Editing Alpha Centauri (section)

== Location and motion ==
Alpha Centauri may be inside the [[G-cloud]] of the [[Local Bubble]],<ref>{{cite journal | last1=Linsky | first1=Jeffrey L. | last2=Redfield | first2=Seth | last3=Tilipman | first3=Dennis | date=November 2019 | title=The interface between the outer heliosphere and the inner local ISM: Morphology of the local interstellar cloud, its hydrogen hole, Strömgren shells, and <sup>60</sup>Fe accretion | journal=The Astrophysical Journal | volume=886 | issue=1 | id=41 | pages=19 | doi=10.3847/1538-4357/ab498a | doi-access=free | arxiv=1910.01243 | bibcode=2019ApJ...886...41L | s2cid=203642080 }}</ref> and its nearest known system is the binary [[brown dwarf]] system [[Luhman 16]], at {{convert|3.6|ly|pc|lk=on|abbr=off}} distance.<ref name=substellarcompanion>{{cite journal |last1=Boffin |first1=Henri M.J. |last2=Pourbaix |first2=D. |last3=Mužić |first3=K. |last4=Ivanov |first4=V.D. |last5=Kurtev |first5=R. |last6=Beletsky |first6=Y. |last7=Mehner|first7=A.|last8=Berger |first8=J.P. |last9=Girard |first9=J.H. |last10=Mawet |first10=D. |display-authors=6 |date=4 December 2013 |title=Possible astrometric discovery of a substellar companion to the closest binary brown dwarf system WISE J104915.57–531906.1 |journal=[[Astronomy and Astrophysics]] |volume=561 |pages=L4 |arxiv=1312.1303 |bibcode=2014A&A...561L...4B |doi=10.1051/0004-6361/201322975 |s2cid=33043358}}</ref>{{failed verification|date=May 2025}}

=== Historical distance estimates ===

:{| class="wikitable sortable mw-collapsible"
|+ Alpha Centauri AB historical distance estimates
|-
! rowspan="2" | Source 
! rowspan="2" |Year
! rowspan="2" |Subject!! rowspan="2" | Parallax ([[milliarcsecond|mas]]) !! colspan="3" | Distance !! rowspan="2" | References
|-
![[parsec]]s !! [[light-year]]s !! [[petametre]]s
|-
| H. Henderson || 1839 || AB || {{val|1160|110}} || {{val|0.86|0.09|0.07}} || 2.81 ± 0.53 || {{val|26.6|2.8|2.3}} ||<ref name="Henderson1839" />
|-
| T. Henderson
|1842
|AB
| {{val|912.8|64}}
| 1.10 ± 0.15
| 3.57 ± 0.5
| {{val|33.8|2.5|2.2}}
|<ref>{{cite journal |last=Henderson|first=T.|title=The parallax of {{nobr|α Centauri,}} deduced from Mr. Maclear's observations at the Cape of Good Hope, in the years 1839 and 1840 |journal=Memoirs of the Royal Astronomical Society |volume=12 |pages=370–371 |year=1842 |bibcode=1842MmRAS..12..329H}}</ref>
|-
| Maclear
|1851
|AB
| {{val|918.7|34}}
| {{val|1.09|0.04}}
| {{val|3.55|0.14|0.13}}
| 32.4 ± 2.5
|<ref>{{cite journal|last=Maclear|first=T.|title=Determination of the Parallax of α 1 and α2 Centauri, from Observations made at the Royal Observatory, Cape of Good Hope, in the Years 1842-3-4 and 1848
|journal=Memoirs of the Royal Astronomical Society|volume=20|page=98|year=1851|bibcode=1851MmRAS..20...70M}}</ref>
|-
| Moesta
|1868
|AB
| {{val|880|68}}
| {{val|1.14|0.10|0.08}}
| {{val|3.71|0.31|0.27}}
| {{val|35.1|2.9|2.5}}
|<ref>{{cite journal|last=Moesta|first=C. G.|title=Bestimmung der Parallaxe von α und β Centauri|trans-title=Determining the parallax of α and β Centauri|language=de|journal=Astronomische Nachrichten|year=1868
|volume=71|issue=8|pages=117–118|bibcode=1868AN.....71..113M|doi=10.1002/asna.18680710802|url=https://zenodo.org/record/1424671}}</ref>
|-
| Gill & Elkin
|1885
|AB
| {{val|750|10}}
| {{val|1.333|0.018}}
| {{val|4.35|0.06}}
| {{val|41.1|0.6|0.5}}
|<ref>{{cite journal|last1=Gill|first1=David|last2=Elkin|first2=W. L.|title=Heliometer-Determinations of Stellar Parallax in the Southern Hemisphere|journal=Memoirs of the Royal Astronomical Society|year=1885
|volume=48|page=188|bibcode=1885MmRAS..48....1G}}</ref>
|-
| Roberts
|1895
|AB
| {{val|710|50}}
| 1.32 ± 0.2
| 4.29 ± 0.65
| {{val|43.5|3.3|2.9}}
|<ref>{{cite journal|last=Roberts|first=Alex W.|title=Parallax of α Centauri from Meridian Observations 1879–1881|journal=Astronomische Nachrichten|year=1895|volume=139|issue=12|pages=189–190 |bibcode=1895AN....139..177R|doi=10.1002/asna.18961391202|url=https://zenodo.org/record/1424747}}</ref>
|-
| Woolley ''et al.''
|1970
|AB
| {{val|743|7}}
| {{val|1.346|0.013}}
| {{val|4.39|0.04}}
| {{val|41.5|0.4}}
|<ref name="Woolley1970">{{cite journal|url=http://webviz.u-strasbg.fr/viz-bin/VizieR-5?-source=V/32A&Woolley=559|title=Woolley 559|volume=5|page=ill|journal=Catalogue of Stars within 25 Parsecs of the Sun
|last1=Woolley|first1=R.|last2=Epps|first2=E. A.|last3=Penston|first3=M. J.|last4=Pocock|first4=S. B.|year=1970|bibcode=1970ROAn....5.....W|access-date=9 May 2014|archive-url=https://web.archive.org/web/20171008231243/http://webviz.u-strasbg.fr/viz-bin/VizieR-5?-source=V%2F32A&Woolley=559|archive-date=8 October 2017|url-status=live}}</ref>
|-
| Gliese & Jahreiß
|1991
|AB
| {{val|749.0|4.7}}
| {{val|1.335|0.008}}
| {{val|4.355|0.027}}
| {{val|41.20|0.26}}
|<ref name="Gliese1991">{{cite web|url=http://webviz.u-strasbg.fr/viz-bin/VizieR-5?-source=V/70A&Name=Gl%20559|title=Gl 559|work=Preliminary Version of the Third Catalogue of Nearby Stars |publisher=Astronomische Rechen-Institut|last1=Gliese|first1=W.|last2=Jahreiß|first2=H.|year=1991|access-date=9 May 2014}}</ref>
|-
| van Altena ''et al.''
| 1995
| AB
| {{val|749.9|5.4}}
| {{val|1.334|0.010}}
| {{val|4.349|0.032|0.031}}
| {{val|41.15|0.30|0.29}}
|<ref>{{cite report |last1=van Altena |first1=W.F. |last2=Lee |first2=J.T. |last3=Hoffleit |first3=E.D. |year=1995 |section=GCTP 3309 |title=The General Catalogue of Trigonometric Stellar Parallaxes |publisher=Yale University Observatory |edition=4th |url=http://webviz.u-strasbg.fr/viz-bin/VizieR-5?-source=I/238A/picat&GCTP=3309 |access-date=9 May 2014}}</ref>
|-
| Perryman ''et al.''
| 1997
| AB
| {{val|742.12|1.40}}
| {{val|1.3475|0.0025}}
| {{val|4.395|0.008}}
| {{val|41.58|0.08}}<ref name=Perryman1997_tyc_A>
{{cite report
 |last=Perryman |first= |year=1997 |display-authors=etal
 |title=HIP 71683
 |series=The Hipparcos and Tycho Catalogues
 |url=http://webviz.u-strasbg.fr/viz-bin/VizieR-5?-source=I/239/tyc_main&HIP=71683
 |access-date=9 May 2014
}}
</ref><ref name=Perryman1997_tyc_B>
{{cite report
 |last=Perryman |first= |year=1997 |display-authors=etal
 |title=HIP 71681
 |series=The Hipparcos and Tycho Catalogues
 |url=http://webviz.u-strasbg.fr/viz-bin/VizieR-5?-source=I/239/tyc_main&HIP=71681
 |access-date=9 May 2014}}</ref>
|-
| Söderhjelm
| 1999
| AB
| {{val|747.1|1.2}}
| {{val|1.3385|0.0022|0.0021}}
| {{val|4.366|0.007}}
| {{val|41.30|0.07}}
|<ref>{{cite report |last =Söderhjelm |first =Staffan |year=1999 |title=HIP 71683 |section=Visual binary orbits and masses post Hipparcos |url=http://webviz.u-strasbg.fr/viz-bin/VizieR-5?-source=J/A+A/341/121&HIP=71683 |access-date=9 May 2014}}</ref>
|-
|rowspan="2"| van Leeuwen
|rowspan="2"| 2007
| A
| {{val|754.81|4.11}}
| {{val|1.325|0.007}}
| {{val|4.321|0.024|0.023}}
| {{val|40.88|0.22}}
|<ref>{{cite report |last=van Leeuwen |first=Floor |year=2007 |title=HIP 71683 |section=Validation of the new Hipparcos reduction |url=http://webviz.u-strasbg.fr/viz-bin/VizieR-5?-source=I/311&HIP=71683}}</ref>
|-
| B
| {{val|796.92|25.90}}
| {{val|1.25|0.04}}
| {{val|4.09|0.14|0.13}}
| 37.5 ± 2.5
|<ref>{{cite report |last=van Leeuwen |first=Floor |year=2007 |title=HIP 71681 |section=Validation of the new Hipparcos reduction |url=http://webviz.u-strasbg.fr/viz-bin/VizieR-5?-source=I/311&HIP=71681 }}</ref>
|-
| RECONS TOP100
|2012
|AB
| {{val|747.23|1.17}}{{efn|
Weighted parallax based on parallaxes from {{harvp|van Altena|Lee||Hoffleit|1995}} and {{harvp|Söderhjelm|1999}}.
}}
| {{val|1.3383|0.0021}}
| {{val|4.365|0.007}}
| {{val|41.29|0.06}}
|<ref name="RECONS">{{cite report |series=Research Consortium on Nearby Stars |publisher=Georgia State University |date=7 September 2007 |title=The one hundred nearest star systems |url=http://www.astro.gsu.edu/RECONS/TOP100.posted.htm |access-date=2 December 2014 |url-status=live |archive-url=https://web.archive.org/web/20071112173559/http://www.chara.gsu.edu/RECONS/TOP100.posted.htm |archive-date=12 November 2007 }}</ref>
|}

=== Kinematics ===
[[File:Angular map of fusors around Sol within 9ly (large).png|thumb|upright=1.2|Alpha Centauri (with unlabeled Proxima Centauri) on a radar map of all known stellar and [[substellar]] objects within 9 light years (ly), arranged clockwise in [[hour angle|hours]] of [[right ascension]], and  marked by distance (▬) and position (◆). Distances are marked outward from the [[Sun|Sun (Sol)]], with concentric circles indicating the distance in one ly steps. Positions are marked inward from their distance markings, connected by lines according to their [[declination]]s (doted when positive), representing the [[Circular arc|arcs]] of the declinations viewed edge-on.]]
All components of {{nobr|α Centauri}} display significant [[proper motion]] against the background sky. Over centuries, this causes their apparent positions to slowly change.<ref>{{cite web |publisher=ESA |website=Hipparcos mission website |url=http://www.rssd.esa.int/index.php?project=HIPPARCOS&page=high_p |title=High-Proper Motion Stars (2004) }}</ref> Proper motion was unknown to ancient astronomers. Most assumed that the stars were permanently fixed on the [[celestial sphere]], as stated in the works of the philosopher Aristotle.<ref>{{cite web |last=Aristotle |url=https://ebooks.adelaide.edu.au/a/aristotle/heavens/book2.html |title=De Caelo |trans-title = On the Heavens |at=Book II Part 11 |publication-date = 2004 |access-date=6 August 2008|archive-url=https://web.archive.org/web/20080823061709/http://ebooks.adelaide.edu.au/a/aristotle/heavens/book2.html|archive-date=23 August 2008|url-status=dead}}</ref> In 1718, [[Edmond Halley]] found that some stars had significantly moved from their ancient [[astrometric]] positions.<ref name=berry>{{cite book |first=Arthur |last=Berry |title=A Short History of Astronomy|url=https://books.google.com/books?id=QBJcswEACAAJ|date=6 February 2018|publisher=Creative Media Partners, LLC|isbn=978-1-376-81951-9|pages=357–358}}</ref>

In the 1830s, [[Thomas Henderson (astronomer)|Thomas Henderson]] discovered the true distance to {{nobr|α Centauri}} by analysing his many astrometric mural circle observations.<ref name="Henderson1839">{{cite journal|last=Henderson |first= H.|year=1839|title=On the parallax of α Centauri|journal=Monthly Notices of the Royal Astronomical Society|volume=4|issue=19|pages=168–169|bibcode=1839MNRAS...4..168H |doi=10.1093/mnras/4.19.168|url=https://zenodo.org/record/1431843|doi-access=free}}</ref><ref>{{cite web |publisher=Astronomical Society of South Africa |url=http://www.saao.ac.za/assa/html/his-astr-henderson_t.html |title=Henderson, Thomas [FRS] |year = 2008 |url-status=dead |archive-url=https://archive.today/20120909154524/http://www.saao.ac.za/assa/html/his-astr-henderson_t.html|archive-date=9 September 2012}}</ref> He then realised this system also likely had a high proper motion.<ref>{{cite book |first=Anton |last=Pannekoek |year=1989 |title=A History of Astronomy |url=https://books.google.com/books?id=I1LGdDe0NYcC |publisher=Courier Corporation|isbn=978-0-486-65994-7 |page=333}}</ref><ref>{{cite journal |last=Maclear |first= M. |year=1851 |title=Determination of parallax of α{{sup|1}} and {{nobr|α{{sup|2}} Centauri}} |journal=Astronomische Nachrichten |volume=32 |issue=16 |pages=243–244 |doi=10.1002/asna.18510321606 |bibcode=1851MNRAS..11..131M }}</ref><ref name=Aitken/> In this case, the apparent stellar motion was found using [[Nicolas Louis de Lacaille]]'s astrometric observations of 1751–1752,<ref>{{cite book |first1=de la Caillé |last1=N. L. |translator=Raven-Hart, R. |year=1976 |title=Travels at the Cape, 1751–1753: An annotated translation of journal historique du voyage fait au Cap de Bonne-Espérance |publisher=Cape Town |isbn=978-0-86961-068-8 }}</ref> by the observed differences between the two measured positions in different epochs.

Calculated proper motion of the centre of mass for {{nobr|α Centauri AB}} is about 3620&nbsp;mas/y (milliarcseconds per year) toward the west and 694&nbsp;mas/y toward the north, giving an overall motion of 3686&nbsp;mas/y in a direction 11° north of west.<ref name=Kervella2016>{{cite journal|first1=Pierre |last1=Kervella |display-authors=etal |year=2016 |title=Close stellar conjunctions of {{nobr|α Centauri A}} and B until 2050 An {{mvar|m}}{{sub|K}} = 7.8 star may enter the Einstein ring of {{nobr|α Cen A}} |journal=Astronomy & Astrophysics |volume=594 |issue=107 |page=A107 |arxiv=1610.06079 |doi=10.1051/0004-6361/201629201 |bibcode=2016A&A...594A.107K |s2cid=55865290}}</ref>{{efn|
Proper motions are expressed in smaller angular units than arcsec, being measured in milliarcsec (mas.) (thousandths of an arcsec). Negative values for proper motion in RA indicate the sky motion is from east to west, and in declination north to south.
}} The motion of the centre of mass is about 6.1&nbsp;[[arcmin]] each century, or 1.02[[degree (angle)|°]] each millennium. The speed in the western direction is {{cvt|23.0|km/s}} and in the northerly direction {{cvt|4.4|km/s}}. Using [[spectroscopy]] the mean radial velocity has been determined to be around {{cvt|22.4|km/s}} towards the Solar System.<ref name=Kervella2016/> This gives a speed with respect to the Sun of {{cvt|32.4|km/s}}, very close to the peak in the distribution of speeds of nearby stars.<ref>{{cite arXiv |last1=Marshall Eubanks |first1=T. |last2=Hein |first2=Andreas M. |last3=Lingam |first3=Manasvi |last4=Hibberd |first4=Adam |last5=Fries |first5=Dan |last6=Perakis |first6=Nikolaos
|last7=Kennedy |first7=Robert |last8=Blase |first8=W.P. |last9=Schneider|first9=Jean |display-authors=6 |year=2021 |title=Interstellar objects in the Solar System: 1. Isotropic kinematics from the Gaia early data release&nbsp;3
|class=astro-ph.EP |eprint=2103.03289}}</ref>

Since {{nobr|α Centauri AB}} is almost exactly in the plane of the [[Milky Way]] as viewed from Earth, many stars appear behind it. In early May&nbsp;2028, {{nobr|α Centauri A}} will pass between the Earth and a distant red star, when there is a 45% probability that an [[Einstein ring]] will be observed. Other [[conjunction (astronomy)|conjunction]]s will also occur in the coming decades, allowing accurate measurement of proper motions and possibly giving information on planets.<ref name=Kervella2016/>

==== Predicted future changes ====
[[File:Near-stars-past-future-en.svg|thumb|upright=1.2|left|Distances of the [[List of nearest stars|nearest stars]] from 20,000&nbsp;years ago until 80,000&nbsp;years in the future{{citation needed|date=January 2025}}|alt=Line graph with x-axis in thousands of years and y-axis in light years, the lines on the graph being labelled with the names of stars.]]
[[File:Alpha Cen proper motions.gif|thumb|upright=1.2|Animation showing motion of {{nobr|α Centauri}} through the sky. (The other stars are held fixed for didactic reasons.) "Oggi" means today; "anni" means years.|alt=Animated image of a sky chart of the southern celestial hemisphere labelled with years.]]

Based on the system's common proper motion and radial velocities, {{nobr|α Centauri}} will continue to change its position in the sky significantly and will gradually brighten. For example, in about 6,200&nbsp;[[Common Era|CE]], α Centauri's true motion will cause an extremely rare [[First-magnitude star|first-magnitude]] stellar conjunction with [[Beta Centauri]], forming a brilliant optical [[double star]] in the southern sky.<ref name=AOST2/> It will then pass just north of the Southern Cross or [[Crux]], before moving northwest and up towards the present [[celestial equator]] and away from the [[galactic plane]]. By about 26,700&nbsp;[[Common Era|CE]], in the present-day constellation of [[Hydra (constellation)|Hydra]], {{nobr|α Centauri}} will reach perihelion at {{cvt|0.90|pc|ly|lk=on|disp=or}} away,<ref name=Matthews>{{cite journal |last=Matthews |first= R.A.J. |year=1994|title=The close approach of stars in the Solar neighbourhood |journal=Quarterly Journal of the Royal Astronomical Society |volume=35|pages=1–8|bibcode=1994QJRAS..35....1M}}</ref> though later calculations suggest that this will occur in 27,000 AD.<ref name=Bailer2015>{{cite journal |first=Bailer-Jones |last=C.A.L. |year=2015|title=Close encounters of the stellar kind |journal=Astronomy and Astrophysics |volume=575 |pages=A35–A48 |bibcode=2015A&A...575A..35B |arxiv=1412.3648 |doi=10.1051/0004-6361/201425221 |s2cid=59039482}}</ref> At its nearest approach, α Centauri will attain a maximum [[apparent magnitude]] of −0.86, comparable to present-day magnitude of [[Canopus]], but it will still not surpass that of [[Sirius]], which will brighten incrementally over the next 60,000&nbsp;years, and will continue to be the brightest star as seen from Earth (other than the Sun) for the next 210,000&nbsp;years.<ref>{{cite magazine |title={{grey|[no title cited]}} |magazine=[[Sky and Telescope]] |date = April 1998 |page=60 |quote= Calculation based on computations from HIPPARCOS data. }}</ref>