Editing Barnard's Star (section)

== Description ==
[[File:Barnard'sStarSize_en.jpg|left|thumb|Size comparison between [[Jupiter]], Barnard's Star, and the [[Sun]]]]
Barnard's Star is a red dwarf of the dim [[spectral type]] M4 and is too faint to see without a [[telescope]]; its [[apparent magnitude]] is 9.5.

At 7–12 billion years of age, Barnard's Star is considerably older than the Sun, which is 4.5 billion years old, and it might be among the oldest stars in the [[Milky Way]] galaxy.<ref name=Riedel2005>{{cite journal|last1=Riedel|first1=A. R.|last2=Guinan|first2=E. F.|last3=DeWarf|first3=L. E.|last4=Engle|first4=S. G.|last5=McCook|first5=G. P.|bibcode=2005AAS...206.0904R|title=Barnard's Star as a Proxy for Old Disk dM Stars: Magnetic Activity, Light Variations, XUV Irradiances, and Planetary Habitable Zones|date=May 2005|journal=[[Bulletin of the American Astronomical Society]]|volume=37|page=442}}</ref> Barnard's Star has lost a great deal of rotational energy; the periodic slight changes in its brightness indicate that it rotates once in 130 days<ref name=Benedict1998/> (the [[Sun]] rotates in 25). Given its age, Barnard's Star was long assumed to be quiescent in terms of stellar activity. In 1998, astronomers observed an intense [[stellar flare]], showing that Barnard's Star is a [[flare star]].<ref name=Flare>{{cite web|first=Ken|last=Croswell|date=November 2005|url=http://www.astronomy.com/news/2005/11/a-flare-for-barnards-star|access-date=10 August 2006|title=A Flare for Barnard's Star|work=Astronomy Magazine|publisher=Kalmbach Publishing Co}}</ref> Barnard's Star has the [[variable star designation]] V2500 Ophiuchi. In 2003, Barnard's Star presented the first detectable change in the [[radial velocity]] of a star caused by its motion. Further variability in the radial velocity of Barnard's Star was attributed to its stellar activity.<ref name=Kurster/>

[[File:Barnard2005.gif|thumb|left|Barnard's Star, showing position every 5 years in the period 1985–2005]]
The proper motion of Barnard's Star corresponds to a relative lateral speed of 90{{nbsp}}km/s. The 10.3 arcseconds it travels in a year amount to a quarter of a degree in a human lifetime, roughly half the angular diameter of the full Moon.<ref name=Kaler/>

The radial velocity of Barnard's Star is {{val|−110|u=km/s}}, as measured from the [[blueshift]] due to its motion toward the Sun. Combined with its proper motion and distance, this gives a [[Stellar kinematics#Space velocity|"space velocity"]] (actual speed relative to the Sun) of {{val|142.6|0.2|u=km/s}}. Barnard's Star will make its closest approach to the Sun around 11,800 CE, when it will approach to within about 3.75 light-years.<ref name=Bobylev/>

[[File:Near-stars-past-future-en.svg|left|thumb|upright=1.2|Distances to the [[List of nearest stars|nearest stars]] from 20,000 years ago until 80,000 years in the future]]
[[Proxima Centauri]] is the closest star to the Sun at a position currently 4.24 light-years distant from it. However, despite Barnard's Star's even closer pass to the Sun in 11,800 CE, it will still not then be the nearest star, since by that time Proxima Centauri will have moved to a yet-nearer proximity to the Sun.<ref>{{cite journal|last1=Matthews|first1=R. A. J.|title=The Close Approach of Stars in the Solar Neighborhood|journal=[[Quarterly Journal of the Royal Astronomical Society]]|year=1994|volume=35|pages=1–9|bibcode=1994QJRAS..35....1M|last2=Weissman|first2=P. R.|last3=Preston|first3=R. A.|last4=Jones|first4=D. L.|last5=Lestrade|first5=J.-F.|last6=Latham|first6=D. W.|last7=Stefanik|first7=R. P.|last8=Paredes|first8=J. M.}}</ref> At the time of the star's closest pass by the Sun, Barnard's Star will still be too dim to be seen with the naked eye, since its apparent magnitude will only have increased by one magnitude to about 8.5 by then, still being 2.5 magnitudes short of visibility to the naked eye.

Barnard's Star has a mass of about 0.16 [[solar mass]]es ({{Solar mass|link=y}}),<ref name=Pineda2021/> and a radius about 0.2 times that of the Sun.<ref name=Dawson/><ref name=Ochsenbein>{{cite journal|last=Ochsenbein|first=F.|date=March 1982|title=A list of stars with large expected angular diameters|journal=[[Astronomy and Astrophysics Supplement Series]]|volume=47|pages=523–531|bibcode=1982A&AS...47..523O}}</ref> Thus, although Barnard's Star has roughly 150 times the mass of Jupiter ({{Jupiter mass|link=y}}), its radius is only roughly 2 times larger, due to its much higher density. Its [[effective temperature]] is about 3,220 [[kelvin]], and it has a luminosity of only 0.0034 [[solar luminosities]].<ref name=Pineda2021/> Barnard's Star is so faint that if it were at the same distance from Earth as the Sun is, it would appear only 100 times brighter than a full moon, comparable to the brightness of the Sun at 80 [[astronomical unit]]s.<ref name=SolStation>{{cite web|url=http://www.solstation.com/stars/barnards.htm|title=Barnard's Star|publisher=Sol Station|access-date=10 August 2006|archive-url=https://web.archive.org/web/20060820111502/http://www.solstation.com/stars/barnards.htm|archive-date=20 August 2006|url-status=live}}</ref>

Barnard's Star has 10–32% of the solar [[metallicity]].<ref name=Gizis/> Metallicity is the proportion of stellar mass made up of elements heavier than [[helium]] and helps classify stars relative to the galactic population. Barnard's Star seems to be typical of the old, red dwarf [[population II star]]s, yet these are also generally metal-poor [[Stellar halo|halo stars]]. While sub-solar, Barnard's Star's metallicity is higher than that of a halo star and is in keeping with the low end of the metal-rich [[disk star]] range; this, plus its high space motion, have led to the designation "intermediate population II star", between a halo and disk star.<ref name=Gizis/><ref name=Kurster>{{cite journal|last1=Kürster|first1=M.|date=23 May 2003|title=The low-level radial velocity variability in Barnard's Star|journal=Astronomy and Astrophysics|bibcode=2003A&A...403.1077K|doi=10.1051/0004-6361:20030396|volume=403|issue=6|pages=1077–1088|last2=Endl|first2=M.|last3=Rouesnel|first3=F.|last4=Els|first4=S.|last5=Kaufer|first5=A.|last6=Brillant|first6=S.|last7=Hatzes|first7=A. P.|last8=Saar|first8=S. H.|last9=Cochran|first9=W. D.|arxiv=astro-ph/0303528|s2cid=16738100}}</ref> However, some recently published scientific papers have given much higher estimates for the metallicity of the star, very close to the Sun's level, between 75 and 125% of the solar metallicity.<ref name=RajpurohitAllard2018>{{cite journal|last1=Rajpurohit|first1=A. S.|last2=Allard|first2=F.|last3=Rajpurohit|first3=S.|last4=Sharma|first4=R.|last5=Teixeira|first5=G. D. C.|last6=Mousis|first6=O.|last7=Kamlesh|first7=R.|display-authors=2|title=Exploring the stellar properties of M dwarfs with high-resolution spectroscopy from the optical to the near-infrared|journal=Astronomy & Astrophysics|volume=620|year=2018|pages=A180|issn=0004-6361|arxiv=1810.13252|bibcode=2018A&A...620A.180R|doi=10.1051/0004-6361/201833500|doi-access=free|s2cid=204200655}}</ref><ref>{{cite web|url=https://vizier.u-strasbg.fr/viz-bin/VizieR-5?-ref=VIZ5e313a3d4cc3&-out.add=.&-source=J/A%2bA/620/A180/table2&recno=213|title=VizieR record for Barnard's Star|website=[[VizieR]]|publisher=[[Centre de Données astronomiques de Strasbourg]]}}</ref>