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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 a 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".[1]

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.[2] 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.[3] Usually members of the Milky Way halo, they frequently have high space velocities relative to the Sun.[4] Cool subdwarfs of spectral type L and T exist, for example ULAS J131610.28+075553.0 with a spectral type of sdT6.5.[4]

Subclasses of cool subdwarfs are as following:[5][6]

cool subdwarf: Examples: Kapteyn's Star (sdM1), SSSPM J1930-4311 (sdM7)
extreme subdwarf: Example: APMPM J0559-2903 (esdM7)[7]
ultrasubdwarf: Example: LSPM J0822+1700 (usdM7.5)[6]

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 object 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.[8][9]
Examples of subdwarfs

Kapteyn's Star
Groombridge 1830
Mu Cassiopeiae
2MASS J05325346+8246465, a possible halo brown dwarf and the first substellar subdwarf.[10]
SSSPM J1549-3544

References

Ken Croswell, The Alchemy of the Heavens, (New York: Oxford UP, 1995), p. 87.
James Kaler, Stars and their Spectra, (Cambridge: Cambridge UP, 1989), p. 122.
Ken Croswell, The Alchemy of the Heavens, (New York: Oxford UP, 1995), pp. 87–92.
Burningham, Ben; Smith, L.; Cardoso, C. V.; Lucas, P. W.; Burgasser, A. J.; Jones, H. R. A.; Smart, R. L. (May 2014). "The discovery of a T6.5 subdwarf". MNRAS. 440 (1): 359–364.arXiv:1401.5982. Bibcode:2014MNRAS.440..359B. doi:10.1093/mnras/stu184. ISSN 0035-8711.
Burgasser, Adam J.; Kirkpatrick, J. Davy (2006). "Discovery of the Coolest Extreme Subdwarf". The Astrophysical Journal. 645 (2): 1485–1497.arXiv:astro-ph/0603382. Bibcode:2006ApJ...645.1485B. doi:10.1086/504375. S2CID 10911965.
Lépine, Sébastien; Rich, R. Michael; Shara, Michael M. (November 2007). "Revised Metallicity Classes for Low-Mass Stars: Dwarfs (dM), Subdwarfs (sdM), Extreme Subdwarfs (esdM), and Ultrasubdwarfs (usdM)". Astrophysical Journal. 669 (2): 1235–1247.arXiv:0707.2993. Bibcode:2007ApJ...669.1235L. doi:10.1086/521614. ISSN 0004-637X.
Schweitzer, A.; Scholz, R.-D.; Stauffer, J.; Irwin, M.; McCaughrean, M. J. (1999). "APMPM J0559-2903: The coolest extreme subdwarf known". Astronomy and Astrophysics. 350: L62. Bibcode:1999A&A...350L..62S.
Jeffery, C. Simon (2005). "Pulsations in Subdwarf B Stars". Journal of Astrophysics and Astronomy. 26 (2–3): 261–271. Bibcode:2005JApA...26..261J. doi:10.1007/BF02702334. S2CID 13814916.
Geier, S.; Edelmann, H.; Heber, U.; Morales-Rueda, L. (2009). "Discovery of a Close Substellar Companion to the Hot Subdwarf Star HD 149382—The Decisive Influence of Substellar Objects on Late Stellar Evolution". The Astrophysical Journal Letters. 702 (1): L96–L99.arXiv:0908.1025. Bibcode:2009ApJ...702L..96G. doi:10.1088/0004-637X/702/1/L96. S2CID 119282460.

Burgasser, Adam J.; Kirkpatrick, J. Davy; Burrows, Adam; Liebert, James; Reid, I. Neill; Gizis, John E.; McGovern, Mark R.; Prato, L.; McLean, Ian S. (2003). "The First Substellar Subdwarf? Discovery of a Metal‐poor L Dwarf with Halo Kinematics". The Astrophysical Journal. 592 (2): 1186–1192.arXiv:astro-ph/0304174. Bibcode:2003ApJ...592.1186B. doi:10.1086/375813. S2CID 11895472.

vte

Stars
Formation

Accretion Molecular cloud Bok globule Young stellar object
Protostar Pre-main-sequence Herbig Ae/Be T Tauri FU Orionis Herbig–Haro object Hayashi track Henyey track

Evolution

Main sequence Red-giant branch Horizontal branch
Red clump Asymptotic giant branch
super-AGB Blue loop Protoplanetary nebula Planetary nebula PG1159 Dredge-up OH/IR Instability strip Luminous blue variable Blue straggler Stellar population Supernova Superluminous supernova / Hypernova

Spectral classification

Early Late Main sequence
O B A F G K M Brown dwarf WR OB Subdwarf
O B Subgiant Giant
Blue Red Yellow Bright giant Supergiant
Blue Red Yellow Hypergiant
Yellow Carbon
S CN CH White dwarf Chemically peculiar
Am Ap/Bp HgMn Helium-weak Barium Extreme helium Lambda Boötis Lead Technetium Be
Shell B[e]

Remnants

White dwarf
Helium planet Black dwarf Neutron
Radio-quiet Pulsar
Binary X-ray Magnetar Stellar black hole X-ray binary
Burster

Hypothetical

Blue dwarf Green Black dwarf Exotic
Boson Electroweak Strange Preon Planck Dark Dark-energy Quark Q Black Gravastar Frozen Quasi-star Thorne–Żytkow object Iron Blitzar

Stellar nucleosynthesis

Deuterium burning Lithium burning Proton–proton chain CNO cycle Helium flash Triple-alpha process Alpha process Carbon burning Neon burning Oxygen burning Silicon burning S-process R-process Fusor Nova
Symbiotic Remnant Luminous red nova

Structure

Core Convection zone
Microturbulence Oscillations Radiation zone Atmosphere
Photosphere Starspot Chromosphere Stellar corona Stellar wind
Bubble Bipolar outflow Accretion disk Asteroseismology
Helioseismology Eddington luminosity Kelvin–Helmholtz mechanism

Properties

Designation Dynamics Effective temperature Luminosity Kinematics Magnetic field Absolute magnitude Mass Metallicity Rotation Starlight Variable Photometric system Color index Hertzsprung–Russell diagram Color–color diagram

Star systems

Binary
Contact Common envelope Eclipsing Symbiotic Multiple Cluster
Open Globular Super Planetary system

Earth-centric
observations

Sun
Solar System Sunlight Pole star Circumpolar Constellation Asterism Magnitude
Apparent Extinction Photographic Radial velocity Proper motion Parallax Photometric-standard

Lists

Proper names
Arabic Chinese Extremes Most massive Highest temperature Lowest temperature Largest volume Smallest volume Brightest
Historical Most luminous Nearest
Nearest bright With exoplanets Brown dwarfs White dwarfs Milky Way novae Supernovae
Candidates Remnants Planetary nebulae Timeline of stellar astronomy

Related articles

Substellar object
Brown dwarf Sub-brown dwarf Planet Galactic year Galaxy Guest Gravity Intergalactic Planet-hosting stars Tidal disruption event

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