Editing Red dwarf (section)

{{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.