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Revision as of 19:23, 26 September 2015 editJorisvS (talk | contribs)Autopatrolled, Extended confirmed users, Pending changes reviewers46,766 editsm clean up, replaced: Subdwarf B star → B-type subdwarf using AWB← Previous edit Revision as of 17:14, 13 January 2016 edit undoLithopsian (talk | contribs)Autopatrolled, Extended confirmed users, New page reviewers68,717 editsm Use cite journal templateNext edit →
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Usually members of the Milky Way's ], they frequently have high space velocities relative to the ]. Usually members of the Milky Way's ], they frequently have high space velocities relative to the ].


Subclasses of cool subdwarfs are as following:<ref>, Burgasser, Adam J. & Kirkpatrick, J. Davy, 2006.</ref> Subclasses of cool subdwarfs are as following:<ref>{{cite journal|bibcode=2006ApJ...645.1485B}}</ref>
* cool subdwarf: Example: SSSPM J1930-4311 (sdM7) * cool subdwarf: Example: SSSPM J1930-4311 (sdM7)
* extreme subdwarf: Example: ] (esdM7)<ref></ref> * extreme subdwarf: Example: ] (esdM7)<ref>{{cite journal|bibcode=1999A&A...350L..62S}}</ref>


==Hot subdwarfs== ==Hot subdwarfs==
{{Main|B-type subdwarf|O-type subdwarf}} {{Main|B-type subdwarf|O-type subdwarf}}
Hot subdwarfs, of spectral types O and B, also termed "extreme ] stars" 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 ] star loses its outer ] layers before the core begins to fuse ]. The reasons why this premature mass loss occurs are unclear, but the interaction of stars in a ] system is thought to be one of the main mechanisms. Single subdwarfs may be the result of a merger of two ]s or gravitational influence from substellar companions. B-type subdwarfs, being more luminous than white dwarfs, are a significant component in the hot star population of old stellar systems, such as ]s and ].<ref>{{cite journal|url=http://www.ias.ac.in/jaa/junsep2005/index.html|author=Jeffery, C. S.|title=Pulsations in Subdwarf B Stars|journal=]|volume=26|issue=2–3|pages=261|year=2005|doi=10.1007/BF02702334|bibcode=2005JApA...26..261J}}</ref><ref>http://arxiv.org/abs/0908.1025</ref> Hot subdwarfs, of spectral types O and B, also termed "extreme ] stars" 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 ] star loses its outer ] layers before the core begins to fuse ]. The reasons why this premature mass loss occurs are unclear, but the interaction of stars in a ] system is thought to be one of the main mechanisms. Single subdwarfs may be the result of a merger of two ]s or gravitational influence from substellar companions. B-type subdwarfs, being more luminous than white dwarfs, are a significant component in the hot star population of old stellar systems, such as ]s and ].<ref>{{cite journal|bibcode=2005JApA...26..261J}}</ref><ref>{{cite journal|arxiv=0908.1025|bibcode=2009ApJ...702L..96G}}</ref>


==Notable subdwarfs== ==Notable subdwarfs==
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*] *]
*] *]
*], a possible ] ] and the first substellar subdwarf.<ref>''The First Substellar Subdwarf? Discovery of a Metal-Poor L Dwarf with Halo Kinematics'', ], et al. 2003 </ref> *], a possible ] ] and the first substellar subdwarf.<ref>{{cite journal|doi=10.1086/375813}}</ref>
*] *]


Revision as of 17:14, 13 January 2016

Hertzsprung–Russell diagram Spectral type O B A F G K M L T Brown dwarfs White dwarfs Red dwarfs Subdwarfs Main sequence
("dwarfs") Subgiants Giants Red giants Blue giants Bright giants Supergiants Red supergiant Hypergiants absolute
magni-
tude
(MV)

A subdwarf, sometimes denoted by "sd", is a star with 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. On an Hertzsprung–Russell diagram subdwarfs appear to lie below the main sequence.

The term "subdwarf" was coined by Gerard Kuiper in 1939, to refer to a series of stars with anomalous spectra that were previously labeled as "intermediate white dwarfs".

Cool subdwarfs

Like ordinary main-sequence stars, cool subdwarfs (of spectral types G to M) produce their energy from hydrogen fusion. 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. This lower opacity also allows them to emit a higher percentage of ultraviolet light for the same spectral type relative to a Population I star, a feature known as the ultraviolet excess. Usually members of the Milky Way's halo, they frequently have high space velocities relative to the Sun.

Subclasses of cool subdwarfs are as following:

  • cool subdwarf: Example: SSSPM J1930-4311 (sdM7)
  • extreme subdwarf: Example: APMPM J0559-2903 (esdM7)

Hot subdwarfs

Main articles: B-type subdwarf and O-type subdwarf

Hot subdwarfs, of spectral types O and B, also termed "extreme horizontal-branch stars" 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 giant star loses its outer hydrogen layers 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 star system is thought to be one of the main mechanisms. Single subdwarfs may be the result of a merger of two white dwarfs or gravitational influence from substellar companions. B-type subdwarfs, 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.

Notable subdwarfs

References

  1. Ken Croswell, The Alchemy of the Heavens, (New York: Oxford UP, 1995), p. 87.
  2. James Kaler, Stars and their Spectra, (Cambridge: Cambridge UP, 1989), p. 122.
  3. Ken Croswell, The Alchemy of the Heavens, (New York: Oxford UP, 1995), pp. 87–92.
  4. . Bibcode:2006ApJ...645.1485B. {{cite journal}}: Cite journal requires |journal= (help); Missing or empty |title= (help)
  5. . Bibcode:1999A&A...350L..62S. {{cite journal}}: Cite journal requires |journal= (help); Missing or empty |title= (help)
  6. . Bibcode:2005JApA...26..261J. {{cite journal}}: Cite journal requires |journal= (help); Missing or empty |title= (help)
  7. . arXiv:0908.1025. Bibcode:2009ApJ...702L..96G. {{cite journal}}: Cite journal requires |journal= (help); Missing or empty |title= (help)
  8. . doi:10.1086/375813. {{cite journal}}: Cite journal requires |journal= (help); Missing or empty |title= (help)
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