Editing Alpha Centauri (section)

== Stellar system ==
Alpha Centauri is a triple star system, with its two main stars, A and B, together comprising a [[Binary star|binary]] component. The ''AB'' designation, or older ''A×B'', denotes the mass centre of a main binary system relative to companion star(s) in a multiple star system.<ref name=DoubleStarsHeintz>{{cite book |last=Heintz |first=W. D. |url=https://archive.org/details/DoubleStars |title=Double Stars |date=1978 |publisher=D. Reidel |isbn=978-90-277-0885-4 |page=[https://archive.org/details/DoubleStars/page/n27 19] }}{{dead link|date=September 2023}}</ref> ''AB-C'' refers to the component of Proxima Centauri in relation to the central binary, being the distance between the centre of mass and the outlying companion. Because the distance between Proxima (C) and either of Alpha Centauri A or B is similar, the AB binary system is sometimes treated as a single gravitational object.<ref name=wds1996>{{cite book |last1=Worley|first1=C.E. |last2=Douglass |first2= G.G. |year=1996 |title=Washington Visual Double Star Catalog, 1996.0 (WDS)
|url=http://adc.gsfc.nasa.gov/adc-cgi/cat.pl?/catalogs/1/1237 |publisher=[[United States Naval Observatory]] |url-status=dead |archive-url=https://web.archive.org/web/20000422224338/http://adc.gsfc.nasa.gov/adc-cgi/cat.pl?%2Fcatalogs%2F1%2F1237 |archive-date=22 April 2000}}</ref>

=== Orbital properties ===
[[File:Orbit Alpha Centauri AB arcsec.png|thumb|upright=1.4|left|Apparent and true orbits of Alpha Centauri. The A component is held stationary, and the relative orbital motion of the B component is shown. The apparent orbit (thin ellipse) is the shape of the orbit as seen by an observer on Earth. The true orbit is the shape of the orbit viewed perpendicular to the plane of the orbital motion. According to the radial velocity versus time,<ref name="Pourbaix">{{cite journal |last=Pourbaix |first= D.|year=2002|title=Constraining the difference in convective blueshift between the components of alpha Centauri with precise radial velocities|journal=Astronomy and Astrophysics|volume=386|issue=1|pages=280–285|bibcode=2002A&A...386..280P|doi=10.1051/0004-6361:20020287|arxiv=astro-ph/0202400|s2cid=14308791|display-authors=et al.}}</ref> the radial separation of A and B along the line of sight had reached a maximum in 2007, with B being further from Earth than A. The orbit is divided here into 80 points: each step refers to a timestep of approx. 0.99888 years or 364.84 days.|alt=Graphic image of a near-circle and a narrow ellipse labelled respectively as "B's real trajectory" and "B's apparent trajectory", with years marked along portions of the ellipses.]]
[[File:Orbital plot of Proxima Centauri.jpg|thumb|upright=1.4|Orbital plot of Proxima Centauri around the bright apparent star Alpha Centauri AB, with position change marked (in thousands of years).]]

The A and B components of Alpha Centauri have an orbital period of 79.762&nbsp;years. Their orbit is moderately [[orbital eccentricity|eccentric]], as it has an eccentricity of almost 0.52;<ref name=Akeson2021/> their closest approach or [[apsis|periastron]] is {{cvt|11.2|AU|e9km}}, or about the distance between the Sun and Saturn; and their furthest separation or [[apsis|apastron]] is {{cvt|35.6|AU|e9km}}, about the distance between the Sun and Pluto.<ref name=SixthCatOrbVisBin/> The most recent [[apsis|periastron]] was in August 1955 and the next will occur in May&nbsp;2035; the most recent [[apsis|apastron]] was in May 1995 and will next occur in 2075.

Viewed from Earth, the apparent orbit of A and B means that their separation and [[position angle]] (PA) are in continuous change throughout their projected orbit. Observed stellar positions in 2019 are separated by 4.92 [[Minute and second of arc|arcsec]] through the PA of 337.1°, increasing to 5.49 arcsec through 345.3° in 2020.<ref name="SixthCatOrbVisBin"/> The closest recent approach was in February 2016, at 4.0 arcsec through the PA of 300°.<ref name="SixthCatOrbVisBin">{{citation-attribution|1={{cite news |last1=Hartkopf|first1=W. |last2=Mason |first2= D. M.|year=2008 |url=http://ad.usno.navy.mil/wds/orb6.html |title=Sixth Catalog of Orbits of Visual Binaries |publisher=U.S. Naval Observatory |access-date=26 May 2008|archive-url=https://web.archive.org/web/20090412084731/http://ad.usno.navy.mil/wds/orb6.html|archive-date=12 April 2009|url-status=dead}} }}</ref><ref>{{cite web |first = Andrew |last = James |url=http://www.southastrodel.com/PageAlphaCen006.htm |title=ALPHA CENTAURI: 6 |website=southastrodel.com |date=11 March 2008 |access-date=12 August 2010}}</ref> The observed maximum separation of these stars is about 22 arcsec, while the minimum distance is 1.7&nbsp;arcsec.<ref name=Aitken>{{cite book |title=The Binary Stars |publisher=Dover |author-link=Robert Grant Aitken |first=R.G. |last=Aitken |pages=235–237 |year=1961}}</ref> The widest separation occurred during February 1976, and the next will be in January 2056.<ref name=SixthCatOrbVisBin/>

Alpha Centauri C is about {{cvt|13000|AU|ly e12km}} from Alpha Centauri AB, equivalent to about 5% of the distance between Alpha Centauri AB and the Sun.<ref name=Kervella2017/><ref name=Matt93/><ref name="adsabs.harvard.edu"/> Until 2017, measurements of its small speed and its trajectory were of too little accuracy and duration in years to determine whether it is bound to Alpha Centauri AB or unrelated.

Radial velocity measurements made in 2017 were precise enough to show that Proxima Centauri and Alpha Centauri AB are gravitationally bound.<ref name=Kervella2017/> The orbital period of Proxima Centauri is approximately {{val|511000|41000|30000}} years, with an eccentricity of 0.5, much more eccentric than [[Mercury (planet)|Mercury]]'s. Proxima Centauri comes within {{val|4100|700|600|ul=AU}} of AB at periastron, and its apastron occurs at {{val|12300|200|100|ul=AU}}.<ref name="Akeson2021"/>

=== Physical properties ===
[[File:ESO - Alpha Centauri in the HR-System (by).jpg|thumb|ESO - Alpha Centauri in the HR-System (by)]]
[[File:Relative sizes of the Alpha Centauri components and other objects (artist’s impression).tif|thumb|Relative sizes and colour of the Alpha Centauri A, B and C (Proxima) and other [[List of nearest stars and brown dwarfs|local stars]], incl. the Sun and Jupiter (artist’s impression)]]

[[Asteroseismology|Asteroseismic]] studies, [[chromospheric activity]], and stellar rotation ([[gyrochronology]]) are all consistent with the Alpha Centauri system being similar in age to, or slightly older than, the Sun.<ref name="Mam08">{{cite journal |first1=E.E. |last1=Mamajek |first2=L.A. |last2=Hillenbrand |author2-link= Lynne Hillenbrand |year=2008 |title=Improved age estimation for Solar-type dwarfs using activity-rotation diagnostics |journal=Astrophysical Journal |volume=687 |issue=2 |pages=1264–1293|arxiv=0807.1686|bibcode=2008ApJ...687.1264M |doi=10.1086/591785 |s2cid=27151456 }}</ref> Asteroseismic analyses that incorporate tight observational constraints on the stellar parameters for the Alpha Centauri stars have yielded age estimates of {{val|4.85|0.5}} Gyr,<ref name=Thevenin02>{{cite journal|last1=Thévenin |first1= F.|last2=Provost |first2= J. |last3=Morel |first3= P. |last4=Berthomieu |first4= G.|last5=Bouchy |first5= F. |last6=Carrier |first6= F. |year=2002 |title=Asteroseismology and calibration of alpha&nbsp;Cen binary system |journal=Astronomy & Astrophysics |volume=392 |page=L9 |arxiv=astro-ph/0206283 |bibcode=2002A&A...392L...9T |doi=10.1051/0004-6361:20021074 |s2cid=17293259}}</ref> {{val|5.0|0.5}} Gyr,<ref name="Bazot12">{{cite journal |last1=Bazot |first1= M. |last2=Bourguignon |first2= S. |last3=Christensen-Dalsgaard |first3= J. |year=2012 |title=A Bayesian approach to the modelling of {{nobr|alpha Cen A}} |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=427 |issue=3 |pages=1847–1866 |arxiv=1209.0222 |bibcode=2012MNRAS.427.1847B |doi=10.1111/j.1365-2966.2012.21818.x |doi-access= free |s2cid=118414505}}</ref> {{nobr|5.2 ± 1.9 Gyr,}}<ref name=Miglio05>{{cite journal |last1=Miglio |first1= A.|last2=Montalbán |first2= J.|year=2005|title=Constraining fundamental stellar parameters using seismology. Application to α Centauri AB |journal=Astronomy & Astrophysics |volume=441|issue=2|pages=615–629|arxiv=astro-ph/0505537 |bibcode=2005A&A...441..615M|doi=10.1051/0004-6361:20052988 |s2cid=119078808}}</ref> 6.4 Gyr,<ref name=Thoul03>{{cite journal |last1=Thoul |first1= A. |last2=Scuflaire |first2= R. |last3=Noels |first3= A. |last4=Vatovez |first4= B. |last5=Briquet |first5= M.|last6=Dupret |first6= M.-A. |last7=Montalban |first7= J.
|year=2003 |title=A new seismic analysis of alpha Centauri |journal=Astronomy & Astrophysics |volume=402 |pages=293–297 |arxiv=astro-ph/0303467 |bibcode=2003A&A...402..293T |doi=10.1051/0004-6361:20030244 |s2cid=15886763}}</ref> and {{val|6.52|0.3}} Gyr.<ref name=Eggenberger04>{{cite journal |last1=Eggenberger |first1= P. |last2=Charbonnel |first2= C. |last3=Talon |first3= S. |last4=Meynet |first4= G. |last5=Maeder |first5= A. |last6=Carrier |first6= F. |last7=Bourban |first7= G. |year=2004 |title=Analysis of {{nobr|α Centauri AB}} including seismic constraints |journal=Astronomy & Astrophysics |volume=417 |pages=235–246|arxiv=astro-ph/0401606|bibcode=2004A&A...417..235E |doi=10.1051/0004-6361:20034203|s2cid=119487043}}</ref> Age estimates for the stars based on chromospheric activity (Calcium H & K emission) yield {{nobr|4.4 ± 2.1 Gyr,}} whereas gyrochronology yields {{val|5.0|0.3}} Gyr.<ref name=Mam08/> [[Stellar evolution]] theory implies both stars are slightly older than the Sun at 5 to 6 billion years, as derived by their mass and spectral characteristics.<ref name=Matt93/><ref name=stellar-model-kim>{{cite journal|last=Kim |first= Y-C. |year=1999 |title=Standard stellar models; {{nobr|alpha Cen A and B}} |journal=Journal of the Korean Astronomical Society |volume=32 |issue=2 |page=119 |bibcode=1999JKAS...32..119K}}</ref>

From the [[orbital elements]], the total mass of Alpha Centauri AB is about {{Solar mass|2.0|link=y}}{{efn|
<math display="block">\begin{smallmatrix} \left( \frac{\ 11.2 + 35.6\ }{ 2 }\right)^3 \frac{ 1 }{~ 79.91^2\ } \approx 2.0 \end{smallmatrix} \qquad </math> – see formula in [[Standard gravitational parameter#Two bodies orbiting each other|standard gravitational parameter]] article.
}}
– or twice that of the Sun.<ref name=Aitken/> The average individual stellar masses are about {{Solar mass|1.08}} and {{Solar mass|0.91}}, respectively,<ref name=Akeson2021/> though slightly different masses have also been quoted in recent years, such as {{Solar mass|1.14}} and {{Solar mass|0.92}},<ref name=RECONS/> totaling {{Solar mass|2.06}}. Alpha Centauri A and B have [[absolute magnitude]]s of +4.38 and +5.71, respectively.

==== Alpha Centauri AB System ====
[[File:Best image of Alpha Centauri A and B.jpg|thumb| {{nobr|α Centauri A}} (left) is of the same [[stellar classification|stellar type]]&nbsp;G2 as the Sun, while {{nobr|α Centauri B}} (right) is a K1-type star.<ref>{{cite web|title=Best image of Alpha Centauri A and B |url=http://www.spacetelescope.org/images/potw1635a/|website=spacetelescope.org|access-date=29 August 2016}}</ref>|alt=Two white disks side by side, each with coloured fringes and prominent diffraction spikes]]
===== Alpha Centauri A =====
'''Alpha Centauri A''', also known as '''Rigil Kentaurus''', is the principal member, or primary, of the binary system. It is a solar-like [[main sequence|main-sequence]] star with a similar yellowish colour,<ref name="csiro">{{cite web |title=The colour of stars |date=21 December 2004 |website=Australia Telescope, Outreach and Education |publisher=Commonwealth Scientific and Industrial Research Organisation |url=http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html|access-date=16 January 2012|url-status=dead|archive-url=https://web.archive.org/web/20120222183238/http://outreach.atnf.csiro.au/education/senior/astrophysics/photometry_colour.html |archive-date=22 February 2012}}</ref> whose [[stellar classification]] is [[spectral type]] G2-V;<ref name=torres2006/> it is about 10% more massive than the Sun,<ref name="Thevenin02"/> with a radius about 22% larger.<ref name=kervella2017>{{cite journal |title=The radii and limb darkenings of α Centauri A and B. Interferometric measurements with VLTI/PIONIER |journal=Astronomy & Astrophysics |first1=P. |last1=Kervella |first2=L. |last2=Bigot |first3=A. |last3=Gallenne |first4=F. |last4=Thévenin |volume=597 |page=A137 |date=January 2017 |doi=10.1051/0004-6361/201629505 |bibcode=2017A&A...597A.137K
|arxiv=1610.06185 |s2cid=55597767}}</ref> When considered among the individual [[List of brightest stars|brightest stars]] in the night sky, it is the fourth-brightest at an apparent magnitude of +0.01,<ref name="ducati"/> being slightly fainter than [[Arcturus]] at an [[apparent magnitude]] of −0.05.

The type of [[magnetic activity]] on Alpha Centauri A is comparable to that of the Sun, showing [[stellar corona|coronal]] variability due to [[star spot]]s, as modulated by the rotation of the star. However, since 2005 the activity level has fallen into a deep minimum that might be similar to the Sun's historical [[Maunder Minimum]]. Alternatively, it may have a very long stellar activity cycle and is slowly recovering from a minimum phase.<ref name="Ayres2014">{{cite journal|title=The Ups and Downs of α Centauri|last1=Ayres|first1=Thomas R.|journal=The Astronomical Journal|arxiv=1401.0847|volume=147|issue=3|id=59
|page=12|date=March 2014|doi=10.1088/0004-6256/147/3/59|bibcode=2014AJ....147...59A|s2cid=117715969}}</ref>

===== Alpha Centauri B =====
{{hatnote group|
{{about-distinguish|α Centauri B, also known as Toliman|Beta Centauri{{!}}β Centauri|HD 102964{{!}}B Centauri|HD 129116{{!}}b Centauri|section=yes}}{{other uses|Toliman (disambiguation)}}
}}
'''Alpha Centauri B''', also known as '''Toliman''', is the secondary star of the binary system. It is a main-sequence star of spectral type K1-V, making it more an orange colour than Alpha Centauri A;<ref name="csiro"/> it has around 90% of the mass of the Sun and a 14% smaller diameter. Although it has a lower luminosity than A, Alpha Centauri B emits more energy in the [[X-ray]] band.<ref name="Xrays"/> Its [[light curve]] varies on a short time scale, and there has been at least one observed [[Flare star|flare]].<ref name="Xrays">{{cite journal|last1=Robrade|first1=J.|last2=Schmitt|first2=J. H. M. M.|last3=Favata|first3=F.
|year=2005|title=X-rays from α Centauri – The darkening of the solar twin|journal=Astronomy and Astrophysics|volume=442|issue=1|pages=315–321|bibcode=2005A&A...442..315R|doi=10.1051/0004-6361:20053314 |arxiv=astro-ph/0508260|s2cid=119120}}</ref> It is more magnetically active than Alpha Centauri A, showing a cycle of {{Val|8.2|0.2|u=yr}} compared to 11 years for the Sun, and has about half the minimum-to-peak variation in coronal luminosity of the Sun.<ref name="Ayres2014"/> This cycle was recently re-estimated based on more than 20 years of high-resolution spectroscopic observations of the CaIIH&K lines showing a cycle of {{Val|7.8|0.2|u=yr}}.<ref name="Cretignier2024">{{cite journal|last1=Cretignier|first1=M.|last2=Hara|first2=N.|last3=Pietrow|first3=A.G.M.|year=2024|title=Stellar surface information from the Ca II H&K lines - II. Defining better activity proxies|journal=Monthly Notices of the Royal Astronomical Society|volume=535|issue=1|pages=2562–2584|bibcode=2024MNRAS.535.2562C|doi= 
10.1093/mnras/stae2508 |doi-access=free |arxiv=astro-ph/0508260|s2cid=119120}}</ref> Alpha Centauri B has an apparent magnitude of +1.35, slightly dimmer than [[Mimosa (star)|Mimosa]].<ref name="IAU-LSN"/>

==== Alpha Centauri C ====
{{Main|Proxima Centauri}}

Alpha Centauri C, better known as Proxima Centauri, is a small main-sequence [[red dwarf]] of spectral class M6-Ve. It has an [[absolute magnitude]] of +15.60, over 20,000 times fainter than the Sun. Its mass is calculated to be {{Solar mass|{{val|0.1221}}}}.<ref name="KervellaThévenin2017">{{cite journal|last1=Kervella|first1=P.|last2=Thévenin|first2=F.|last3=Lovis|first3=C.|title=Proxima's orbit around α Centauri |journal=Astronomy & Astrophysics|volume=598|year=2017|pages=L7|issn=0004-6361|doi=10.1051/0004-6361/201629930|arxiv=1611.03495|bibcode=2017A&A...598L...7K|s2cid=50867264}}</ref> It is the closest star to the Sun but is too faint to be visible to the naked eye.<ref name=ESA>{{cite web |title=Proxima Centauri UV flux distribution |url=http://sdc.cab.inta-csic.es/ines/Ines_PCentre/Demos/Fluxdist/pcentauri.html |access-date=July 11, 2007 |publisher=[[ESA]]|work=The Astronomical Data Centre}}</ref>

<div style="overflow:auto;">[[File:Relative positions of Sun, Alpha Centauri AB and Proxima Centauri.png|thumb|upright=5|center|Relative positions of Sun, Alpha Centauri AB and Proxima Centauri. Grey dot is projection of Proxima Centauri, located at the same distance as Alpha Centauri AB.|alt=Very wide rectangle with a dot labelled "Sun" on the left and two dots respectively labelled "Alpha Centauri AB" and "Proxima Centauri" on the right side, joined by lines labelled with distances and angles]]</div>