Editing Alpha Centauri (section)

== Observation ==
{{Location map |100x100
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| caption = Location of {{nobr|α Cen}} in Centaurus
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| lat   = 17 | long = 24
| mark  = Cercle rouge 100%.svg | marksize=20
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To the naked eye, {{nobr|α Centauri AB}} appear to be a single star, the [[List of stars in Centaurus|brightest in the southern constellation of Centaurus]].<ref name=Moore-2002>{{cite book |editor1-last=Moore |editor1-first=Patrick |year=2002 |title=Astronomy Encyclopedia |publisher=Philip's |isbn=978-0-540-07863-9 |url=https://books.google.com/books?id=uJxWDwAAQBAJ&pg=PP5 }}{{dead link|date=May 2024}}</ref> Their apparent angular separation varies over about 80 years between 2 and 22 [[arcsecond]]s (the [[naked eye]] has a resolution of 60 arcsec),<ref>{{cite book |first=Johannes Ebenhaezer |last=van Zyl |year=1996 |title=Unveiling the Universe: An introduction to astronomy |publisher=Springer |isbn=978-3-540-76023-8 |url-access=registration |url=https://archive.org/details/unveilingunivers01vanz }}</ref> but through much of the orbit, both are easily resolved in binoculars or small telescopes.<ref name=AOST2a>{{cite book |last1=Hartung |first1=E.J. |last2=Frew |first2= David |last3=Malin |first3= David |year=1994 |title=Astronomical Objects for Southern Telescopes |publisher=Cambridge University Press}}</ref> At −0.27 [[apparent magnitude]] (combined for A and B magnitudes {{crossreference|1=(see {{Slink|Apparent magnitude|Magnitude addition}})}}), Alpha Centauri is a [[first-magnitude star]] and is fainter only than [[Sirius]] and [[Canopus]].<ref name=Moore-2002/> It is the outer star of The Pointers or The Southern Pointers,<ref name=AOST2a/> so called because the line through [[Beta Centauri]] (Hadar/Agena),<ref name=NortonSA>{{cite book |last1=Norton |first1=A.P. |first2=Ed. I. |last2=Ridpath |year=1986 |title=Norton's 2000.0: Star Atlas and Reference Handbook|publisher=Longman Scientific and Technical|pages=39–40}}</ref> some 4.5° west,<ref name=AOST2a/> points to the constellation [[Crux]]—the Southern Cross.<ref name=AOST2a/><ref name="AOST2">{{cite book|last1=Hartung |first1=E.J. |last2=Frew |first2=D. |last3=Malin |first3=D. |year=1994 |title=Astronomical Objects for Southern Telescopes |page=194 |publisher=Melbourne University Press|isbn=978-0-522-84553-2 |url=https://books.google.com/books?id=FTsDDQAAQBAJ&pg=PT3}}</ref> The Pointers easily distinguish the true Southern Cross from the fainter [[asterism (astronomy)|asterism]] known as the [[False Cross]].<ref>{{cite book |last=Mitton |first=Jacquelin |year=1993 |title=The Penguin Dictionary of Astronomy |url=https://archive.org/details/penguindictionar00mitt |url-access=limited |page=[https://archive.org/details/penguindictionar00mitt/page/148 148] |publisher=Penguin Books |isbn=9780140512267}}</ref>

South of about 29° South latitude, {{nobr|α Cen}} is [[Circumpolar star|circumpolar]] and never sets below the horizon.{{efn|
This is calculated for a fixed latitude by knowing the star's [[declination]] ({{mvar|δ}}) using the formulae (90°+ {{mvar|δ}}). {{nobr|α Centauri's}} declination is −60° 50′, so the observed [[latitude]] where the star is circumpolar will be south of −29° 10′ South or 29°. Similarly, the place where Alpha Centauri never rises for northern observers is north of the latitude (90°+ {{mvar|δ}}) N or +29° North.
}} North of about 29° N latitude, Alpha Centauri never rises. Alpha Centauri lies close to the southern horizon when viewed from latitude 29° N to the [[equator]] (close to [[Hermosillo]] and [[Chihuahua City]] in [[Mexico]]; [[Galveston, Texas]]; [[Ocala, Florida]]; and [[Lanzarote]], the [[Canary Islands]] of [[Spain]]), but only for a short time around its [[culmination]].<ref name=NortonSA/> The star culminates each year at local midnight on 24 April and at local 9 p.m. on 8 June.<ref name=NortonSA/><ref>{{cite web |last=James |first= Andrew  |title=Culmination Times |department = The Constellations, Part&nbsp;2 |website=Southern Astronomical Delights (southastrodel.com) |location=Sydney, New South Wales |url=http://www.southastrodel.com/Page20502.htm |access-date=6 August 2008 }}</ref>

As seen from Earth, Proxima Centauri is 2.2° southwest from {{nobr|α Centauri AB;}} this distance is about four times the [[angular diameter]] of the [[Moon]].<ref name=Matt93>{{cite journal |last1=Matthews |first1= R.A.J. |last2=Gilmore |first2= Gerard |year=1993 |title=Is Proxima really in orbit about {{nobr|α Cen A/B ?}} |journal=Monthly Notices of the Royal Astronomical Society |volume=261|pages=L5–L7|bibcode=1993MNRAS.261L...5M |doi=10.1093/mnras/261.1.l5 |doi-access=free}}</ref> Proxima Centauri appears as a deep-red star of a typical apparent magnitude of 11.1 in a sparsely populated star field, requiring moderately sized telescopes to be seen. Listed as V645&nbsp;Cen in the ''[[General Catalogue of Variable Stars]]'', version&nbsp;4.2, this [[UV Ceti star]] or "flare star" can unexpectedly brighten rapidly by as much as 0.6 [[Magnitude (astronomy)|magnitude]] at visual wavelengths, then fade after only a few minutes.<ref name=bendict1998>{{cite conference |last1=Benedict |first1=G. Fritz |last2=McArthur |first2=Barbara |last3=Nelan |first3=E. |last4=Story |first4=D. |last5=Whipple |first5=A.L. |last6=Shelus |first6=P.J. |last7=Jefferys |first7=W. H.|last8=Hemenway|first8=P.D. |last9=Franz |first9=Otto G. |last10=Wasserman |first10=L.H. |last11=van Altena |first11=W. |last12=Fredrick |first12=L.W. |display-authors=6 |year=1998 |title=Proxima Centauri: Time-resolved astrometry of a flare site using HST fine guidance sensor&nbsp;3 |editor1-first=R.A. |editor1-last=Donahue |editor2-first=J.A. |editor2-last=Bookbinder |conference=The Tenth Cambridge Workshop on Cool Stars, Stellar Systems and the Sun |series = [[Astronomical Society of the Pacific|ASP]] Conference Series |volume = 154 |page = 1212 |bibcode=1998ASPC..154.1212B}}</ref> Some amateur and professional astronomers regularly monitor for outbursts using either optical or radio telescopes.<ref>{{cite journal |last=Page |first= A.A. |year=1982 |title=Mount Tamborine Observatory |journal=International Amateur-Professional Photoelectric Photometry Communication |volume=10|page=26|bibcode=1982IAPPP..10...26P}}</ref> In August&nbsp;2015, the largest recorded flares of the star occurred, with the star becoming 8.3&nbsp;times brighter than normal on 13&nbsp;August, in the [[Apparent magnitude#Standard reference values|B&nbsp;band (blue light region)]].<ref name="proximaflare">{{cite web |title=Light curve generator (LCG) |website = [[American Association of Variable Star Observers]] (aavso.org) |url=https://www.aavso.org/lcg/plot?auid=000-BCV-333&starname=V645%20CEN&lastdays=200&start=2457230&stop=2457270&obscode=&obscode_symbol=2&obstotals=yes&calendar=calendar&forcetics=&pointsize=1&width=800&height=450&mag1=&mag2=&mean=&vmean=&grid=on&visual=on&uband=on&bband=on&v=on |access-date=7 June 2017 |url-status=dead |archive-url=https://web.archive.org/web/20200725055311/https://www.aavso.org/lcg/plot?auid=000-BCV-333&starname=V645%20CEN&lastdays=200&start=2457230&stop=2457270&obscode=&obscode_symbol=2&obstotals=yes&calendar=calendar&forcetics=&pointsize=1&width=800&height=450&mag1=&mag2=&mean=&vmean=&grid=on&visual=on&uband=on&bband=on&v=on |archive-date=25 July 2020 }}</ref>

=== Observational history ===
[[File:The Very Large Telescope and the star system Alpha Centauri.jpg|thumb|upright=1.2|The [[Very Large Telescope]] open to the night sky, with the Milky Way running diagonally across the sky above it and many southern stars and constellations labelled and connected by lines, including Alpha Centauri and the not visible Proxima Centauri.|alt=Image of a very large telescope dome open to the night sky, with the Milky Way running diagonally across the sky above it and many southern stars and constellations labelled and connected by lines]]

Alpha Centauri is listed in the 2nd&nbsp;century star catalog appended to [[Claudius Ptolemaeus|Ptolemy]]'s ''[[Almagest]]''.  Ptolemy gave its [[ecliptic coordinates]], but texts differ as to whether the ecliptic latitude reads {{nobr|44° 10′ South}} or {{nobr|41° 10′ South}}.<ref>{{cite book|url=https://isidore.co/calibre/get/pdf/Ptolemy%26%2339%3Bs%20Almagest%20-%20Ptolemy%2C%20Claudius%20%26amp%3B%20Toomer%2C%20G.%20J__5114.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://isidore.co/calibre/get/pdf/Ptolemy%26%2339%3Bs%20Almagest%20-%20Ptolemy%2C%20Claudius%20%26amp%3B%20Toomer%2C%20G.%20J__5114.pdf |archive-date=2022-10-09 |url-status=live|title=Ptolemy's Almagest
|publisher=Gerald Duckworth & Co.|location=London|first=Claudius|last=Ptolemaeus|translator-first=G. J.|translator-last=Toomer|page=368, note 136|date=1984|isbn=978-0-7156-1588-1|access-date=22 December 2017}}{{dead link|date=June 2021|bot=medic}}{{cbignore|bot=medic}}</ref> (Presently the ecliptic latitude is {{nowrap|43.5° South}}, but it has decreased by a fraction of a degree since [[Ptolemy]]'s time due to [[proper motion]].) In Ptolemy's time, Alpha Centauri was visible from [[Alexandria, Egypt]], at {{nobr|31° N,}}<!-- Its southerly declination was about –53° S --> but, due to [[Precession of the equinoxes|precession]], its declination is now {{nobr|–60° 51′ South}}, and it can no longer be seen at that latitude. English explorer [[Robert Hues]] brought Alpha Centauri to the attention of European observers in his 1592 work ''Tractatus de Globis'', along with Canopus and [[Achernar]], noting:

{{Blockquote|Now, therefore, there are but three [[First magnitude star|Stars of the first magnitude]] that I could perceive in all those parts which are never seene here in [[England]]. The first of these is that bright Star in the sterne of [[Argo Navis|Argo]] which they call Canobus [Canopus]. The second [Achernar] is in the end of [[Eridanus (constellation)|Eridanus]]. The third [Alpha Centauri] is in the right foote of the [[Centaurus|Centaure]].<ref name="knobel">{{cite journal |last=Knobel |first= Edward B. |author-link=Edward Knobel |year=1917 |title=On Frederick de Houtman's ''Catalogue of Southern Stars'', and the origin of the southern constellations |journal=Monthly Notices of the Royal Astronomical Society |volume=77 |issue=5|pages=414–432 [416] |bibcode=1917MNRAS..77..414K |doi=10.1093/mnras/77.5.414 |doi-access=free}}</ref>}}

The [[Binary star|binary]] nature of Alpha Centauri AB was recognized in December 1689 by Jean Richaud, while observing a passing [[comet]] from his station in [[Puducherry (union territory)|Puducherry]]. Alpha Centauri was only the third binary star to be discovered, preceded by [[Mizar|Mizar AB]] and [[Acrux]].<ref name=KameswaraRao1984>{{cite journal |last1=Kameswara-Rao |first1=N. |last2=Vagiswari |first2=A. |last3=Louis |first3=C. |year=1984  |title=Father J. Richaud and early telescope observations in India |journal=Bulletin of the Astronomical Society of India |volume=12 |page=81 |bibcode=1984BASI...12...81K}}</ref>

The large proper motion of Alpha Centauri AB was discovered by [[Manuel John Johnson]], observing from [[Saint Helena]], who informed [[Thomas Henderson (astronomer)|Thomas Henderson]] at the [[Royal Observatory, Cape of Good Hope]] of it. The [[Parallax in astronomy|parallax]] of Alpha Centauri was subsequently determined by Henderson from many exacting positional observations of the AB system between April&nbsp;1832 and May&nbsp;1833. He withheld his results, however, because he suspected they were too large to be true, but eventually published them in 1839 after [[Friedrich Wilhelm Bessel|Bessel]] released his own accurately determined parallax for {{nobr|[[61 Cygni]]}} in 1838.<ref name="autogenerated345">{{cite book |first=Anton |last=Pannekoek |year=1989 |orig-year=1961 |title=A History of Astronomy |edition=reprint |pages=345–346 |publisher=Dover |isbn=978-0-486-65994-7  |url=https://books.google.com/books?id=I1LGdDe0NYcC&pg=PA2 }}</ref> For this reason, Alpha Centauri is sometimes considered as the second star to have its distance measured because Henderson's work was not fully acknowledged at first.<ref name="autogenerated345"/> (The distance of Alpha Centauri from the Earth is now reckoned at 4.396&nbsp;[[light-year]]s or {{cvt|4.396|ly|km|disp=out}}.)

[[File:South celestial pole.png|thumb|Alpha Centauri (Rigel Kentaurus) around the [[South celestial pole]]]]

[[John Herschel]] made the first [[Micrometer (device)|micrometrical]] observations in 1834.<ref name=JHerschel1847>{{cite book |last=Herschel |first=J.F.W. |author-link=John Herschel |year=1847 |title=Results of astronomical observations made during the years 1834, 5, 6, 7, 8 at the Cape of Good Hope; being the completion of a telescopic survey of the whole surface of the visible heavens, commenced in 1825 |publisher=Smith, Elder and Co, London|bibcode=1847raom.book.....H}}</ref> Since the early 20th century, measures have been made with [[photographic plates]].<ref name="adsabs.harvard.edu">{{cite journal|last1=Kamper|first1=K. W. |last2=Wesselink |first2=A. J. |year=1978 |title=Alpha and Proxima Centauri |journal=Astronomical Journal|volume=83 |page=1653 |bibcode=1978AJ.....83.1653K |doi=10.1086/112378 |doi-access=free}}</ref>

By 1926, [[William Stephen Finsen]] calculated the approximate [[Orbital elements|orbit elements]] close to those now accepted for this system.<ref name="Aitken"/> All future positions are now sufficiently accurate for visual observers to determine the relative places of the stars from a binary star [[ephemeris]].<ref>{{citation-attribution|1={{cite web|url=http://ad.usno.navy.mil/wds/orb6/orb6ephem.html|title=Sixth Catalogue of Orbits of Visual Binary Stars: Ephemeris (2008)|publisher=U.S. Naval Observatory|access-date=13 August 2008|archive-url=https://web.archive.org/web/20090113210000/http://ad.usno.navy.mil/wds/orb6/orb6ephem.html
|archive-date=13 January 2009|url-status=dead}} }}</ref> Others, like D. Pourbaix (2002), have regularly refined the precision of new published orbital elements.<ref name="SixthCatOrbVisBin"/>

[[Robert T. A. Innes]] discovered [[Proxima Centauri]] in 1915 by blinking photographic plates taken at different times during a [[proper motion]] survey. These showed large proper motion and parallax similar in both size and direction to those of {{nobr|α Centauri AB,}} which suggested that Proxima Centauri is part of the {{nobr|α Centauri}} system and slightly closer to Earth than {{nobr|α Centauri AB}}. As a result, Innes concluded that Proxima Centauri was the closest star to Earth yet discovered.