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Proxima Centauri, the closest star tothe Sun at 4.2 ly, is a red dwarf

Red dwarfFrom Wikipedia, the free encyclopedia

This article is about the type of star. For the television sitcom, see Red Dwarf.

A red dwarf is a small and relatively cool star on the mainsequence, either late K or M spectral type. Red dwarfs range inmass from a low of 0.075 solar masses (M) to about 0.50 M andhave a surface temperature of less than 4,000 K.

Red dwarfs are by far the most common type of star in the MilkyWay, at least in the neighborhood of the Sun, but because of theirlow luminosity, individual red dwarfs cannot easily be observed.From Earth, not one is visible to the naked eye.[1] ProximaCentauri, the nearest star to the Sun, is a red dwarf (Type M5,apparent magnitude 11.05), as are twenty of the next thirty nearest.According to some estimates, red dwarfs make up three­quarters ofthe stars in the Milky Way.[2]

Stellar models indicate that red dwarfs lessthan 0.35 M are fully convective.[3] Hencethe helium produced by the thermonuclearfusion of hydrogen is constantly remixedthroughout the star, avoiding a buildup at thecore. Red dwarfs therefore develop veryslowly, having a constant luminosity andspectral type for, in theory, some trillions ofyears, until their fuel is depleted. Because ofthe comparatively short age of the universe,no red dwarfs of advanced evolutionarystages exist.

Contents

1 Description and characteristics2 Planets3 Habitability4 Spectral standard stars5 See also6 References7 External links

Description and characteristics

Hertzsprung–Russell diagram

Spectral type

Browndwarfs

White dwarfsRed

dwarfs

Subdwarfs

Main sequence("dwarfs")

SubgiantsGiants

Bright giants

Supergiants

Hypergiants

absolutemagni­tude(MV)

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The predicted main­sequence lifetimeof a red dwarf plotted against its massrelative to the Sun.[7]

Typical characteristics[10]

Stellarclass

Mass(M)

Radius(R)

Luminosity(L)

Teff(K)

M0V 60% 62% 7.2% 3,800M1V 49% 49% 3.5% 3,600M2V 44% 44% 2.3% 3,400M3V 36% 39% 1.5% 3,250M4V 20% 26% 0.55% 3,100M5V 14% 20% 0.22% 2,800M6V 10% 15% 0.09% 2,600M7V 9% 12% 0.05% 2,500M8V 8% 11% 0.03% 2,400M9V 7.5% 8% 0.015% 2,300

Red dwarfs are very­low­mass stars.[4] Consequently they have relatively low temperatures in their coresand energy is generated at a slow rate through nuclear fusion of hydrogen into helium by the proton–proton(PP) chain mechanism. Hence these stars emit little light, sometimes as little as 1∕10,000 that of the Sun. Eventhe largest red dwarfs (for example HD 179930, HIP 12961 and Lacaille 8760) have only about 10% of theSun's luminosity.[5] In general, red dwarfs less than 0.35 M transport energy from the core to the surfaceby convection. Convection occurs because of opacity of the interior, which has a high density compared tothe temperature. As a result, energy transfer by radiation is decreased, and instead convection is the mainform of energy transport to the surface of the star. Above this mass, the red dwarfs will have a regionaround their core where convection does not occur.[6]

Because late­type red dwarfs are fully convective, helium does notaccumulate at the core and, compared to larger stars such as the Sun,they can burn a larger proportion of their hydrogen before leavingthe main sequence. As a result, red dwarfs have estimated lifespansfar longer than the present age of the universe, and stars less than0.8 M have not had time to leave the main sequence. The lower themass of a red dwarf, the longer the lifespan. It is believed that thelifespan of these stars exceeds the expected 10 billion year lifespanof our Sun by the third or fourth power of the ratio of the solar massto their masses; thus a 0.1 M red dwarf may continue burning for

10 trillion years.[4][8] As the proportion of hydrogen in a red dwarf isconsumed, the rate of fusion declines and the core starts to contract.The gravitational energy released by this size reduction is convertedinto heat, which is carried throughout the star by convection.[9]

According tocomputer simulations, the minimum mass a red dwarf musthave in order to become a red giant is 0.25 M; less massiveobjects, as they age, increase their surface temperatures andluminosities becoming blue dwarfs and finally become whitedwarfs.[7]

The less massive the star, the longer this evolutionaryprocess takes; for example, it has been calculated that a0.16 M red dwarf (approximately the mass of the nearbyBarnard's Star) would stay on the main sequence during 2.5trillion years that would be followed by five billion years as ablue dwarf, in which the star would have 1/3 of the Sun'sluminosity (L)[7] and a surface temperature of 6,500‒8,500Kelvin.

The fact that red dwarfs and other low­mass stars still remainon the main sequence when more massive stars have movedoff the main sequence allows the age of star clusters to be estimated by finding the mass at which the stars

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Artist's conception of a red dwarf, the mostcommon type of star in the Sun's stellarneighborhood, and in the universe. Althoughtermed a red dwarf, the surface temperature of thisstar would give it an orange hue when viewed fromclose proximity

turn off the main sequence. This provides a lower, stellar, age limit to the Universe and also allowsformation timescales to be placed upon the structures within the Milky Way, namely the Galactic halo andGalactic disk.

One mystery which has not been solved as of 2009 is the absence of red dwarfs with no metals. (Inastronomy, a metal is any element heavier than hydrogen or helium.) The Big Bang model predicts the firstgeneration of stars should have only hydrogen, helium, and trace amounts of lithium. If such stars includedred dwarfs, they should still be observable today, but none have yet been identified. The preferredexplanation is that without heavy elements only large and not yet observed population III stars can form,and these rapidly burn out, leaving heavy elements which then allow for the formation of red dwarfs.Alternative explanations, such as the idea that zero­metal red dwarfs are dim and could be few in number,are considered much less likely because they seem to conflict with stellar evolution models.

Planets

Many red dwarfs are orbited by extrasolar planets butlarge Jupiter­sized planets are comparatively rare.Doppler surveys around a wide variety of stars indicateabout 1 in 6 stars having twice the mass of the Sun areorbited by one or more Jupiter­sized planets, vs. 1 in 16for Sun­like stars and only 1 in 50 for red dwarfs. Onthe other hand, microlensing surveys indicate that long­period Neptune­mass planets are found around 1 in 3red dwarfs. [11] Observations with HARPS furtherindicate 40% of red dwarfs have a "super­Earth" classplanet orbiting in the habitable zone where liquid watercan exist on the surface of the planet.[12]

At least four and possibly up to six extrasolar planetswere discovered orbiting the red dwarf Gliese 581between 2005–2010. One planet has about the mass ofNeptune, or 16 Earth masses (M⊕). It orbits just 6million kilometers (0.04 AU) from its star, and so isestimated to have a surface temperature of 150 °C,despite the dimness of the star. In 2006, an even smallerextrasolar planet (only 5.5 M⊕) was found orbiting the red dwarf OGLE­2005­BLG­390L; it lies 390million km (2.6 AU) from the star and its surface temperature is −220 °C (56 K).

In 2007, a new, potentially habitable extrasolar planet, Gliese 581 c, was found, orbiting Gliese 581. If theminimum mass estimated by its discoverers (a team led by Stephane Udry), namely 5.36 M⊕, is correct, it isthe smallest extrasolar planet revolving around a main­sequence star discovered to date and since thenGliese 581 d, which is also potentially habitable, was discovered. (There are smaller planets known arounda neutron star, named PSR B1257+12.) The discoverers estimate its radius to be 1.5 times that of Earth(R⊕).

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An artist's impression of a planet with twoexomoons orbiting in the habitable zone ofa red dwarf.

Gliese 581 c and d are within the habitable zone of the host star, and are two of the most likely candidatesfor habitability of any extrasolar planets discovered so far.[13] Gliese 581 g, detected September 2010,[14]has a near­circular orbit in the middle of the star's habitable zone. However, the planet's existence iscontested.[15]

Habitability

Main article: Habitability of red dwarf systems

Planetary habitability of red dwarf systems is subject to somedebate. In spite of their great numbers and long lifespans, thereare several factors which may make life difficult on planetsaround a red dwarf. First, planets in the habitable zone of a reddwarf would be so close to the parent star that they would likelybe tidally locked. This would mean that one side would be inperpetual daylight and the other in eternal night. This couldcreate enormous temperature variations from one side of theplanet to the other. Such conditions would appear to make itdifficult for forms of life similar to those on Earth to evolve.And it appears there is a great problem with the atmosphere ofsuch tidally locked planets: the perpetual night zone would becold enough to freeze the main gases of their atmospheres, leaving the daylight zone nude and dry. On theother hand, recent theories propose that either a thick atmosphere or planetary ocean could potentiallycirculate heat around such a planet. Alternatively, a moon in orbit around a gas giant may be habitable. Itwould circumvent the tidal lock problem by becoming tidally locked to its planet. This way there would bea day/night cycle as the moon orbited its primary, and there would be distribution of heat.

In addition, red dwarfs emit most of their radiation as infrared light, while on Earth plants use energymostly in the visible spectrum. Red dwarfs emit almost no ultraviolet light, which would be a problem,should this kind of light be required for life to exist. Variability in stellar energy output may also havenegative impacts on development of life. Red dwarfs are often covered by starspots, reducing stellar outputby as much as 40% for months at a time. At other times, some red dwarfs, called flare stars, can emitgigantic flares, doubling their brightness in minutes. This variability may also make it difficult for life todevelop and persist near a red dwarf. Gibor Basri of the University of California, Berkeley claims a planetorbiting close to a red dwarf could keep its atmosphere even if the star flares.[16]

Spectral standard stars

The spectral standards for M­type stars have changed slightly over the years, but settled down somewhatsince the early 1990s. Part of this is due to the fact that even the nearest M dwarfs are fairly faint, and thestudy of mid­ to late­M dwarfs has only taken off in the past few decades due to evolution of astronomicaltechniques, from photographic plates to charged­couple devices (CCDs) to infrared­sensitive arrays.

The revised Yerkes Atlas system (Johnson & Morgan 1953)[17] listed only 2 M­type spectral standard stars:HD 147379 (M0 V) and HD 95735/Lalande 21185 (M2 V). While HD 147379 was not considered astandard by expert classifiers in later compendia of standards, Lalande 21185 is still a primary standard for

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Gliese 623b is right of center

M2 V. Robert Garrison[18] does not list any "anchor" standards among the M dwarf stars, but Lalande21185 has survived as a M2 V standard through many compendia.[17][19][20] The review on MKclassification by Morgan & Keenan (1973) did not contain M dwarf standards. In the mid­1970s, M dwarfstandard stars were published by Keenan & McNeil (1976)[21] and Boeshaar (1976),[22] but unfortunatelythere was little agreement among the standards. As later cooler stars were identified through the 1980s, itwas clear that an overhaul of the M dwarf standards was needed. Building primarily upon the Boeshaarstandards, a group at Steward Observatory (Kirkpatrick, Henry, & McCarthy 1991)[23] filled in the spectralsequence from K5 V to M9 V. It is these M type dwarf standardstars which have largely survived intact as the main standards to themodern day. There have been negligible changes in the M dwarfspectral sequence since 1991. Additional M dwarf standards werecompiled by Henry et al. (2002),[24] and D. Kirkpatrick has recentlyreviewed the classification of M dwarf stars and standard stars inGray & Corbally's 2009 monograph.[25] The M­dwarf primaryspectral standards are: GJ 270 (M0 V), GJ 229A (M1 V), Lalande21185 (M2 V), Gliese 581 (M3 V), GJ 402 (M4 V), GJ 51 (M5 V),Wolf 359 (M6 V), Van Biesbroeck 8 (M7 V), VB 10 (M8 V), LHS2924 (M9 V).

See also

Aurelia and Blue MoonBlue dwarf (red­dwarf stage)Cataclysmic variable starStellar classification, ClassM

Habitability of red dwarfsystemsHertzsprung–RusselldiagramFlare starNemesis (hypothetical star)Red giantStar count, star survey

Stellar evolutionWhite dwarfKapteyn's StarYerkes luminosityclassification

References1. "The Brightest Red Dwarf" (http://kencroswell.com/thebrightestreddwarf.html), by Ken Croswell (Accessed

6/7/08)2. Exoplanets near red dwarfs suggest another Earth nearer (http://www.bbc.co.uk/news/science­environment­

21350899), 6 February 2013, Jason Palmer, BBC, retrieved at 11 April 20133. Reiners, A.; Basri, G. (March 2009). "On the magnetic topology of partially and fully convective stars".Astronomy and Astrophysics 496 (3): 787–790. arXiv:0901.1659. Bibcode:2009A&A...496..787R.doi:10.1051/0004­6361:200811450.

4. Richmond, Michael (November 10, 2004). "Late stages of evolution for low­mass stars". Rochester Institute ofTechnology. Retrieved 2007­09­19.

5. Chabrier, G.; Baraffe, I.; Plez, B. (1996). "Mass­Luminosity Relationship and Lithium Depletion for Very LowMass Stars". Astrophysical Journal Letters 459 (2): L91–L94. Bibcode:1996ApJ...459L..91C.doi:10.1086/309951.

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6. Padmanabhan, Thanu (2001). Theoretical Astrophysics. Cambridge University Press. pp. 96–99. ISBN 0­521­56241­4.

7. Adams, Fred C.; Laughlin, Gregory; Graves, Genevieve J. M. (2004). "Red Dwarfs and the End of the MainSequence" (PDF). Gravitational Collapse: From Massive Stars to Planets. Revista Mexicana de Astronomía yAstrofísica. pp. 46–49. Bibcode:2004RMxAC..22...46A.

8. Fred C. Adams & Gregory Laughlin (1996). "A Dying Universe: The Long Term Fate and Evolution ofAstrophysical Objects". Reviews of Modern Physics 69 (2): 337–372. arXiv:astro­ph/9701131.Bibcode:1997RvMP...69..337A. doi:10.1103/RevModPhys.69.337.

9. Koupelis, Theo (2007). In Quest of the Universe. Jones & Bartlett Publishers. ISBN 0­7637­4387­9.10. Kaltenegger, Lisa; Traub, Wesley A. (June 2009). "Transits of Earth­like Planets". The Astrophysical Journal

698 (1): 519–527. arXiv:0903.3371. Bibcode:2009ApJ...698..519K. doi:10.1088/0004­637X/698/1/519.11. J. A. Johnson (2011). "The Stars that Host Planets". Sky & Telescope (April): 22–27.12. Billions of Rocky Planets in Habitable Zones Around Red Dwarfs (http://www.spaceref.com/news/viewpr.html?

pid=36565) European Southern Observatory March 28, 201213. Major Discovery: New Planet Could Harbor Water and Life

(http://www.space.com/scienceastronomy/070424_hab_exoplanet.html) By Ker Than (Staff Writer) 24 April 2007SPACE.com

14. "Scientists find potentially habitable planet near Earth". Physorg.com. Retrieved 2013­03­26.15. Mikko Tuomi (2011). "Bayesian re­analysis of the radial velocities of Gliese 581. Evidence in favour of only

four planetary companions". arXiv:1102.3314 [astro­ph.EP].16. Scientific American, "Red Star Rising" (http://www.sciam.com/article.cfm?id=red­star­rising), Mark Alpert,

2005 November 717. Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas

(http://adsabs.harvard.edu/abs/1953ApJ...117..313J) H.L. Johnson & W.W. Morgan, 1953, AstrophysicalJournal, 117, 313

18. MK ANCHOR POINTS (http://www.astro.utoronto.ca/~garrison/mkstds.html), Robert F. Garrison19. The Perkins Catalog of Revised MK Types for the Cooler Stars

(http://adsabs.harvard.edu/abs/1989ApJS...71..245K), P.C. Keenan & R.C McNeil, "Astrophysical JournalSupplement Series" 71 (October 1989), pp. 245–266.

20. A standard stellar spectral sequence in the red/near­infrared – Classes K5 to M9(http://adsabs.harvard.edu/abs/1991ApJS...77..417K) Kirkpatrick, J. D.; Henry, Todd J.; McCarthy, Donald W.,Jr., 1991, Astrophysical Journal Supplement Series, vol. 77, Nov. 1991, p. 417

21. An atlas of spectra of the cooler stars: Types G,K,M,S, and C. Part 1: Introduction and tables(http://adsabs.harvard.edu/abs/1976aasc.book.....K) P.C. Keenan & R.C. McNeil, 1976, Columbus: Ohio StateUniversity Press

22. The spectral classification of M­dwarf stars (http://adsabs.harvard.edu/abs/1976PhDT........14B) Boeshaar, P.C,1976, Ph.D. Thesis Ohio State Univ., Columbus.

23. A standard stellar spectral sequence in the red/near­infrared – Classes K5 to M9(http://adsabs.harvard.edu/abs/1991ApJS...77..417K) Kirkpatrick, J. D.; Henry, Todd J.; McCarthy, Donald W.,Jr., 1991, Astrophysical Journal Supplement Series, vol. 77, Nov. 1991, p. 417­440

24. The Solar Neighborhood. VI. New Southern Nearby Stars Identified by Optical Spectroscopy(http://adsabs.harvard.edu/abs/2002AJ....123.2002H) Henry, Todd J.; Walkowicz, Lucianne M.; Barto, Todd C.;Golimowski, David A., 2002, The Astronomical Journal, Volume 123, Issue 4, pp. 2002–2009

25. Stellar Spectral Classification (http://adsabs.harvard.edu/abs/2009ssc..book.....G) Richard O. Gray andChristopher J. Corbally, 2009, Princeton University Press. ISBN 978­0­691­12511­4

Notes

A. Burrows; W. B. Hubbard; D. Saumon; J. I. Lunine (1993). "An expanded set of brown dwarf and very lowmass star models". Astrophysical Journal 406 (1): 158–171. Bibcode:1993ApJ...406..158B. doi:10.1086/172427."VLT Interferometer Measures the Size of Proxima Centauri and Other Nearby Stars". European SouthernObservatory. November 19, 2002. Retrieved 2007­01­12.Neptune­Size Planet Orbiting Common Star Hints at Many More(http://space.com/scienceastronomy/051130_small_planet.html)

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Look up red dwarf inWiktionary, the freedictionary.

Wikimedia Commons hasmedia related to Red dwarfs.

External links

Variable stars (http://www.aavso.org/variable­stars­main)Stellar Flares(http://www.ucm.es/info/Astrof/invest/actividad/flares.html) –D. Montes, UCM.Red Dwarfs (http://jumk.de/astronomie/about­stars/red­dwarfs.shtml)Red Star Rising : Small, cool stars may be hot spots for life(http://www.nature.com/scientificamerican/journal/v293/n5/full/scientificamerican1105­28.html) –Scientific American (November 2005)

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