View source for Red dwarf

{{short description|Dim, low mass stars on the main sequence}}
{{About|the type of star|the British comedy franchise|Red Dwarf}}
{{redirect|M dwarf|substellar objects|brown dwarf}}
{{Infobox astronomical formation|
|image=New shot of Proxima Centauri, our nearest neighbour.jpg
|caption=[[Proxima Centauri]], the closest star to the Sun, at a distance of {{cvt|4.2|ly|pc|lk=on}}, is a red dwarf.
|thing=Class of small [[main sequence]] star.
|commonscat=Red dwarfs
|qid=Q5893
|Mass=< 1.0 [[Solar mass|''M''<sub>☉</sub>]]
|comp=Hydrogen, helium
|luminosity=[[Luminosity class|Class V]]
|head=}}
A '''red dwarf''' is the smallest kind of [[star]] on the [[main sequence]]. Red dwarfs are by far the most common type of [[Fusor (astronomy)|fusing]] star in the [[Milky Way]], at least in the neighborhood of the [[Sun]]. However, due to their low luminosity, individual red dwarfs are not easily observed. From Earth, not one star that fits the stricter definitions of a red dwarf is visible to the naked eye.<ref>{{cite web |url=http://kencroswell.com/thebrightestreddwarf.html |title=The Brightest Red Dwarf |author=Ken Croswell |access-date=2019-07-10}}</ref> [[Proxima Centauri]], the star nearest to the Sun, is a red dwarf, as are fifty of the [[List of nearest stars|sixty nearest stars]]. According to some estimates, red dwarfs make up three-quarters of the fusing stars in the Milky Way.<ref name="bbcrd2013-04-11">{{cite web |url=https://www.bbc.co.uk/news/science-environment-21350899 |title=Exoplanets near red dwarfs suggest another Earth nearer |date=6 February 2013 |author=Jason Palmer |publisher=BBC |access-date=2019-07-10}}</ref>
 
The coolest red dwarfs near the Sun have a surface temperature of about {{val|2,000|fmt=commas|ul=K}} and the smallest have radii about 9% [[solar radius|that of the Sun]], with masses about 7.5% [[solar mass|that of the Sun]]. These red dwarfs have [[Stellar classification#Spectral types|spectral type]]s of L0 to L2. There is some overlap with the properties of [[brown dwarf]]s, since the most massive brown dwarfs at lower metallicity can be as hot as {{val|3,600|fmt=commas|u=K}} and have late M spectral types.

Definitions and usage of the term "red dwarf" vary on how inclusive they are on the hotter and more massive end. One definition is synonymous with stellar '''M dwarfs''' ('''M-type main sequence stars'''), yielding a maximum temperature of {{val|3,900|u=K|fmt=commas}} and {{Solar mass|0.6|link=y}}. Another includes all stellar M-type main-sequence and all [[K-type main-sequence star]]s (K dwarf), yielding a maximum temperature of {{val|5,200|u=K|fmt=commas}} and {{Solar mass|0.8}}. Some definitions include any stellar M dwarf and part of the K dwarf classification. Other definitions are also in use. Many of the coolest, lowest mass M dwarfs are expected to be brown dwarfs, not true stars, and so those would be excluded from any definition of red dwarf.

Stellar models indicate that red dwarfs less than {{Solar mass|0.35}} are fully [[Convection zone|convective]].<ref name=aaa496_3_787>{{cite journal
 |last1=Reiners |first1=Ansgar |last2=Basri |first2=Gibor
 | title=On the magnetic topology of partially and fully convective stars
 | journal=Astronomy and Astrophysics | volume=496 | issue=3 | pages=787–790
 |date=March 2009 | doi=10.1051/0004-6361:200811450
 | bibcode=2009A&A...496..787R |arxiv = 0901.1659 | s2cid=15159121 }}</ref> Hence, the helium produced by the [[thermonuclear fusion]] of hydrogen is constantly remixed throughout the star, avoiding helium buildup at the core, thereby prolonging the period of fusion. Low-mass red dwarfs therefore develop very slowly, maintaining a constant [[luminosity]] and spectral type for trillions of years, until their fuel is depleted, and turn into [[blue dwarf (red dwarf stage)|blue dwarf]]. Because of the comparatively short [[age of the universe]], no red dwarfs yet exist at advanced stages of evolution.

==Definition==
The term "red dwarf" when used to refer to a star does not have a strict definition. One of the earliest uses of the term was in 1915, used simply to contrast "red" dwarf stars from hotter "blue" dwarf stars.<ref name=lindemann>{{cite journal|bibcode=1915Obs....38..299L|title=The age of the Earth|journal=The Observatory|volume=38|pages=299|last1=Lindemann|first1=F. A.|year=1915}}</ref> It became established use, although the definition remained vague.<ref name=edgeworth>{{cite journal|bibcode=1946Natur.157..481E|title=Red Dwarf Stars|journal=Nature|volume=157|issue=3989|pages=481|last1=Edgeworth|first1=K. E.|year=1946|doi=10.1038/157481d0|s2cid=4106298|doi-access=free}}</ref> In terms of which spectral types qualify as red dwarfs, different researchers picked different limits, for example K8–M5<ref name=dyer>{{cite journal|bibcode=1956AJ.....61..228D|title=An analysis of the space motions of red dwarf stars|journal=Astronomical Journal|volume=61|pages=228|last1=Dyer|first1=Edward R.|year=1956|doi=10.1086/107332}}</ref> or "later than K5".<ref name=mumford>{{cite journal|bibcode=1956AJ.....61..224M|title=The motions and distribution of dwarf M stars|journal=Astronomical Journal|volume=61|pages=224|last1=Mumford|first1=George S.|year=1956|doi=10.1086/107331|doi-access=free}}</ref>  ''Dwarf M star'', abbreviated dM, was also used, but sometimes it also included stars of spectral type K.<ref name=Vyssotsky>{{cite journal|bibcode=1956AJ.....61..201V|title=Dwarf M stars found spectrophotometrically|journal=Astronomical Journal|volume=61|pages=201|last1=Vyssotsky|first1=A. N.|year=1956|doi=10.1086/107328}}</ref>

In modern usage, the definition of a ''red dwarf'' still varies. When explicitly defined, it typically includes [[Late-type star|late K-]] and early to mid-M-class stars,<ref name=engle>{{cite journal|bibcode=2011ASPC..451..285E|arxiv=1111.2872|title=Red Dwarf Stars: Ages, Rotation, Magnetic Dynamo Activity and the Habitability of Hosted Planets|journal=9th Pacific Rim Conference on Stellar Astrophysics. Proceedings of a Conference Held at Lijiang|volume=451|pages=285|last1=Engle|first1=S. G.|last2=Guinan|first2=E. F.|year=2011}}</ref> but in many cases it is restricted just to M-class stars.<ref name=heath>{{cite journal|doi=10.1023/A:1006596718708|pmid=10472629|year=1999|last1=Heath|first1=Martin J.|title=Habitability of planets around red dwarf stars|journal=Origins of Life and Evolution of the Biosphere|volume=29|issue=4|pages=405–24|last2=Doyle|first2=Laurance R.|last3=Joshi|first3=Manoj M.|last4=Haberle|first4=Robert M.|bibcode=1999OLEB...29..405H|s2cid=12329736|doi-access=free}}</ref><ref name=farihi>{{cite journal|bibcode=2006ApJ...646..480F|arxiv=astro-ph/0603747|title=White Dwarf-Red Dwarf Systems Resolved with the Hubble Space Telescope. I. First Results|journal=The Astrophysical Journal|volume=646|issue=1|pages=480–492|last1=Farihi|first1=J.|last2=Hoard|first2=D. W.|last3=Wachter|first3=S.|year=2006|doi=10.1086/504683|s2cid=16750158}}</ref>  In some cases all K stars are included as red dwarfs,<ref name=petterson>{{cite journal|bibcode=1989A&A...217..187P|title=A spectroscopic survey of red dwarf flare stars|journal=Astronomy and Astrophysics|volume=217|pages=187|last1=Pettersen|first1=B. R.|last2=Hawley|first2=S. L.|year=1989}}</ref> and occasionally even earlier stars.<ref name=alekseev>{{cite journal|bibcode=2002A&A...396..203A|title=Starspots and active regions on the emission red dwarf star LQ Hydrae|journal=Astronomy and Astrophysics|volume=396|pages=203–211|last1=Alekseev|first1=I. Yu.|last2=Kozlova|first2=O. V.|year=2002|doi=10.1051/0004-6361:20021424|doi-access=free}}</ref>

The most recent surveys place the coolest true main-sequence stars into spectral types L2 or L3. At the same time, many objects cooler than about M6 or M7 are brown dwarfs, insufficiently massive to sustain [[Isotopes of hydrogen#Hydrogen-1 (Protium)|hydrogen-1]] fusion.<ref name=dietrich/> This gives a significant overlap in spectral types for red and brown dwarfs. Objects in that spectral range can be difficult to categorize.

==Description and characteristics==
{{Star nav}}
Red dwarfs are [[Star formation#Low mass and high mass star formation|very-low-mass stars]].<ref name="richmond">{{cite web
 | last=Richmond | first=Michael | date=November 10, 2004
 | url=http://spiff.rit.edu/classes/phys230/lectures/planneb/planneb.html
 | title=Late stages of evolution for low-mass stars
 | publisher=Rochester Institute of Technology
 | access-date=2019-07-10 }}</ref> As a result, they have relatively low pressures, a low fusion rate, and hence, a low temperature. The energy generated is the product of [[nuclear fusion]] of [[hydrogen]] into [[helium]] by way of the [[Proton–proton chain reaction|proton–proton (PP) chain]] mechanism. Hence, these stars emit relatively little light, sometimes as little as {{frac|10,000}} that of the Sun, although this would still imply a power output on the order of 10<sup>22</sup>&nbsp;watts (10 trillion gigawatts or 10 [[Metric prefix|ZW]]). Even the largest red dwarfs (for example [[HD 179930]], [[HIP 12961]] and [[Lacaille 8760]]) have only about 10% of the [[Solar luminosity|Sun's luminosity]].<ref>{{cite journal
 | author=Chabrier, G.
 | author2=Baraffe, I.
 | author3=Plez, B.
 | title=Mass-Luminosity Relationship and Lithium Depletion for Very Low Mass Stars
 | journal=Astrophysical Journal Letters
 | date=1996 | volume=459
 | issue=2 | pages=L91–L94
 | bibcode=1996ApJ...459L..91C
 | doi = 10.1086/309951
| doi-access=free
 }}</ref> In general, red dwarfs less than {{Solar mass|0.35}} transport energy from the core to the surface by [[convection]]. Convection occurs because of [[Opacity (optics)|opacity]] of the interior, which has a high density compared to the temperature. As a result, energy transfer by [[radiation]] is decreased, and instead convection is the main form of energy transport to the surface of the star. Above this mass, a red dwarf will have a region around its core where convection does not occur.<ref>{{cite book
 | first=Thanu | last=Padmanabhan
 | date=2001 | pages=96–99
 | title=Theoretical Astrophysics
 | publisher=Cambridge University Press
 | isbn=0-521-56241-4 }}</ref>

[[File:Red dwarf lifetime.png|left|thumb|The predicted main-sequence lifetime of a red dwarf plotted against its mass relative to the Sun.<ref name="Adams2004">{{cite conference
 | last=Adams | first=Fred C.
 |author2=Laughlin, Gregory |author3=Graves, Genevieve J. M. 
 | title=Red Dwarfs and the End of the Main Sequence
 | book-title=Gravitational Collapse: From Massive Stars to Planets
 | date=2004
 | pages=46–49
 | publisher=Revista Mexicana de Astronomía y Astrofísica
 | bibcode=2004RMxAC..22...46A
 | url=http://www.astroscu.unam.mx/rmaa/RMxAC..22/PDF/RMxAC..22_adams.pdf
}}</ref>]]
Because low-mass red dwarfs are fully convective, helium does not accumulate at the core, and compared to larger stars such as the Sun, they can burn a larger proportion of their hydrogen before leaving the [[main sequence]]. As a result, red dwarfs have estimated lifespans far longer than the present age of the universe, and stars less than {{Solar mass|0.8}} have not had time to leave the main sequence. The lower the mass of a red dwarf, the longer the lifespan. It is believed that the lifespan of these stars exceeds the expected 10-billion-year lifespan of the Sun by the third or fourth power of the ratio of the solar mass to their masses; thus, a {{Solar mass|0.1}} red dwarf may continue burning for 10 trillion years.<ref name="richmond"/><ref>{{cite journal
 |title=A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects
 |author=Fred C. Adams|author2=Gregory Laughlin
 |name-list-style=amp|year=1997|doi=10.1103/RevModPhys.69.337
 |journal=Reviews of Modern Physics|volume=69|issue=2|pages=337–372
 |arxiv=astro-ph/9701131
 |bibcode = 1997RvMP...69..337A |s2cid=12173790}}</ref> As the proportion of hydrogen in a red dwarf is consumed, the rate of fusion declines and the core starts to contract. The gravitational energy released by this size reduction is converted into heat, which is carried throughout the star by convection.<ref>{{cite book | first=Theo | last=Koupelis | date=2007 | title=In Quest of the Universe | publisher=Jones & Bartlett Publishers | isbn=978-0-7637-4387-1 | url-access=registration | url=https://archive.org/details/inquestofunivers00koup }}</ref>

{| class="wikitable floatright" style="text-align:center; font-size:smaller;"
|+ Properties of typical M-type main-sequence stars<ref>{{cite journal |last1=Pecaut |first1=Mark J. |last2=Mamajek |first2=Eric E. |title=Intrinsic Colors, Temperatures, and Bolometric Corrections of Pre-main-sequence Stars |journal=The Astrophysical Journal Supplement Series |date=1 September 2013 |volume=208 |issue=1 |pages=9 |doi=10.1088/0067-0049/208/1/9 |issn=0067-0049|arxiv=1307.2657 |bibcode=2013ApJS..208....9P |s2cid=119308564 }}</ref><ref>{{cite web |last1=Mamajek |first1=Eric |title=A Modern Mean Dwarf Stellar Color and Effective Temperature Sequence |url=http://www.pas.rochester.edu/~emamajek/EEM_dwarf_UBVIJHK_colors_Teff.txt |publisher=University of Rochester, Department of Physics and Astronomy |access-date=5 July 2021 |date=2 March 2021}}</ref><ref>{{cite journal |first1=C. |last1=Cifuentes|first2=J.A. |last2=Caballero|first3=M. |last3=Cortés-Contreras|first4=D.|last4=Montes|first5=F.J.|last5=Abellán|first6=R.|last6=Dorda|first7=G.|last7=Holgado|year=2020|title=CARMENES input catalogue of M dwarfs. V. Luminosities, colours, and spectral energy distributions|journal=Astronomy and Astrophysics |volume=642 |pages=32|issue=October 2020|doi=10.1051/0004-6361/202038295 |bibcode=2020A&A...642A.115C |arxiv=2007.15077}}</ref>
|-
![[Spectral type|Spectral<br/>type]]<ref>[[Brown dwarf#Spectral class M|Younger brown dwarfs]] may also exhibit spectra similar to late M-type stars.</ref>
![[Stellar mass|Mass]] ({{Solar mass|link=y}})
![[Stellar radius|Radius]] ({{Solar radius|link=y}})
![[Luminosity]] ({{Solar luminosity|link=y}})
![[Effective temperature|Effective<br/>temperature]]<br/>(K)
![[Color index|Color<br/>index]]<br/>{{nowrap|(B − V)}}
|-
| M0V || 0.57 || 0.588 || 0.069 || style="background-color:#{{Color temperature|3850 |hexval}}"|3,850 || 1.42
|-
| M1V || 0.50 || 0.501 || 0.041 || style="background-color:#{{Color temperature|3660 |hexval}}"|3,660 || 1.49
|-
| M2V || 0.44 || 0.446 || 0.029 || style="background-color:#{{Color temperature|3560 |hexval}}"|3,560 || 1.51
|-
| M3V || 0.37 || 0.361 || 0.016 || style="background-color:#{{Color temperature|3430 |hexval}}"|3,430 || 1.53
|-
| M4V || 0.23 || 0.274 || 7.2x10<sup>−3</sup> || style="background-color:#{{Color temperature|3210 |hexval}}"|3,210 || 1.65
|-
| M5V || 0.162 || 0.196 || 3.0x10<sup>−3</sup> || style="background-color:#{{Color temperature|3060 |hexval}}"|3,060 || 1.83
|-
| M6V || 0.102 || 0.137 || 1.0x10<sup>−3</sup> || style="background-color:#{{Color temperature|2810 |hexval}}"|2,810 || 2.01
|-
| M7V || 0.090 || 0.120 || 6.5x10<sup>−4</sup> || style="background-color:#{{Color temperature|2680 |hexval}}"|2,680 || 2.12
|-
| M8V || 0.085 || 0.114 || 5.2x10<sup>−4</sup> || style="background-color:#{{Color temperature|2570 |hexval}}"|2,570 || 2.15
|-
| M9V || 0.079 || 0.102 || 3.0x10<sup>−4</sup> || style="background-color:#{{Color temperature|2380 |hexval}}"|2,380 || 2.17
|}

According to computer simulations, the minimum mass a red dwarf must have to eventually evolve into a [[red giant]] is {{Solar mass|0.25}}; less massive objects, as they age, would increase their surface temperatures and luminosities becoming [[Blue dwarf (red-dwarf stage)|blue dwarfs]] and finally [[white dwarf]]s.<ref name="Adams2004" />

The less massive the star, the longer this evolutionary process takes. A {{Solar mass|0.16}} red dwarf (approximately the mass of the nearby [[Barnard's Star]]) would stay on the main sequence for 2.5 trillion years, followed by five billion years as a blue dwarf, during which the star would have one third of the [[Solar luminosity|Sun's luminosity]] ({{Solar luminosity|link=y}}) and a surface temperature of 6,500–8,500 [[kelvin]]s.<ref name="Adams2004" />

The fact that red dwarfs and other low-mass stars still remain on the main sequence when more massive stars have moved off the main sequence allows the age of [[star cluster]]s to be estimated by finding the mass at which the stars move off the main sequence. This provides a lower limit to the age of the [[Universe]] and also allows formation timescales to be placed upon the structures within the [[Milky Way]], such as the [[Galactic spheroid|Galactic halo]] and [[Galactic plane|Galactic disk]].

All observed red dwarfs contain [[Metallicity|"metals"]], which in astronomy are elements heavier than hydrogen and helium. The [[Big Bang]] model predicts that the first generation of stars should have only hydrogen, helium, and trace amounts of lithium, and hence would be of low metallicity. With their extreme lifespans, any red dwarfs that were a part of that first generation ([[population III stars]]) should still exist today. Low-metallicity red dwarfs, however, are rare. The accepted model for the chemical evolution of the universe anticipates such a scarcity of metal-poor dwarf stars because only giant stars are thought to have formed in the metal-poor environment of the early universe.{{why|date=December 2023}} As giant stars end their short lives in [[supernova]] explosions, they spew out the heavier elements needed to form smaller stars. Therefore, dwarfs became more common as the universe aged and became enriched in metals. While the basic scarcity of ancient metal-poor red dwarfs is expected, observations have detected even fewer than predicted. The sheer difficulty of detecting objects as dim as red dwarfs was thought to account for this discrepancy, but improved detection methods have only confirmed the discrepancy.<ref>{{cite news |url=https://astrobites.org/2012/02/15/and-now-theres-a-problem-with-m-dwarfs-too/ |title=And now there's a problem with M dwarfs, too |author=Elisabeth Newton |newspaper=[[Astrobites]] |date=Feb 15, 2012 |access-date=2019-07-10}}</ref>

The boundary between the least massive red dwarfs and the most massive brown dwarfs depends strongly on the metallicity. At solar metallicity the boundary occurs at about {{solar mass|0.07}}, while at zero metallicity the boundary is around {{solar mass|0.09}}. At solar metallicity, the least massive red dwarfs theoretically have temperatures around {{val|1,700|fmt=commas|ul=K}}, while measurements of red dwarfs in the solar neighbourhood suggest the coolest stars have temperatures of about {{val|2,075|fmt=commas|u=K}} and spectral classes of about L2. Theory predicts that the coolest red dwarfs at zero metallicity would have temperatures of about {{val|3,600|fmt=commas|u=K}}. The least massive red dwarfs have radii of about {{solar radius|0.09}}, while both more massive red dwarfs and less massive brown dwarfs are larger.<ref name=dietrich>
{{cite journal
| bibcode = 2014AJ....147...94D
| arxiv = 1312.1736
| title = The Solar Neighborhood. XXXII. The Hydrogen Burning Limit
| journal = The Astronomical Journal
| volume= 147
| issue = 5
| pages = 94 
| last1 = Dieterich|first1=Sergio B.|last2=Henry|first2=Todd J.|last3=Jao|first3=Wei-Chun|last4=Winters|first4=Jennifer G.|last5=Hosey|first5=Altonio D.|last6=Riedel|first6=Adric R.|last7=Subasavage|first7=John P.
| year = 2014
| doi = 10.1088/0004-6256/147/5/94
| s2cid = 21036959
}}</ref><ref name=burrows>{{cite journal |doi=10.1103/RevModPhys.73.719 |title=The theory of brown dwarfs and extrasolar giant planets |journal=Reviews of Modern Physics |volume=73 |issue=3 |pages=719–765 |year=2001 |last1=Burrows |first1=Adam |last2=Hubbard |first2=William B. |last3=Lunine |first3=Jonathan I. |last4=Liebert |first4=James |bibcode=2001RvMP...73..719B |arxiv=astro-ph/0103383 |s2cid=204927572 }}</ref>

==Spectral standard stars==
[[File:Gliese 623.jpg|right|thumb|[[Gliese 623]] is a pair of red dwarfs, with GJ&nbsp;623a on the left and the fainter GJ&nbsp;623b to the right of center.]]
The spectral standards for M&nbsp;type stars have changed slightly over the years, but settled down somewhat since the early 1990s. Part of this is due to the fact that even the nearest red dwarfs are fairly faint, and their colors do not register well on [[photographic emulsion]]s used in the early to mid 20th century. The study of mid- to late-M&nbsp;dwarfs has significantly advanced only in the past few decades, primarily due to development of new [[astrograph]]ic and [[spectroscopic]] techniques, dispensing with photographic plates and progressing to charged-couple devices (CCDs) and infrared-sensitive arrays.

The revised Yerkes Atlas system (Johnson & Morgan, 1953)<ref name="adsabs.harvard.edu">{{cite journal |bibcode=1953ApJ...117..313J |title=Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas |journal=Astrophysical Journal |volume=117 |pages=313 |last1=Johnson |first1=H.L. |last2=Morgan |first2=W.W. |year=1953 |doi=10.1086/145697}}</ref> listed only two M&nbsp;type spectral standard stars: [[HD&nbsp;147379]] (M0V)
and HD&nbsp;95735/[[Lalande 21185]] (M2V). While HD&nbsp;147379 was not considered a standard by expert classifiers in later compendia of standards, [[Lalande 21185]] is still a primary standard for M2V. Robert Garrison<ref name="Garrison">{{cite web |url=http://www.astro.utoronto.ca/~garrison/mkstds.html |title=MK anchor-point standards table |first=Robert F. |last=Garrison |website=astro.utoronto.ca |publisher=University of Toronto |department=Department of Astronomy & Astrophysics |access-date=2011-12-18 |archive-date=2019-06-25 |archive-url=https://web.archive.org/web/20190625094716/http://www.astro.utoronto.ca/~garrison/mkstds.html |url-status=dead }}</ref> does not list any "anchor" standards among the red dwarfs, but [[Lalande 21185]] has survived as a M2V standard through many compendia.<ref name="adsabs.harvard.edu"/><ref name="Keenan89">{{cite journal |bibcode=1989ApJS...71..245K |title=The Perkins catalog of revised MK types for the cooler stars |journal=Astrophysical Journal Supplement Series |volume=71 |pages=245 |last1=Keenan |first1=Philip C. |last2=McNeil |first2=Raymond C. |year=1989 |doi=10.1086/191373|s2cid=123149047 }}</ref><ref name="ReferenceA">{{cite journal |bibcode=1991ApJS...77..417K |title=A standard stellar spectral sequence in the red / near-infrared - Classes&nbsp;K5 to M9 |journal=Astrophysical Journal Supplement Series |volume=77 |pages=417 |last1=Kirkpatrick |first1=J.D. |last2=Henry |first2=Todd J. |last3=McCarthy |first3=Donald W. |year=1991 |doi=10.1086/191611|doi-access=free }}</ref> The review on MK classification by Morgan & Keenan (1973) did not contain red dwarf standards. 

In the mid-1970s, red dwarf standard stars were published by Keenan & McNeil (1976)<ref>{{cite book |last1=Keenan |first1=Philip Childs |last2=McNeil |first2=Raymond C. |year=1976 |title=An atlas of spectra of the cooler stars: Types G, K, M, S, and C. Part&nbsp;1: Introduction and tables |place=Columbus, OH |publisher=Ohio State University Press |bibcode=1976aasc.book.....K}}</ref> and Boeshaar (1976),<ref>{{cite thesis |last1=Boeshaar |first1=P.C. |year=1976 |title=The spectral classification of M&nbsp;dwarf stars |degree=Ph.D. |publisher=Ohio State University |place=Columbus, OH |bibcode=1976PhDT........14B}}</ref> but there was little agreement among the standards. As later cooler stars were identified through the 1980s, it was clear that an overhaul of the red dwarf standards was needed. Building primarily upon the Boeshaar standards, a group at Steward Observatory (Kirkpatrick, Henry, & McCarthy, 1991)<ref name="ReferenceA"/> filled in the spectral sequence from K5V to M9V. It is these M&nbsp;type dwarf standard stars which have largely survived as the main standards to the modern day. There have been negligible changes in the red dwarf spectral sequence since 1991. Additional red dwarf standards were compiled by Henry et al. (2002),<ref>{{cite journal |bibcode=2002AJ....123.2002H |title=The Solar neighborhood. VI.&nbsp;New southern nearby stars identified by optical spectroscopy |journal=The Astronomical Journal |volume=123 |issue=4 |page=2002 |last1=Henry |first1=Todd J .|author-link2=Lucianne Walkowicz |last2=Walkowicz |first2=Lucianne M. |last3=Barto |first3=Todd C. |last4=Golimowski |first4=David A. |year=2002 |doi=10.1086/339315 |arxiv = astro-ph/0112496|s2cid=17735847 }}</ref> and D. Kirkpatrick has recently
reviewed the classification of red dwarfs and standard stars in Gray & Corbally's 2009 monograph.<ref>{{cite book |last1=Gray |first1=Richard O. |last2=Corbally |first2=Christopher |year=2009 |title=Stellar Spectral Classification |publisher=Princeton University Press |bibcode=2009ssc..book.....G}}</ref> The M&nbsp;dwarf primary spectral standards are: [[GJ 270]] (M0V), [[GJ 229A]] (M1V), [[Lalande 21185]] (M2V), [[Gliese 581]] (M3V), [[Gliese 402]] (M4V), [[GJ 51]] (M5V), [[Wolf 359]] (M6V), [[van Biesbroeck 8]] (M7V), [[VB 10]] (M8V), [[LHS 2924]] (M9V).

==Planets==
[[File:AU MIc M-dwarf artist's conception.jpg|thumb|Illustration depicting [[AU Mic]], an M-type (spectral class M1Ve) red dwarf star less than 0.7% the age of the Sun. The dark areas represent huge sunspot-like regions.]]
Many red dwarfs are orbited by [[exoplanet]]s, but large [[Jupiter]]-sized planets are comparatively rare. Doppler surveys of a wide variety of stars indicate about 1 in 6&nbsp;stars with twice the mass of the Sun are orbited by one or more of Jupiter-sized planets, versus 1 in 16 for Sun-like stars and the frequency of close-in giant planets (Jupiter size or larger) orbiting red dwarfs is only 1 in 40.<ref name="Mawet">{{cite conference|last1=Mawet|first1=Dimitri|last2=Jovanovic|first2=Nemanja|last3=Delorme|first3=Jacques-Robert|last4=Wizinowich|first4=Peter L.|last5=Wallace|first5=James K.|last6=Bond|first6=Charlotte Z.|last7=Chun|first7=Mark R.|last8=Cetre|first8=Sylvain|last9=Lilley|first9=Scott J.|last10=Hall|first10=Donald N. B.|last11=Echeverri|first11=Daniel |title=Adaptive Optics Systems VI |display-authors=3|editor-last=Schmidt|editor-first=Dirk|editor2-last=Schreiber|editor2-first=Laura|editor3-last=Close|editor3-first=Laird M.|chapter=Keck Planet Imager and Characterizer (KPIC): status update|publisher=SPIE|date=2018-07-10|page=6 |chapter-url=https://authors.library.caltech.edu/87801/1/1070306.pdf<!--If terms of use conflict -"https://web.archive.org/web/20200702221850/https://www.astro.caltech.edu/~dmawet/research/exoplanets-around-m-stars.html"-->|doi=10.1117/12.2314037|isbn=9781510619593 |quote=<span style="font-family:LatinModern;">Close separations {{nowrap|(< 1 AU)}} have been extensively probed by Doppler and transit surveys with the following results: the frequency of close-in giant planets (1−10&nbsp;{{mvar|M}}{{sub|Jup}}) is only {{nowrap|2.5 ± 0.9%}}, consistent with core accretion plus migration models.</span>}}</ref> On the other hand, [[microlensing]] surveys indicate that long-orbital-period [[Neptune]]-mass planets are found around one in three red dwarfs.<ref name="JAJohnson">{{cite magazine |last=Johnson |first=J.A. |date=April 2011 |title=The stars that host planets |magazine=[[Sky & Telescope]] |pages=22–27}}</ref> Observations with [[HARPS]] further indicate 40% of red dwarfs have a "[[super-Earth]]" class planet orbiting in the habitable zone where liquid water can exist on the surface.<ref>{{cite web |url=http://www.spaceref.com/news/viewpr.html?pid=36565 |title=Billions of rocky planets in habitable zones around red dwarfs |publisher=[[European Southern Observatory]] |date=28 March 2012 |access-date=2019-07-10 |df=dmy-all }}{{Dead link|date=February 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Computer simulations of the formation of planets around low-mass stars predict that Earth-sized planets are most abundant, but more than 90% of the simulated planets are at least 10% water by mass, suggesting that many Earth-sized planets orbiting red dwarf stars are covered in deep oceans.<ref name="AlibertBenz">
{{cite journal
|first=Yann |last=Alibert
|year=2017
|title=Formation and composition of planets around very low mass stars
|journal=Astronomy and Astrophysics |volume=539 |pages=8
|issue=12 October 2016
|doi=10.1051/0004-6361/201629671 |bibcode=2017A&A...598L...5A |arxiv=1610.03460|s2cid=54002704
}}</ref>

At least four and possibly up to six exoplanets were discovered orbiting within the [[Gliese 581 planetary system|Gliese&nbsp;581 planetary system]] between 2005 and 2010. One planet has about the mass of [[Neptune]], or 16&nbsp;[[Earth mass]]es ({{Earth mass|link=y}}). It orbits just {{Convert|6|e6km|AU|lk=on}} from its star, and is estimated to have a surface temperature of {{cvt|150|C|K F|lk=on}}, despite the dimness of its star. In 2006, an even smaller exoplanet (only {{Earth mass|5.5}}) was found orbiting the red dwarf [[OGLE-2005-BLG-390L]]; it lies {{convert|390|e6km|AU}} from the star and its surface temperature is {{cvt|−220|C|K F}}.

In 2007, a new, potentially [[planetary habitability|habitable]] exoplanet, {{nowrap|[[Gliese 581c]]}}, was found, orbiting [[Gliese 581]]. The minimum mass estimated by its discoverers (a team led by [[Stephane Udry]]) is {{Earth mass|5.36}}. The discoverers estimate its radius to be 1.5&nbsp;times that of Earth ({{Earth radius|link=y}}). Since then [[Gliese 581d]], which is also potentially habitable, was discovered.

Gliese&nbsp;581c and d are within the [[habitable zone]] of the host star, and are two of the most likely candidates for habitability of any exoplanets discovered so far.<ref>{{cite web |url=http://www.space.com/scienceastronomy/070424_hab_exoplanet.html |title=Major discovery: New planet could harbor water and life |author=Than, Ker |date=24 April 2007 |publisher=SPACE.com |access-date=2019-07-10}}</ref> [[Gliese 581g]], detected September 2010,<ref>{{cite web |url=http://www.physorg.com/news204999128.html |title=Scientists find potentially habitable planet near Earth |publisher=Physorg.com |access-date=2013-03-26}}</ref> has a near-circular orbit in the middle of the star's habitable zone. However, the planet's existence is contested.<ref>{{cite journal |author=Tuomi, Mikko |date=2011 |title=Bayesian re-analysis of the radial velocities of Gliese&nbsp;581. Evidence in favour of only four planetary companions |journal=Astronomy & Astrophysics |volume=528 |pages=L5 |arxiv=1102.3314 |doi=10.1051/0004-6361/201015995 |bibcode=2011A&A...528L...5T|s2cid=11439465 }}</ref>

On 23&nbsp;February 2017 NASA announced the discovery of seven Earth-sized planets orbiting the red dwarf star [[TRAPPIST-1]] approximately 39&nbsp;light-years away in the constellation Aquarius. The planets were discovered through the transit method, meaning we have mass and radius information for all of them. [[TRAPPIST-1e]], [[TRAPPIST-1f|f]], and [[TRAPPIST-1g|g]] appear to be within the habitable zone and may have liquid water on the surface.<ref>{{cite web |url=https://www.nasa.gov/press-release/nasa-telescope-reveals-largest-batch-of-earth-size-habitable-zone-planets-around |website=www.nasa.gov |title=NASA telescope reveals record-breaking exoplanet discovery |date=2017-02-22 |df=dmy-all}}</ref>

==Habitability==
{{Main article|Habitability of red dwarf systems}}
[[File:NASA-RedDwarfPlanet-ArtistConception-20130728.jpg|thumb|left|upright=1.2|An artist's impression of a planet with two [[exomoon]]s orbiting in the [[habitable zone]] [[Planets orbiting red dwarfs|of a red dwarf]].]]

Modern evidence suggests that planets in red dwarf systems are extremely unlikely to be habitable. In spite of their great numbers and long lifespans, there are several factors which may make life difficult on planets around a red dwarf. First, planets in the habitable zone of a red dwarf would be so close to the parent star that they would likely be [[Tidal locking|tidally locked]]. For a nearly circular orbit, this would mean that one side would be in perpetual daylight and the other in eternal night. This could create enormous temperature variations from one side of the planet to the other. Such conditions would appear to make it difficult for forms of life similar to those on Earth to evolve. And it appears there is a great problem with the atmosphere of such tidally locked planets: the perpetual night zone would be cold enough to freeze the main gases of their atmospheres, leaving the daylight zone bare and dry. On the other hand, a theory proposes that either a thick atmosphere or planetary ocean could potentially circulate heat around such a planet.<ref>{{cite web|url=http://www.astrobio.net/news-exclusive/planets-orbiting-red-dwarfs-may-stay-wet-enough-life/|title=Planets Orbiting Red Dwarfs May Stay Wet Enough for Life|publisher=Astrobiology|language=en|author=Charles Q. Choi|date=9 February 2015|access-date=15 January 2017 |archive-url=https://web.archive.org/web/20150921050613/http://www.astrobio.net/news-exclusive/planets-orbiting-red-dwarfs-may-stay-wet-enough-life/ |archive-date=2015-09-21 |url-status=usurped}}</ref> Furthermore, even if a red dwarf's characteristics render most of its planet's surface uninhabitable, there is a chance for life to exist around a limited region, such as the planet's [[Terminator (solar)|terminator]].<ref>{{Cite web |last=Raymond |first=Sean |date=2015-02-20 |title=Forget "Earth-Like" Worlds |url=https://nautil.us/forget-earth_likewell-first-find-aliens-on-eyeball-planets-235308/ |access-date=2025-01-26 |website=Nautilus |language=en-US}}</ref>

Variability in stellar energy output may also have negative impacts on the development of life. Red dwarfs are often [[flare star]]s, which can emit gigantic flares, doubling their brightness in minutes. This variability makes it difficult for life to develop and persist near a red dwarf.<ref>{{cite journal |last1=Vida |first1=K. |last2=Kővári |first2=Zs. |last3=Pál |first3=A. |last4=Oláh |first4=K. |last5=Kriskovics |first5=L.  |display-authors=etal|title=Frequent Flaring in the TRAPPIST-1 System - Unsuited for Life? |journal=The Astrophysical Journal |date=2017 |volume=841 |issue=2 |page=2 |doi=10.3847/1538-4357/aa6f05 |bibcode=2017ApJ...841..124V|arxiv=1703.10130 |s2cid=118827117 |doi-access=free }}</ref> While it may be possible for a planet orbiting close to a red dwarf to keep its atmosphere even if the star flares, more-recent research suggests that these stars may be the source of constant high-energy flares and very large magnetic fields, diminishing the possibility of life as we know it.<ref>{{cite magazine|magazine = Scientific American |url = https://www.scientificamerican.com/article/red-star-rising/ |title = Red Star Rising|first = Mark|last =  Alpert|date = 1 November 2005}}</ref><ref>{{cite web |website=Gizmodo |url=https://gizmodo.com/this-stormy-star-means-alien-life-may-be-rarer-than-we-1743540362 |title=This Stormy Star Means Alien Life May Be Rarer Than We Thought |author=George Dvorsky |date=2015-11-19 |access-date=2019-07-10}}</ref>

==See also==
* {{annotated link|List of red dwarfs}}
* {{annotated link|Cataclysmic variable star}}
* {{annotated link|Nemesis (hypothetical star)}}
* {{annotated link|Star count}}
* {{annotated link|Stellar evolution}}
* {{annotated link|Kapteyn's Star}}
* {{annotated link|Yerkes luminosity classification}}

==References==
{{reflist}}

==Sources==
{{Refbegin}}
* {{cite journal
 |author=Burrows, Adam |author2=Hubbard, William B. |author3=Saumon, Didier |author4=Lunine, Jonathan I.
 |title=An expanded set of brown dwarf and very low mass star models
 |journal=Astrophysical Journal
 |date=1993 |volume=406 |issue=1 |pages=158–71 |bibcode=1993ApJ...406..158B |doi=10.1086/172427 |doi-access=free }}
* {{cite news|title=VLT Interferometer Measures the Size of Proxima Centauri and Other Nearby Stars |publisher=European Southern Observatory |date=November 19, 2002 |url=http://www.eso.org/outreach/press-rel/pr-2002/pr-22-02.html |access-date=2007-01-12 |url-status=dead |archive-url=https://web.archive.org/web/20070103234953/http://www.eso.org/outreach/press-rel/pr-2002/pr-22-02.html |archive-date=January 3, 2007 }}
* [http://space.com/scienceastronomy/051130_small_planet.html Neptune-Size Planet Orbiting Common Star Hints at Many More]
{{Refend}}

==External links==
{{Wiktionary}}
{{Commons category|Red dwarfs}}
* [http://www.aavso.org/variable-stars-main Variable stars] AAVSO
* [http://www.ucm.es/info/Astrof/invest/actividad/flares.html Stellar Flares] Publications about Flares by the Stellar Activity Group (UCM)
* [http://jumk.de/astronomie/about-stars/red-dwarfs.shtml Red Dwarfs] Jumk.de
* [http://www.nature.com/scientificamerican/journal/v293/n5/full/scientificamerican1105-28.html Red Star Rising : Small, cool stars may be hot spots for life] – ''Scientific American'' (November 2005)

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[[Category:Red dwarfs| ]]
[[Category:Star types]]
[[Category:Stellar phenomena]]