Editing Red dwarf (section)

==Description and characteristics==
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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>