot Subdwarf Stars and Related Objects, eds.
Prof Warner attended the conference "Hot Subdwarf Stars and related Objects" in Bamberg (D) in July 2007 and gave the conference summary.
Only a few dozen ultracool subdwarfs, which are up to 10,000 times fainter than the Sun, have been identified.