View source for Proxima Centauri

{{Starbox begin
| name=Proxima Centauri
}}
{{Starbox image
| image = [[Image:Position from Proxima Centauri.png|250px]]
| caption = Position of Proxima Centauri
}}
{{Starbox observe
| epoch=[[J2000.0]]
| equinox=[[J2000.0]] ([[International Celestial Reference System|ICRS]])
| constell = [[Centaurus]]
| pronounce = {{IPA-en|ˈprɒksɪmə sɛnˈtɔriː|}}<ref group="nb">Proxima is pronounced {{IPA-en|ˈprɒksɪmə|}}. Centauri may be pronounced {{IPA|/sɛnˈtɔriː/}} or {{IPA|/sɛnˈtɔraɪ/}}.</ref>
| ra = {{RA|14|29|42.9487}}<ref name="SIMBAD">{{cite web
| url =http://simbad.u-strasbg.fr/simbad/sim-id?Ident=proxima%20centauri
| title =SIMBAD query result: V* V645 Cen -- Flare Star
| publisher =Centre de Données astronomiques de Strasbourg
| accessdate = 2008-08-11 }}—some of the data is located under "Measurements".</ref>
| dec = {{DEC|−62|40|46.141}}<ref name="SIMBAD" />
| appmag_v = 11.05<ref name="SIMBAD" />
}}
{{Starbox character
| class=M5.5&nbsp;Ve<ref name="SIMBAD" />
| b-v=1.90<ref name="SIMBAD" />
| u-b=1.43<ref name="SIMBAD" />
| variable=[[Flare star]]
}}
{{Starbox astrometry
| radial_v=−21.7&nbsp;±&nbsp;1.8<ref name=aaa379/>
| prop_mo_ra=−3775.40<ref name="SIMBAD" />
| prop_mo_dec=769.33<ref name="SIMBAD" />
| parallax=768.7
| p_error=0.3
| parallax_footnote=<ref name="apj118" />
| absmag_v=15.49<ref name="abs_mag">{{cite journal
 | last=Kamper | first=K. W. | coauthors=Wesselink, A. J.
 | title=Alpha and Proxima Centauri
 | journal=Astronomical Journal | year=1978
 | volume=83 | pages=1653–1659
 | url=http://adsabs.harvard.edu/abs/1978AJ.....83.1653K
 | accessdate=2008-08-03
 | doi=10.1086/112378 }}</ref>
}}
{{Starbox detail
| age=4.85{{Esp|9}}<ref name="ESO2003">{{cite news
 | author=Kervella, Pierre; Thevenin, Frederic
 | title=A Family Portrait of the Alpha Centauri System: VLT Interferometer Studies the Nearest Stars
 | publisher=ESO | date=2003-03-15
 | url=http://www.eso.org/public/outreach/press-rel/pr-2003/pr-05-03.html
 | accessdate=2007-07-09 }}</ref>
| metal=<!-- assumed to be the same as Alpha Cen A/B -->
| mass=0.123&nbsp;±&nbsp;0.006<ref name=aaa397>{{cite journal
 | last=Ségransan | first=D.
 | coauthors=Kervella, P.; Forveille, T.; Queloz, D.
 | title=First radius measurements of very low mass stars with the VLTI
 | journal=Astronomy and Astrophysics | year=2003
 | volume=397 | pages=L5–L8
 | url=http://adsabs.harvard.edu/abs/2003A&A...397L...5S
 | accessdate=2008-08-07
 | doi=10.1051/0004-6361:20021714 }}</ref>
| radius=0.145&nbsp;±&nbsp;0.011<ref name=aaa397/>
| rotation=83.5&nbsp;days<ref>{{cite journal
 | author=Benedict, G. Fritz ''et al.''
 | title=Photometry of Proxima Centauri and Barnard's Star Using Hubble Space Telescope Fine Guidance Sensor 3: A Search for Periodic Variations
 | journal=The Astronomical Journal
 | year=1998 | volume=116 | issue=1 | pages=429–439
 | url=http://adsabs.harvard.edu/abs/1998AJ....116..429B
 | accessdate=2007-07-09
 | doi=10.1086/300420 }}</ref>
| luminosity_bolometric=0.0017<ref name=lbol>See Table 1, {{cite journal
 | last=Doyle | first=J. G. | coauthors=Butler, C. J.
 | title=Optical and infrared photometry of dwarf M and K stars
 | journal=Astronomy and Astrophysics | year=1990
 | volume=235 | pages=335–339
 | bibcode=1990A&A...235..335D}} and p. 57, {{cite book
 | title=Principles of Physical Cosmology
 | last=Peebles | first=P. J. E.
 | year=1993
 | location=Princeton, New Jersey
 | publisher=Princeton University Press
 | isbn=0691019339
}}</ref>
| temperature=3,042&nbsp;±&nbsp;117<ref name=aaa397/>
| gravity=5.20&nbsp;±&nbsp;0.23<ref name=aaa397/>
}}
{{Starbox catalog
| names = [[Bayer designation|Alpha Centauri C]], [[Catalog of Components of Double and Multiple Stars|CCDM]]&nbsp;J14396-6050C, [[General Catalogue of Trigonometric Parallaxes|GCTP]]&nbsp;3278.00, [[Gliese-Jahreiss catalogue|GJ]]&nbsp;551, [[Hipparcos catalogue|HIP]]&nbsp;70890, [[Luyten Five-Tenths catalogue|LFT]]&nbsp;1110, [[Luyten Half-Second catalogue|LHS]]&nbsp;49, [[Luyten Proper-Motion catalogue|LPM]]&nbsp;526, [[Luyten Two-Tenths catalogue|LTT]]&nbsp;5721, [[New Luyten Two-Tenths catalogue|NLTT]]&nbsp;37460, [[Variable star designation|V645 Centauri]]<ref name="SIMBAD" />
}}
{{Starbox reference
| Simbad = V645+Cen
| ARICNS = <!--Code-->
}}
{{Starbox end}}

'''Proxima Centauri''' ([[Latin language|Latin]] {{lang|la|''proxima''}}: meaning 'next to' or 'nearest to')<ref>{{cite web
 | url=http://www.jact.org/subjects/vocablist.htm
 | title=Latin Resources
 | publisher =Joint Association of Classical Teachers
 | accessdate=2007-07-15}}</ref> is a [[red dwarf]] [[star]] approximately 4.2 [[light-year]]s distant in the [[constellation]] of [[Centaurus]]. It was discovered in 1915 by [[Robert Innes]], the Director of the [[Union Observatory]] in [[South Africa]]. The star may be part of the [[Alpha Centauri]] system, and it is the [[List of nearest stars|nearest star]] to the [[Sun]].<ref name="ESO2003"/>

Because of the proximity of this star, its [[angular diameter]] can be measured directly, yielding a diameter one-seventh that of the Sun.<ref name="ESO2003"/> Proxima Centauri's mass is about an eighth of the Sun's, and its average [[density]] is about 40 times that of the Sun.<ref group="nb" name="density" /> Although it has a very low average [[luminosity]], Proxima Centauri is a [[flare star]] that undergoes random increases in brightness because of magnetic activity.<ref name=apj612>{{cite journal
 | last=Christian | first=D. J.
 | coauthors=Mathioudakis, M.; Bloomfield, D. S.; Dupuis, J.; Keenan, F. P.
 | title=A Detailed Study of Opacity in the Upper Atmosphere of Proxima Centauri
 | journal=The Astrophysical Journal
 | year=2004 | volume=612 | issue=2 | pages=1140–1146
 | url=http://adsabs.harvard.edu/abs/2004ApJ...612.1140C
 | accessdate=2008-06-13
 | doi=10.1086/422803 }}</ref> The star's [[magnetic field]] is created by [[convection]] throughout the stellar body, and the resulting flare activity generates a total [[X-ray]] emission similar to that produced by the Sun.<ref name=apj547/> The mixing of the fuel at Proxima Centauri's core through convection and the star's relatively low energy production rate means that it will be a [[main sequence|main-sequence star]] for another four trillion years,<ref name=adams/> or nearly 300 times the current [[age of the universe]].<ref name="NASA">{{cite web
 | author=Dunkley, J. ''et al.''
 | title = Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Data Processing, Sky Maps, and Basic Results
 | url =http://arxiv.org/abs/0803.0586
 | publisher=NASA | accessdate=2008-03-06 }}</ref>

Searches for companions orbiting Proxima Centauri have been unsuccessful, ruling out the presence of [[brown dwarfs]] and [[Gas giants|supermassive planets]].<ref name="aaal344"/><ref name=apj119/> Precision [[radial velocity]] surveys have also ruled out the presence of [[super-Earth]]s within the star's habitable zone.<ref name="aaa488">{{cite journal|title=Toward detection of terrestrial planets in the habitable zone of our closest neighbor: proxima Centauri|author=Endl, M. and Kürster, M.|year=2008|journal=[[Astronomy and Astrophysics]]|volume=488|issue=3|pages=1149–1153|doi=10.1051/0004-6361:200810058|bibcode=2008A&A...488.1149E}}</ref><ref group="nb" name="m_sin_i">This is actually an upper limit on the quantity ''m'' sin ''i'', where ''i'' is the angle between the orbit normal and the line of sight. If the planetary orbits are close to face-on as observed from Earth, more massive planets could have evaded detection by the radial velocity method.</ref> The detection of smaller objects will require the use of new instruments, such as the proposed [[Space Interferometry Mission]].<ref name="numbers"/> Since Proxima Centauri is a red dwarf and a flare star, whether a planet orbiting this star could [[Habitability of red dwarf systems|support life]] is disputed.<ref name=tarter/><ref name=Khodachenko/> Because of the star's proximity, it has been proposed as a destination for [[interstellar travel]].<ref name=gilster>{{cite book
 | first=Paul | last=Gilster | year=2004
 | title=Centauri Dreams: Imagining and Planning
 | publisher=Springer | isbn=038700436X }}</ref>

== Observation ==
[[Robert Innes]], Director of the [[Union Observatory]] in [[Johannesburg]], [[South Africa]] in 1915, discovered that Proxima Centauri had the same [[proper motion]] as Alpha Centauri.<ref name="ESO2002">{{cite web
 | first=Didier | last=Queloz | date=2002-11-29
 | url =http://www.eso.org/outreach/press-rel/pr-2002/pr-22-02.html
 | title =How Small are Small Stars Really? VLT Interferometer Measures the Size of Proxima Centauri and Other Nearby Stars
 | publisher =European Southern Observatory
 | accessdate = 2007-07-09 }}</ref> He also suggested it be named ''Proxima Centauri''.<ref>{{cite journal
 | last=Alden | first=Harold L.
 | title=Alpha and Proxima Centauri
 | journal=Astronomical Journal
 | year=1928 | volume=39 | issue=913 | pages=20–23
 | url=http://adsabs.harvard.edu/abs/1928AJ.....39...20A
 | accessdate=2008-06-28
 | doi=10.1086/104871 }}</ref> In 1917, at the [[Royal Observatory, Cape of Good Hope|Royal Observatory]] at the [[Cape of Good Hope]], the Dutch astronomer [[Joan Voûte]] measured the star's trigonometric [[parallax]] and determined that Proxima Centauri was indeed the same distance from the Sun as Alpha Centauri. It was also found to be the lowest-[[luminosity]] star known at the time.<ref>{{cite journal
 | last = Voûte | first = J.
 | title=A 13th magnitude star in Centaurus with the same parallax as α Centauri
 | journal=Monthly Notices of the Royal Astronomical Society
 | year=1917 | volume=77 | pages=650–651
 | url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1917MNRAS..77..650V&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=444c5eb29022736
 | accessdate=2007-07-09 }}</ref> In 1951, American astronomer [[Harlow Shapley]] announced that Proxima Centauri was a flare star. Examination of past photographic records showed that the star displayed a measurable increase in magnitude on about 8% of the images, making it the most active flare star then known.<ref>{{cite journal
 | last = Shapley | first = Harlow
 | title=Proxima Centauri as a Flare Star
 | journal=Proceedings of the National Academy of Sciences of the United States of America
 | year=1951 | volume=37 | issue=1 | pages=15–18
 | url=http://adsabs.harvard.edu/abs/1951PNAS...37...15S
 | accessdate=2007-07-11 | doi=10.1073/pnas.37.1.15 }}</ref>

The proximity of the star allows for detailed observation of its flare activity. In 1980, the [[Einstein Observatory]] produced a detailed X-ray energy curve of a stellar flare on Proxima Centauri. Further observations of flare activity were made with the [[EXOSAT]] and [[ROSAT]] [[satellite]]s, and the X-ray emissions of smaller, solar-like flares were observed by the Japanese [[Advanced Satellite for Cosmology and Astrophysics|ASCA]] satellite in 1995.<ref>{{cite journal
 | last=Haisch | first=Bernhard
 | coauthors=Antunes, A.; Schmitt, J. H. M. M.
 | title=Solar-Like M-Class X-ray Flares on Proxima Centauri Observed by the ASCA Satellite
 | journal=Science | year=1995
 | volume=268 | issue=5215 | pages=1327–1329
 | doi=10.1126/science.268.5215.1327
 | pmid=17778978 }}</ref> Proxima Centauri has since been the subject of study by most X-ray observatories, including [[XMM-Newton]] and [[Chandra X-ray Observatory|Chandra]].<ref name=aaa416/>

Because of Proxima Centauri's southern declination, it can only be viewed south of [[latitude]] [[27th parallel north|27°&nbsp;N]].<ref group="nb">For a star south of the zenith, the angle to the zenith is equal to the Latitude minus the Declination. The star is hidden from site when the zenith angle is 90° or more, i.e. below the horizon. Thus, for Proxima Centauri:
:Highest latitude = 90° + −62.68° = 27.32°.
See: {{cite book
 | first=William Wallace | last=Campbell
 | year=1899 | title=The Elements of Practical Astronomy
 | publisher=Macmillan | location=London
 | url=http://books.google.com/books?id=v2tEAAAAIAAJ
 | accessdate=2008-08-12 }}</ref> Red dwarfs such as Proxima Centauri are far too faint to be seen with the naked eye; even from Alpha Centauri A or B, Proxima would only be seen as a fifth magnitude star.<ref>{{cite web
 | url =http://sdc.laeff.inta.es/ines/Ines_PCentre/Demos/Fluxdist/pcentauri.html
 | title =Proxima Centauri UV Flux Distribution
 | publisher =ESA/Laboratory for Space Astrophysics and Theoretical Physics
 | accessdate = 2007-07-11
}}</ref><ref>{{cite web
 | first=Jim | last=Kaler | title=Rigil Kentaurus
 | url=http://www.astro.uiuc.edu/~kaler/sow/rigil-kent.html
 | publisher=University of Illinois
 | accessdate=2008-08-03 }}</ref> It has an [[apparent visual magnitude]] of 11, so a [[telescope]] with an [[aperture]] of at least 8&nbsp;cm (3.1&nbsp;[[inch|in.]]) is needed to observe this star even under ideal viewing conditions—under clear, dark skies with Proxima Centauri well above the horizon.<ref>{{cite book
 | first=P. Clay | last=Sherrod
 | coauthors=Koed, Thomas L.; Aleichem, Thomas L. Sholem
 | year=2003 | title=A Complete Manual of Amateur Astronomy: Tools and Techniques for Astronomical Observations
 | publisher=Courier Dover Publications
 | isbn=0486428206 }}</ref>

== Characteristics ==

Proxima Centauri is classified as a red dwarf star because it belongs to the [[main sequence]] on the [[Hertzsprung-Russell diagram]] and it is of [[Stellar classification|spectral class M5.5]]. It is further classified as a "late M-dwarf star", meaning that at M5.5 it falls to the low-mass extreme of M-type stars.<ref name="ESO2003"/> This star's [[absolute visual magnitude]], or its visual magnitude as viewed from a distance of 10 parsecs, is 15.5.<ref name="abs_mag"/> Its total luminosity over all [[wavelength]]s is 0.17% that of the Sun,<ref name=lbol /> although when observed in the wavelengths of [[visible light]] the eye is most sensitive to, it is only 0.0056% as luminous as the Sun.<ref>p. 8, {{cite book
 | last=Binney | first=James
 | coauthors=Scott Tremaine
 | title=Galactic Dynamics
 | publisher=Princeton University Press
 | location=Princeton, New Jersey
 | year=1987
 | isbn=0691084459}}</ref> More than 85% of its radiated power is at [[infrared]] wavelengths.<ref>p. 357, {{cite journal
 | last=Leggett | first=S. K.
 | title=Infrared colors of low-mass stars
 | journal=Astrophysical Journal Supplement Series
 | year=1992 | volume=82 | issue=1 | pages=351–394
 | url=http://adsabs.harvard.edu/abs/1992ApJS...82..351L
 | accessdate=2008-08-13
 | doi=10.1086/191720 }}</ref>

[[File:Alpha centauri size.svg|right|thumb|280px|This illustration shows the comparative sizes of (from left to right) the Sun, α Centauri A, α Centauri B and Proxima Centauri]]

In 2002, [[optical interferometry]] with the [[Very Large Telescope]] (VLTI) found that the [[angular diameter]] of Proxima Centauri was 1.02&nbsp;±&nbsp;0.08 [[arcsecond|milliarcsec]]. Because its distance is known, the actual diameter of Proxima Centauri can be calculated to be about 1/7 that of the Sun, or 1.5 times that of [[Jupiter]].<ref name="ESO2002"/> The star's estimated mass is only 12.3% of a [[solar mass]], or 129 [[Jupiter mass]]es.<ref name="ESO2003"/> The mean [[density]] of a main sequence star increases with decreasing mass,<ref>{{cite book
 | first=Martin V. | last=Zombeck | year=2007
 | title=Handbook of Space Astronomy and Astrophysics
 | publisher=Cambridge University Press
 | edition=Third | pages=109
 | location=Cambridge, UK | isbn=0521782422 }}</ref> and Proxima Centauri is no exception: it has a mean density of 56,800&nbsp;kg/m<sup>3</sup>, compared with the Sun's mean density of 1,409&nbsp;kg/m<sup>3</sup>.<ref group="nb" name="density">The density (''ρ'') is given by the mass divided by the volume. Relative to the Sun, therefore, the density is:
:{|
| <math>\rho</math>
| = <math>\begin{smallmatrix}\frac{M}{M_{\odot}} \cdot \left( \frac{R}{R_{\odot}} \right)^{-3} \cdot \rho_{\odot}\end{smallmatrix}</math>
|-
| || = 0.123 · 0.145<sup>-3</sup> · 1.41{{Esp|3}}&nbsp;kg/m<sup>3</sup>
|-
| || = 40.3 · 1.41{{Esp|3}}&nbsp;kg/m<sup>3</sup>
|-
| || = 5.68{{Esp|4}}&nbsp;kg/m<sup>3</sup>
|}
where <math>\begin{smallmatrix}\rho_{\odot}\end{smallmatrix}</math> is the average solar density.
See: {{cite web
 | authors=Munsell, Kirk; Smith, Harman; Davis, Phil; Harvey, Samantha
 | date=2008-06-11
 | url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sun&Display=Facts&System=Metric
 | title=Sun: Facts & Figures | work=Solar System Exploration
 | publisher=NASA | accessdate=2008-07-12 }}</ref>

Because of its low mass, the interior of the star is completely [[Convection zone|convective]], causing energy to be transferred to the exterior by the physical movement of plasma rather than through [[Radiation zone|radiative processes]]. This convection means that the helium ash left over from the [[thermonuclear fusion]] of hydrogen does not accumulate at the core, but is instead circulated throughout the star. Unlike the Sun, which will only burn through about 10% of its total hydrogen supply before leaving the main sequence, Proxima Centauri will consume a much higher proportion of its fuel before the fusion of hydrogen comes to an end.<ref name=adams/>

Convection is associated with the generation and persistence of a [[Stellar magnetic field|magnetic field]]. The magnetic energy from this field is released at the surface through [[stellar flare]]s that briefly increase the overall luminosity of the star. These flares can grow as large as the star and reach temperatures measured as high as 27&nbsp;million [[Kelvin|K]]<ref name=aaa416>{{cite journal
 | last=Guedel | first=M.
 | coauthors=Audard, M.; Reale, F.; Skinner, S. L.; Linsky, J. L.
 | title=Flares from small to large: X-ray spectroscopy of Proxima Centauri with XMM-Newton
 | journal=Astronomy and Astrophysics | year=2004
 | volume=416 | pages=713–732
 | url=http://arxiv.org/abs/astro-ph/0312297
 | accessdate=2008-07-11
 | doi=10.1051/0004-6361:20031471 }}</ref>—hot enough to radiate [[X-ray]]s.<ref>{{cite web
 | author=Staff | date =2006-08-30
 | url =http://chandra.harvard.edu/photo/2004/proxima/
 | title =Proxima Centauri: The Nearest Star to the Sun
 | publisher =Harvard-Smithsonian Center for Astrophysics
 | accessdate = 2007-07-09 }}</ref> Indeed, the quiescent X-ray luminosity of this star, approximately (4–16){{Esp|26}}&nbsp;[[erg]]/s ((4–16){{Esp|19}}&nbsp;[[watt|W]]), is roughly equal to that of the much larger Sun. The peak X-ray luminosity of the largest flares can reach 10<sup>28</sup>&nbsp;erg/s (10<sup>21</sup> W.)<ref name=aaa416/>

The [[chromosphere]] of this star is active, and its [[spectrum]] displays a strong [[Spectral line|emission line]] of singly ionized [[magnesium]] at a [[wavelength]] of 280&nbsp;[[Nanometre|nm]].<ref>{{cite journal
 | first=Guinan | last=E. F. | coauthors=Morgan, N. D.
 | title=Proxima Centauri: Rotation, Chromosperic Activity, and Flares
 | journal=Bulletin of the American Astronomical Society
 | year=1996 | volume=28 | pages=942
 | url=http://adsabs.harvard.edu/abs/1996BAAS...28S.942G
 | accessdate=2008-06-14 }}</ref> About 88% of the surface of Proxima Centauri may be active, a percentage that is much higher than that of the Sun even at the peak of the [[solar cycle]]. Even during quiescent periods with few or no flares, this activity increases the [[corona]] temperature of Proxima Centauri to 3.5&nbsp;million K, compared to the 2&nbsp;million K of the Sun's corona.<ref>{{cite journal
 | last=Wargelin | first=Bradford J.
 | coauthors=Drake, Jeremy J.
 | title=Stringent X-Ray Constraints on Mass Loss from Proxima Centauri
 | journal=The Astrophysical Journal | year=2002
 | volume=578 | pages=503–514
 | doi=10.1086/342270 }}</ref> However, the overall activity level of this star is considered low compared to other M-class dwarfs,<ref name=apj547>{{cite journal
 | author=Wood, B. E.; Linsky, J. L.; Müller, H.-R.; Zank, G. P.
 | title=Observational Estimates for the Mass-Loss Rates of α Centauri and Proxima Centauri Using Hubble Space Telescope Lyα Spectra
 | journal=The Astrophysical Journal | year=2001
 | volume=547 | issue=1 | pages=L49–L52
 | url=http://adsabs.harvard.edu/abs/2001ApJ...547L..49W
 | accessdate=2007-07-09
 | doi=10.1086/318888 }}</ref> which is consistent with the star's estimated age, since the activity level of a red dwarf is expected to steadily wane over billions of years as its [[stellar rotation]] rate decreases.<ref>{{cite journal
 | last=Stauffer | first=J. R. | coauthors=Hartmann, L. W.
 | title=Chromospheric activity, kinematics, and metallicities of nearby M dwarfs
 | journal=Astrophysical Journal Supplement Series
 | year=1986 | volume=61 | issue=2 | pages=531–568
 | url=http://articles.adsabs.harvard.edu/abs/1986ApJS...61..531S
 | accessdate=2008-06-29
 | doi=10.1086/191123 }}</ref> The activity level also appears to vary with a period of roughly 442 days, which is shorter than the [[solar cycle]] of 11 years.<ref>{{cite journal
 | author=Cincunegui, C.; Díaz, R. F.; Mauas, P. J. D.
 | title=A possible activity cycle in Proxima Centauri
 | journal=Astronomy and Astrophysics | year=2007
 | volume=461 | issue=3 | pages=1107–1113
 | url=http://adsabs.harvard.edu/abs/2007A&A...461.1107C
 | accessdate=2007-07-11
 | doi = 10.1051/0004-6361:20066027 }}</ref>

Proxima Centauri has a relatively weak [[stellar wind]], resulting in no more than 20% of the Sun's mass loss rate from the [[solar wind]]. Because the star is much smaller than the Sun, however, the mass loss per unit surface area from Proxima Centauri may be eight times that from the solar surface.<ref>{{cite journal
 | last=Wood | first=B. E.
 | coauthors=Linsky, J. L.; Muller, H.-R.; Zank, G. P.
 | title=Observational Estimates for the Mass-Loss Rates of Alpha Centauri and Proxima Centauri Using Hubble Space Telescope Lyman-alpha Spectra
 | journal=Astrophysical Journal | year=2000
 | volume=537 | issue=2 | pages=L49–L52
 | url=http://arxiv.org/abs/astro-ph/0011153
 | accessdate=2008-07-11
 | doi=10.1086/309026 }}</ref>

A red dwarf with the mass of Proxima Centauri will remain on the main sequence for about four trillion years. As the proportion of helium increases because of hydrogen fusion, the star will become smaller and hotter, gradually transforming from red to blue. Near the end of this period it will become significantly more luminous, reaching 2.5% of the Sun's luminosity and warming up any orbiting bodies for a period of several billion years. Once the hydrogen fuel is exhausted, Proxima Centauri will then evolve into a [[white dwarf]] (without passing through the [[red giant]] phase) and steadily lose any remaining heat energy.<ref name=adams>{{cite conference
 | last=Adams | first=Fred C.
 | coauthors=Laughlin, Gregory; Graves, Genevieve J. M.
 | title=Red Dwarfs and the End of the Main Sequence
 | booktitle=Gravitational Collapse: From Massive Stars to Planets
 | pages=46–49
 | publisher=Revista Mexicana de Astronomía y Astrofísica
 | url=http://adsabs.harvard.edu/abs/2004RMxAC..22...46A
 | accessdate = 2008-06-24 }}</ref>

=== Distance and motion ===
<!-- Astrometry -->
Based on the [[parallax]] of 772.3&nbsp;±&nbsp;2.4 [[Minute of arc|milliarcseconds]] measured by [[Hipparcos]] (and the more precise parallax determined using the [[Fine Guidance Sensors]] on the [[Hubble Space Telescope]] of 768.7&nbsp;±&nbsp;0.3<ref name="apj118">{{cite journal
 | author=Benedict, G. Fritz ''et al.''
 | title=Interferometric Astrometry of Proxima Centauri and Barnard's Star Using HUBBLE SPACE TELESCOPE Fine Guidance Sensor 3: Detection Limits for Substellar Companions
 | journal=The Astronomical Journal
 | year=1999 | volume=118 | issue=2 | pages=1086–1100
 | url=http://adsabs.harvard.edu/abs/1999astro.ph..5318B
 | accessdate=2007-07-21
 | doi=10.1086/300975 }}</ref> milliarcseconds), Proxima Centauri is roughly 4.2&nbsp;light years from Earth, or 270,000 times more distant than the Sun. From Earth's vantage point, Proxima is separated by 2.18°<ref name=apj121>{{cite journal
 | author=Kirkpatrick, J. Davy ''et al.''
 | title=Brown Dwarf Companions to G-type Stars. I: Gliese 417B and Gliese 584C
 | journal=The Astronomical Journal
 | year=1999 | volume=121 | pages=3235–3253
 | url=http://arxiv.org/abs/astro-ph/0103218
 | accessdate=2008-06-23
 | doi=10.1086/321085 }}</ref> from Alpha Centauri, or four times the [[angular diameter]] of the full [[Moon]].<ref>{{cite web
 | last=Williams | first=D. R. | date=2006-02-10
 | url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html
 | title=Moon Fact Sheet | publisher=NASA
 | accessdate=2007-10-12 }}</ref> Proxima also has a relatively large [[proper motion]]—moving 3.85 [[arcseconds]] per year across the sky.<ref>{{cite conference
 | author=Benedict, G. F. ''et al.''
 | title =Astrometric Stability and Precision of Fine Guidance Sensor #3: The Parallax and Proper Motion of Proxima Centauri
 | booktitle =Proceedings of the HST Calibration Workshop
 | pages =380–384
 | url =http://clyde.as.utexas.edu/SpAstNEW/Papers_in_pdf/%7BBen93%7DEarlyProx.pdf
 |format=PDF| accessdate = 2007-07-11 }}</ref> It has a [[radial velocity]] toward the Sun of 21.7&nbsp;km/s.<ref name="SIMBAD" />

<!-- Motion through space -->
Among the known stars, Proxima Centauri has been the closest star to the Sun for about 32,000&nbsp;years and will be so for about another 33,000&nbsp;years, after which the closest star to the Sun will be [[Ross 248]].<ref name="qjras35">{{cite journal
 | last=Matthews | first=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 }}</ref> Proxima will make its closest approach to the Sun, coming within 3.11&nbsp;light years of the latter, in approximately 26,700&nbsp;years.<ref name=aaa379>{{cite journal
 | last=García-Sánchez | first=J.
 | coauthors=Weissman, P. R.; Preston, R. A.; Jones, D. L.; Lestrade, J.-F.; Latham, D. W.; Stefanik, R. P.; Paredes, J. M.
 | title=Stellar encounters with the solar system
 | journal=Astronomy and Astrophysics
 | year=2001 | volume=379 | pages=634–659
 | url=http://adsabs.harvard.edu/abs/2001A&A...379..634G
 | accessdate=2008-06-12
 | doi=10.1051/0004-6361:20011330 }}</ref> Proxima Centauri is orbiting through the [[Milky Way]] at a distance from the [[galactic core]] that varies from 8.3 to 9.5&nbsp;[[Parsec|kpc]], and with an [[orbital eccentricity]] of 0.07.<ref>{{cite journal
 | last=Allen | first=C. | coauthors=Herrera, M. A.
 | title=The galactic orbits of nearby UV Ceti stars
 | journal=Revista Mexicana de Astronomia y Astrofisica
 | year=1998 | volume=34 | pages=37–46
 | url=http://adsabs.harvard.edu/abs/1998RMxAA..34...37A
 | accessdate=2008-06-13 }}</ref>

<!-- Possible orbit -->
From the time of the discovery of Proxima, it was suspected to be a true companion of the Alpha Centauri [[binary star]] system. At a distance to Alpha Centauri of just 0.21&nbsp;ly (15,000&nbsp;±&nbsp;700 [[astronomical unit]]s [AU]),<ref name="apj132">{{cite journal
 | author=Wertheimer, Jeremy G.; Laughlin, Gregory
 | title=Are Proxima and α Centauri Gravitationally Bound?
 | journal=The Astronomical Journal
 | year=2006 | volume=132 | issue=5 | pages=1995–1997
 | url=http://adsabs.harvard.edu/abs/2006astro.ph..7401W
 | accessdate=2007-07-09 | doi = 10.1086/507771
}}</ref> Proxima Centauri may be in orbit about Alpha Centauri, with an [[orbital period]] of the order of 500,000&nbsp;years or more. For this reason, Proxima is sometimes referred to as Alpha Centauri C. Modern estimates, taking into account the small separation between and relative velocity of the stars, suggest that the chance of the observed alignment being a coincidence is roughly one in a million.<ref>{{cite journal
 | last = Matthews, Robert; Gilmore, Gerard
 | title = Is Proxima really in orbit about Alpha CEN A/B?
 | journal = MNRAS | volume = 261 | pages = L5 | year = 1993
 | url = http://adsabs.harvard.edu/abs/1993MNRAS.261L...5M}}</ref> Data from the [[Hipparcos]] satellite, combined with ground-based observations, is consistent with the hypothesis that the three stars are truly a bound system. If so, Proxima would currently be near [[apsis|apastron]], the farthest point in its orbit from the Alpha Centauri system. More accurate measurement of the radial velocity is needed to confirm this hypothesis.<ref name="apj132" />

If Proxima was bound to the Alpha Centauri system during its formation, the stars are likely to share the same [[element (chemistry)|element]]al composition. The gravitational influence of Proxima may also have stirred up the Alpha Centauri [[protoplanetary disk]]s. This would have increased the delivery of [[volatiles]] such as water to the dry inner regions. Any [[terrestrial planet]]s in the system may have been enriched by this material.<ref name="apj132" />

Six single stars, two binary star systems, and a triple star share a common motion through space with Proxima Centauri and the Alpha Centauri system. The [[space velocity|space velocities]] of these stars are all within 10&nbsp;km/s of Alpha Centauri's peculiar motion. Thus, they may form a [[moving group]] of stars, which would indicate a common point of origin,<ref>{{cite journal
 | last=Johnston | first=Kathryn V.
 | title=Fossil Signatures of Ancient Accretion Events in the Halo
 | journal=Bulletin of the American Astronomical Society
 | year=1995 | volume=27 | pages=1370
 | url=http://arxiv.org/abs/astro-ph?papernum=9602060
 | accessdate=2008-08-10 }}</ref> such as in a [[star cluster]]. If it is determined that Proxima Centauri is not gravitationally bound to Alpha Centauri, then such a moving group would help explain their relatively close proximity.<ref>{{cite journal
 | last=Anosova | first=J.
 | coauthors=Orlov, V. V.; Pavlova, N. A.
 | title=Dynamics of nearby multiple stars. The alpha Centauri system
 | journal=Astronomy and Astrophysics
 | year=1994 | volume=292 | issue=1 | pages=115–118
 | url=http://adsabs.harvard.edu/abs/1994A%26A...292..115A
 | accessdate=2008-08-10 }}</ref>

=== Possible companions ===
{{seealso|Habitability of red dwarf systems}}

{| class="wikitable" style="float: right;"
|+ RV-derived Upper Mass<br>Limits of Companion<ref name="aaa488" />
![[Orbital period|Orbital<br>period]]<br>(days)
![[Semi-major axis|Separation]]<br>(AU)
!Maximum<br>Mass<ref group="nb" name="m_sin_i" /><br>(×&nbsp;[[Earth]])
|-
|style="text-align: center;"| 3.6–13.8
|style="text-align: center;"| 0.022–0.054
|style="text-align: center;"| 2–3
|-
|style="text-align: center;"| &lt;100
|style="text-align: center;"| &lt;0.21
|style="text-align: center;"| 8.5
|-
|style="text-align: center;"| &lt;1000
|style="text-align: center;"| &lt;1
|style="text-align: center;"| 16
|}

If a massive planet is orbiting Proxima Centauri, some displacement of the star would occur over the course of each orbit. If the [[Orbital plane (astronomy)|orbital plane]] of the planet is not perpendicular to the line of sight from the Earth then this displacement would cause periodic changes in the radial velocity of Proxima Centauri. The fact that multiple measurements of the star's radial velocity have detected no such shifts has lowered the maximum mass that a possible companion to Proxima Centauri could possess.<ref name="apj118" /><ref name="aaal344">{{cite journal
 | author=Kürster, M. ''et al.''
 | title=Precise radial velocities of Proxima Centauri. Strong constraints on a substellar companion
 | journal=Astronomy & Astrophysics Letters
 | year=1999 | volume=344 | pages=L5–L8
 | url=http://arxiv.org/abs/astro-ph/9903010v1
 | bibcode=1999A&A...344L...5K
 | accessdate=2008-12-19 }}</ref> Unfortunately, the activity level of the star adds noise to the radial velocity measurements, limiting future prospects for detection of a companion using this method.<ref>{{cite journal
 | last=Saar | first=Steven H.
 | coauthors=Donahue, Robert A.
 | title=Activity-related Radial Velocity Variation in Cool Stars
 | journal=Astrophysical Journal
 | year=1997 | volume=485 | pages=319–326
 | url=http://adsabs.harvard.edu/abs/1997ApJ...485..319S
 | accessdate=2008-07-11
 | doi=10.1086/304392 }}</ref>

In 1998, an examination of Proxima Centauri using the [[Faint Object Spectrograph]] on board the Hubble Space Telescope appeared to show evidence of a companion orbiting at a distance of about 0.5&nbsp;AU.<ref>{{cite journal
 | last=Schultz | first=A. B.
 | coauthors=Hart, H. M.; Hershey, J. L.; Hamilton, F. C.; Kochte, M.; Bruhweiler, F. C.; Benedict, G. F.; Caldwell, John; Cunningham, C.; Wu, Nailong; Franz, O. G.; Keyes, C. D.; Brandt, J. C.
 | title=A possible companion to Proxima Centauri
 | journal=Astronomical Journal
 | year=1998 | volume=115 | pages=345–350
 | url=http://adsabs.harvard.edu/abs/1998AJ....115..345S
 | accessdate=2008-06-25
 | doi=10.1086/300176 }}</ref> However a subsequent search using the [[Wide Field Planetary Camera 2]] failed to locate any companions.<ref name=apj119>{{cite journal
 | last=Schroeder | first=Daniel J.
 | coauthors=Golimowski, David A.; Brukardt, Ryan A.; Burrows, Christopher J.; Caldwell, John J.; Fastie, William G.; Ford, Holland C.; Hesman, Brigette; Kletskin, Ilona; Krist, John E.; Royle, Patricia; Zubrowski, Richard. A.
 | title=A Search for Faint Companions to Nearby Stars Using the Wide Field Planetary Camera 2
 | journal=The Astronomical Journal
 | year=2000 | volume=119 | issue=2 | pages=906–922
 | url=http://adsabs.harvard.edu/abs/2000AJ....119..906S
 | accessdate=2008-06-25
 | doi=10.1086/301227 }}</ref> Proxima Centauri, along with Alpha Centauri A and B, are among the "Tier 1" target stars for [[NASA]]'s proposed [[Space Interferometry Mission]] (SIM), which will theoretically be able to detect planets as small as three Earth-masses within two AU of a "Tier 1" target star.<ref name="numbers">{{cite web
 | last=Watanabe | first=Susan | date=2006-10-18
 | url=http://www.jpl.nasa.gov/news/features.cfm?feature=1209
 | title=Planet-Finding by Numbers | publisher = NASA JPL
 | accessdate=2007-07-09 }}</ref>

[[Image:RedDwarfNASA.jpg|right|thumb|Artist's concept of a red dwarf star. ''[[NASA]] illustration''.]]
The TV documentary ''[[Aurelia and Blue Moon#Aurelia|Alien Worlds]]'' hypothesized that a life-sustaining planet could exist in orbit around Proxima Centauri or other red dwarf stars. Such a planet would lie within the [[habitable zone]] of Proxima Centauri, about 0.023–0.054&nbsp;AU from the star, and would have an orbital period of 3.6–14&nbsp;days.<ref>{{cite conference
 | last=Endl | first=M.
 | coauthors=Kuerster, M.; Rouesnel, F.; Els, S.; Hatzes, A. P.; Cochran, W. D.
 | editor=Drake Deming
 | title=Extrasolar Terrestrial Planets: Can We Detect Them Already?
 | booktitle=Conference Proceedings, Scientific Frontiers in Research on Extrasolar Planets
 | pages=75–79 | date=June 18–21, 2002
 | location=Washington DC
 | url=http://arxiv.org/abs/astro-ph/0208462
 | accessdate=2008-06-23 }}</ref> A planet orbiting within this zone will experience [[tidal locking]] to the star, so that Proxima Centauri moves little in the planet's sky, and most of the surface experiences either day or night perpetually. However, the presence of an atmosphere could serve to redistribute the energy from the star-lit side to the far side of the planet.<ref name=tarter>{{cite journal
 | author=Tarter, Jill C. ''et al.''
 | title=A Reappraisal of The Habitability of Planets around M Dwarf Stars
 | journal=Astrobiology | year=2007 | volume=7 | issue=1
 | pages=30–65 | doi=10.1089/ast.2006.0124 }}</ref>

While Proxima Centauri being a flare star means that its flares could cause problems with the atmosphere of any planet in its habitable zone, the documentary's scientists thought that this obstacle could be overcome (see [[Aurelia and Blue Moon#Continued theories|Continued theories]]). Gibor Basri of the [[University of California]], Berkeley, even mentioned that "no one [has] found any showstoppers to habitability." For example, one concern was that the torrents of charged particles from the star's flares could strip the atmosphere off any nearby planet. However, if the planet had a magnetic field, the field would deflect the particles from the atmosphere; even the slow rotation of a tidally locked M-dwarf planet—it spins once for every time it orbits its star—would be enough to generate a magnetic field, as long as part of the planet's interior remained molten.<ref>{{cite web
 | last=Alpert | first=Mark | month=November | year=2005
 | url=http://www.sciam.com/article.cfm?id=red-star-rising
 | title=Red Star Rising | publisher=Scientific American
 | accessdate=2008-05-19 }}</ref>

Other scientists, especially proponents of the [[Rare Earth hypothesis]],<ref>{{cite book
 | first=Peter D. | last=Ward
 | coauthors=Brownlee, Donald | year=2000
 | title=Rare Earth: Why Complex Life is Uncommon in the Universe
 | publisher=Springer | isbn=0387987010 }}</ref> disagree that red dwarf stars can sustain life. The tide-locked rotation may result in a relatively weak planetary [[magnetic moment]], leading to strong atmospheric erosion by [[coronal mass ejections]] from Proxima Centauri.<ref name=Khodachenko>{{cite journal
 | author=Khodachenko, Maxim L. ''et al.''
 | title=Coronal Mass Ejection (CME) Activity of Low Mass M Stars as An Important Factor for The Habitability of Terrestrial Exoplanets. I. CME Impact on Expected Magnetospheres of Earth-Like Exoplanets in Close-In Habitable Zones
 | journal=Astrobiology | year=2007
 | volume=7 | issue=1 | pages=167–184
 | doi=10.1089/ast.2006.0127
}}</ref>

== Interstellar travel ==
[[Image:Sol View from AlpCenA1.png|right|thumb|The Sun as seen from the Alpha Centauri system, using [[Celestia]]]]

Proxima Centauri has been suggested as a possible first destination for [[interstellar travel]].<ref name=gilster/> A flyby en route to [[Alpha Centauri]] is a similar alternative that does not require a deceleration phase. [[Nuclear pulse propulsion]] encompasses several technologies which might enable such interstellar travel with a trip timescale of a century, beginning within the next century, inspiring several studies such as [[Project Orion]], [[Project Daedalus]], and [[Project Longshot]].<ref name=longshot>{{cite web
 | last=Beals | first=K. A.
 | coauthors=Beaulieu, M.; Dembia, F. J.; Kerstiens, J.; Kramer, D. L.; West, J. R.; Zito, J. A.
 | year=1988
 | url=http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19890007533_1989007533.pdf
 |format=PDF| title=Project Longshot, an Unmanned Probe to Alpha Centauri
 | work=NASA-CR-184718 | publisher=U. S. Naval Academy
 | accessdate=2008-06-13 }}</ref>

Although the [[Voyager program]] spacecraft are anticipated to become the first spacecraft to enter interstellar space, they move relatively slowly, at about 17&nbsp;km/s, requiring well over 10,000 years to travel each light-year.<ref>{{cite book
 | author=Mallove, Eugene F.; Gregory L. Matloff
 | title=The starflight handbook: a pioneer's guide to interstellar travel
 | page=6 | publisher=Wiley | year=1989
 | isbn=0471619124 }}. Voyager 1 has an asymptotic velocity of 3.5 AU/yr, Voyager 2 an asymptotic velocity of 3.4 AU/yr.</ref> In comparison, Proxima is presently approaching at a rate of 21.7&nbsp;km/s.<ref name="SIMBAD" /> However, it will only come as close as 3.11 light-years, and then move further away after 26,700 years.<ref name=aaa379/> Thus, a slow-moving probe would have only several tens of thousands of years to catch Proxima Centauri near its closest approach, and could end up watching it recede into the distance. The advent of [[ion thruster]] engines enables interstellar spacecraft faster than the Voyagers, such as the [[Innovative Interstellar Explorer]] proposal, but ion thrusters are believed to be too slow to get a working probe to another star.<ref name="longshot"/>

From Proxima Centauri, the Sun would appear as a bright, 0.4 magnitude star in the constellation [[Cassiopeia (constellation)|Cassiopeia]].<ref>The coordinates of the Sun would be diametrically opposite Proxima, at α={{RA|02|29|42.9487}}, δ={{DEC|+62|40|46.141}}. The absolute magnitude ''M<sub>v</sub>'' of the Sun is 4.83, so at a parallax ''π'' of 0.77199 the apparent magnitude ''m'' is given by 4.83 - 5(log<sub>10</sub>(0.77199) + 1) = 0.40.
See: {{cite book
 | first=Roger John | last=Tayler | year=1994
 | title=The Stars: Their Structure and Evolution
 | publisher=Cambridge University Press
 | pages=16 | isbn=0521458854 }}</ref> If current, non-nuclear propulsion were used, a voyage of thousands of years would likely require a ship large enough to carry a population of people that could be used for colonization of a planet.<ref>{{cite journal
 | last=Crawford | first=I. A.
 | title=Interstellar Travel: A Review for Astronomers
 | journal=Quarterly Journal of the Royal Astronomical Society | volume=31 | pages=377&ndash;400
 | year=1990 | month=September
 | bibcode=1990QJRAS..31..377C }}</ref>

== See also ==

* [[List of nearest stars]]
* [[Orders of magnitude (length)]]
* [[Stars and planetary systems in fiction#Proxima Centauri|Proxima Centauri in fiction]]

== Notes ==
{{reflist|group=nb}}

== References ==
{{reflist|2}}

== External links ==

*{{cite web
 | url=http://antwrp.gsfc.nasa.gov/apod/ap020715.html
 | title= Proxima Centauri: The Closest Star
 | accessdate=2008-06-25 | work=[[NASA]]
 | publisher=Astronomy Picture of the Day
 | date=2002-07-15 }}
*{{cite web
 | url=http://chandra.harvard.edu/photo/2004/proxima/
 | title= Proxima Centauri: The Nearest Star to the Sun
 | accessdate=2008-07-01 | work=Chandra X-ray Observatory
 | publisher=Astronomy Picture of the Day
 | date=2008-07-01 }}
*{{cite web
 | last=James | first=Andrew | date=2008-03-11
 | url=http://homepage.mac.com/andjames/PageAlphaCen006.htm
 | title=A Voyage to Alpha Centauri
 | work=The Imperial Star - Alpha Centauri
 | publisher=Southern Astronomical Delights
 | accessdate=2008-08-05 }}
*{{cite web
 | url=http://www.solstation.com/stars/alp-cent3.htm
 | title=Alpha Centauri 3 | publisher=SolStation
 | accessdate=2008-08-05 }}
*{{cite web
 | url=http://www.extrasolar.net/startour.asp?StarCatId=&StarId=35
 | title=Proxima Centauri | accessdate=2008-06-25
 | work=Extrasolar Visions }}
*{{cite web
 | url=http://www.extrasolar.net/planettour.asp?PlanetID=67
 | title=Proxima Centauri b | accessdate=2008-06-25
 | work=Extrasolar Visions }}
*{{cite web
 | url=http://www.uranometrianova.pro.br/astronomia/AA002/alphacen.htm
 | title=O Sistema Alpha Centauri | accessdate=2008-06-25
 | work=Astronomia & Astrofísica | language=Portuguese }}

{{featured article}}
{{Sky|14|29|42.9487|-|62|40|46.141|4.243}}

[[Category:Centaurus constellation]]
[[Category:Flare stars]]
[[Category:M-type main sequence stars]]
[[Category:Stars with proper names]]
[[Category:HIP objects|070890]]
[[Category:Objects named with variable star designations|Centauri, V645]]

{{Link FA|it}}
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