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Proxima Centauri

Proxima Centauri as seen by Hubble

Observation dataEpoch J2000.0 Equinox J2000.0 (ICRS)

Constellation Centaurus

Pronunciation /ˈprɒksɪmә sɛnˈtɔriː/[nb 1]

Right ascension 14h 29m 42.94853s[1]

Declination −62° 40′ 46.1631″[1]

Apparent magnitude (V) 11.13[2]

Characteristics

Spectral type M6 Ve[3]

Apparent magnitude (J) 5.357 ± 0.023[4]

U−B color index 1.26[2]

B−V color index 1.82[2]

Variable type Flare star

Astrometry

Radial velocity (Rv) −22.4 ± 0.5[3] km/s

Proper motion (μ) RA: −3775.75[1] mas/yr

Proxima CentauriFrom Wikipedia, the free encyclopedia

This article is about the star in the Alpha Centauri system. For other uses, see Proxima Centauri(disambiguation).

Proxima Centauri (Latin proxima, meaning "next to"or "nearest to")[13] is a red dwarf about 4.24 light­yearsfrom the Sun, inside the G­cloud, in the constellation ofCentaurus.[14][15] It was discovered in 1915 by theScottish astronomer Robert Innes, the Director of theUnion Observatory in South Africa, and is the nearestknown star to the Sun,[11] although it is too faint to beseen with the naked eye, with an apparent magnitude of11.05. Its distance to the second­ and third­nearest stars,which form the bright binary Alpha Centauri, is0.237 ± 0.011 ly (15,000 ± 700 AU).[16] ProximaCentauri is very likely part of a triple star system withAlpha Centauri A and B, but its orbital period may begreater than 500,000 years.

Because of Proxima Centauri's proximity, its distancefrom Earth and angular diameter can be measureddirectly, from which it can be determined that itsdiameter is about one­seventh of that of the Sun.[11]Proxima Centauri's mass is about an eighth of the Sun's(M), and its average density is about 40 times that of

the Sun.[nb 2] Although it has a very low averageluminosity, Proxima is a flare star that undergoesrandom dramatic increases in brightness because ofmagnetic activity.[17] The star's magnetic field is createdby convection throughout the stellar body, and theresulting flare activity generates a total X­ray emissionsimilar to that produced by the Sun.[18] The mixing ofthe fuel at Proxima Centauri's core through convectionand its relatively low energy­production rate mean thatit will be a main­sequence star for another four trillionyears,[19] or nearly 300 times the current age of theuniverse.[20]

Searches for companions orbiting Proxima Centaurihave been unsuccessful, ruling out the presence ofbrown dwarfs and supermassive planets.[21][22] Precisionradial velocity surveys have also ruled out the presence

Coordinates: 14h 29m 42.9487s, −62° 40′ 46.141″

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Dec.: 765.54[1] mas/yr

Parallax (π) 768.13 ± 1.04[2] mas

Distance 4.246 ± 0.006 ly(1.302 ± 0.002 pc)

Absolute magnitude (MV) 15.60[5]

Details

Mass 0.123 ± 0.006[6] M

Radius 0.141 ± 0.007[7] R

Luminosity (bolometric) 0.0017[8] L

Surface gravity (log g) 5.20 ± 0.23[6] cgs

Temperature 3,042 ± 117[6] K

Metallicity [Fe/H] 0.21[9] dex

Rotation 83.5 days[10]

Rotational velocity(v sin i)

< 0.09[3] km/s

Age 4.85[11] Gyr

Other designations

Alpha Centauri C, CCDM J14396­6050C,GCTP 3278.00, GJ 551, HIP 70890,LFT 1110, LHS 49, LPM 526, LTT 5721,

NLTT 37460, V645 Centauri[12]

Database references

SIMBAD data (http://simbad.u­strasbg.fr/simbad/sim­id?Ident=V645+Cen)

of super­Earths within the star's habitable zone.[23][nb 3]The detection of smaller objects will require the use ofnew instruments, such as the James Webb SpaceTelescope, which is scheduled for deployment in2018.[24] Because Proxima Centauri is a red dwarf and aflare star, whether a planet orbiting it could support lifeis disputed.[25][26] Nevertheless, because of the star'sproximity to Earth, it has been proposed as a destinationfor interstellar travel.[27]

Contents

1 Observation2 Characteristics

2.1 Distance and motion2.2 Possible companions2.3 Habitable zone

3 Interstellar travel4 See also5 References

5.1 Explanatory notes5.2 Citations

6 External links

Observation

In 1915, the Scottish astronomer Robert Innes, Directorof the Union Observatory in Johannesburg, SouthAfrica, discovered a star that had the same propermotion as Alpha Centauri.[28][29][30][31] He suggested itbe named Proxima Centauri[32] (actually ProximaCentaurus).[33] In 1917, at the Royal Observatory at theCape of Good Hope, the Dutch astronomer Joan Voûtemeasured the star's trigonometric parallax at0.755 ± 0.028″ and determined that Proxima Centauri was approximately the same distance from the Sun asAlpha Centauri. It was also found to be the lowest­luminosity star known at the time.[34] An equallyaccurate parallax determination of Proxima Centauri was made by American astronomer Harold L. Alden in1928, who confirmed Innes's view that it is closer, with a parallax of 0.783 ± 0.005″.[29][32]

In 1951, American astronomer Harlow Shapley announced that Proxima Centauri is a flare star.Examination of past photographic records showed that the star displayed a measurable increase inmagnitude on about 8% of the images, making it the most active flare star then known.[36][37] The proximityof the star allows for detailed observation of its flare activity. In 1980, the Einstein Observatory produced a

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Stars closest to the Sun, including ProximaCentauri (April 25, 2014).[35]

detailed X­ray energy curve of a stellar flare on Proxima Centauri. Further observations of flare activitywere made with the EXOSAT and ROSAT satellites, and the X­ray emissions of smaller, solar­like flareswere observed by the Japanese ASCA satellite in 1995.[38] Proxima Centauri has since been the subject ofstudy by most X­ray observatories, including XMM­Newton and Chandra.[39]

Because of Proxima Centauri's southern declination, itcan only be viewed south of latitude 27° N.[nb 4] Reddwarfs such as Proxima Centauri are far too faint to beseen with the naked eye. Even from Alpha Centauri Aor B, Proxima would only be seen as a fifth magnitudestar.[40][41] It has an apparent visual magnitude of 11, soa telescope with an aperture of at least 8 cm (3.1 in.) isneeded to observe it, even under ideal viewingconditions—under clear, dark skies with ProximaCentauri well above the horizon.[42]

Characteristics

Proxima Centauri is a red dwarf, because it belongs tothe main sequence on the Hertzsprung–Russell diagram and is of spectral class M6. M6 means that it fallsin the low­mass end of M­type stars.[11] Its absolute visual magnitude, or its visual magnitude as viewedfrom a distance of 10 parsecs, is 15.5.[43] Its total luminosity over all wavelengths is 0.17% that of theSun,[8] although when observed in the wavelengths of visible light the eye is most sensitive to, it is only0.0056% as luminous as the Sun.[44] More than 85% of its radiated power is at infrared wavelengths.[45]

In 2002, optical interferometry with the Very Large Telescope (VLTI) found that the angular diameter ofProxima Centauri was 1.02 ± 0.08 milliarcsec. Because its distance is known, the actual diameter ofProxima Centauri can be calculated to be about 1/7 that of the Sun, or 1.5 times that of Jupiter. The star'sestimated mass is 12.3% M, or 129 Jupiter masses (MJ).[46] The mean density of a main­sequence star

increases with decreasing mass,[47] and Proxima Centauri is no exception: it has a mean density of56.8 × 103 kg/m3 (56.8 g/cm3), compared with the Sun's mean density of 1.411 × 103 kg/m3

(1.411 g/cm3).[nb 2]

Because of its low mass, the interior of the star is completely convective, causing energy to be transferredto the exterior by the physical movement of plasma rather than through radiative processes. This convectionmeans that the helium ash left over from the thermonuclear fusion of hydrogen does not accumulate at thecore, 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 nearly all ofits fuel before the fusion of hydrogen comes to an end.[19]

Convection is associated with the generation and persistence of a magnetic field. The magnetic energy fromthis field is released at the surface through stellar flares that briefly increase the overall luminosity of thestar. These flares can grow as large as the star and reach temperatures measured as high as 27 million K[39]

—hot enough to radiate X­rays.[48] Indeed, Proxima Centauri's quiescent X­ray luminosity, approximately

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This illustration shows the comparative sizes of(from left to right) the Sun, α Centauri A, αCentauri B, and Proxima Centauri

The two bright stars are (left) Alpha Centauriand (right) Beta Centauri. The faint red star inthe center of the red circle is Proxima Centauri.

(4–16) × 1026 erg/s ((4–16) × 1019 W), is roughly equal tothat of the much larger Sun. The peak X­ray luminosity ofthe largest flares can reach 1028 erg/s (1021 W.)[39]

Proxima Centauri's chromosphere is active, and itsspectrum displays a strong emission line of singly ionizedmagnesium at a wavelength of 280 nm.[49] About 88% ofthe surface of Proxima Centauri may be active, apercentage that is much higher than that of the Sun even atthe peak of the solar cycle. Even during quiescent periodswith few or no flares, this activity increases the coronatemperature of Proxima Centauri to 3.5 million K,compared to the 2 million K of the Sun's corona.[50]However, Proxima Centauri's overall activity level isconsidered low compared to other red dwarfs,[18] which isconsistent with the star's estimated age of4.85 × 109 years,[11] since the activity level of a red dwarfis expected to steadily wane over billions of years as itsstellar rotation rate decreases.[51] The activity level alsoappears to vary with a period of roughly 442 days, whichis shorter than the solar cycle of 11 years.[52]

Proxima Centauri has a relatively weak stellar wind, nomore than 20% of mass loss rate of the solar wind.Because the star is much smaller than the Sun, however,the mass loss per unit surface area from Proxima Centaurimay be eight times that from the solar surface.[53]

A red dwarf with the mass of Proxima Centauri willremain on the main sequence for about four trillion years. As the proportion of helium increases because ofhydrogen fusion, the star will become smaller and hotter, gradually transforming from red to blue. Near theend of this period it will become significantly more luminous, reaching 2.5% of the Sun's luminosity (L)and warming up any orbiting bodies for a period of several billion years. Once the hydrogen fuel isexhausted, Proxima Centauri will then evolve into a white dwarf (without passing through the red giantphase) and steadily lose any remaining heat energy.[19]

Distance and motion

Based on the parallax of 768.7 ± 0.3 milliarcseconds, measured using the Hipparcos astrometry satellite,[54]

and more precisely with the Fine Guidance Sensors on the Hubble Space Telescope,[5] Proxima Centauri isabout 4.24 light­years (ly) from the Sun, or 270,000 times more distant than Earth is from the Sun. FromEarth's vantage point, Proxima is separated by 2.18°[55] from Alpha Centauri, or four times the angulardiameter of the full Moon.[56] Proxima also has a relatively large proper motion—moving 3.85 arcsecondsper year across the sky.[57] It has a radial velocity toward the Sun of 22.4 km/s.

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Distances of the nearest stars from 20,000 years ago until80,000 years in the future. Proxima Centauri is in yellow

Among the known stars, Proxima Centauri hasbeen the closest star to the Sun for about32,000 years and will be so for about another33,000 years, after which the closest star to theSun will be Ross 248.[58] In 2001, J. García­Sánchez et al. predicted that Proxima will makeits closest approach to the Sun, coming within3.11 ly of the latter, in approximately26,700 years.[59] A 2010 study by V. V.Bobylev predicted a closest approach distanceof 2.90 ly in about 27,400 years.[60] ProximaCentauri is orbiting through the Milky Way at adistance from the Galactic Center that variesfrom 8.3 to 9.5 kpc, with an orbital eccentricityof 0.07.[61]

Ever since the discovery of Proxima it has beensuspected to be a true companion of the AlphaCentauri binary star system. At a distance to Alpha Centauri of just 0.21 ly (15,000 ± 700 AU),[16] ProximaCentauri may be in orbit around Alpha Centauri, with an orbital period of the order of 500,000 years ormore. For this reason, Proxima is sometimes referred to as Alpha Centauri C. Modern estimates, taking intoaccount the small separation between and relative velocity of the stars, suggest that the chance of theobserved alignment being a coincidence is roughly one in a million.[62] Data from the Hipparcos satellite,combined with ground­based observations, is consistent with the hypothesis that the three stars are truly abound system. If so, Proxima would currently be near apastron, the farthest point in its orbit from the AlphaCentauri system. Such a triple system can form naturally through a low­mass star being dynamicallycaptured by a more massive binary of 1.5–2 M within their embedded star cluster before the cluster

disperses.[63] More accurate measurement of the radial velocity is needed to confirm this hypothesis.[16]

If Proxima was bound to the Alpha Centauri system during its formation, the stars would be likely to sharethe same elemental composition. The gravitational influence of Proxima may also have stirred up the AlphaCentauri protoplanetary disks. This would have increased the delivery of volatiles such as water to the dryinner regions. Any terrestrial planets in the system may have been enriched by this material.[16]

Six single stars, two binary star systems, and a triple star share a common motion through space withProxima Centauri and the Alpha Centauri system. The space velocities of these stars are all within 10 km/sof Alpha Centauri's peculiar motion. Thus, they may form a moving group of stars, which would indicate acommon point of origin,[64] such as in a star cluster. If it is determined that Proxima Centauri is notgravitationally bound to Alpha Centauri, then such a moving group would help explain their relatively closeproximity.[65]

Though Proxima Centauri is the nearest bona fide star, it is still possible that one or more as­yet undetectedsub­stellar brown dwarfs may lie closer.[66]

Possible companions

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RV­derived upper mass limits ofhypothetical companion[23]

Orbitalperiod(days)

Separation(AU)

Maximummass[nb 3](× Earth)

3.6–13.8 0.022–0.054 2–3<100 <0.21 8.5<1000 <1 16

If a massive planet is orbiting Proxima Centauri, it would cause somedisplacement of Proxima Centauri over the course of the planet's orbit.If the orbital plane of the planet is not perpendicular to the line of sightfrom Earth, then this displacement would cause periodic changes in theradial velocity of Proxima Centauri. The fact that multiplemeasurements of the star's radial velocity have detected no such shiftshas lowered the maximum mass that a possible companion to ProximaCentauri could possess.[5][21] The activity level of the star adds noise tothe radial velocity measurements, limiting future prospects for detectionof a companion using this method.[67]

In 1998, an examination of Proxima Centauri using the Faint Object Spectrograph on board the HubbleSpace Telescope appeared to show evidence of a companion orbiting at a distance of about 0.5 AU.[68]However, a subsequent search using the Wide Field Planetary Camera 2 failed to locate anycompanions.[22] Astrometric measurements at the Cerro Tololo Inter­American Observatory appear to ruleout a Jovian companion with an orbital period of 2−12 years.[69]

Proxima Centauri, along with Alpha Centauri A and B, was among the "Tier 1" target stars for NASA'snow­canceled Space Interferometry Mission (SIM), which would theoretically have been able to detectplanets as small as three Earth masses (M⊕) within two AU of a "Tier 1" target star.[24]

Habitable zone

See also: Habitability of red dwarf systems

The TV documentary Alien Worlds hypothesized that a life­sustaining planet could exist in orbit aroundProxima Centauri or other red dwarfs. Such a planet would lie within the habitable zone of ProximaCentauri, about 0.023–0.054 AU from the star, and would have an orbital period of 3.6–14 days.[70] Aplanet orbiting within this zone will experience tidal locking to the star, so that Proxima Centauri moveslittle in the planet's sky, and most of the surface experiences either day or night perpetually. However, thepresence of an atmosphere could serve to redistribute the energy from the star­lit side to the far side of theplanet.[25]

Proxima Centauri's flare outbursts could erode the atmosphere of any planet in its habitable zone, but thedocumentary's scientists thought that this obstacle could be overcome (see continued theories). Gibor Basriof the University of California, Berkeley, even mentioned that "no one [has] found any showstoppers tohabitability." For example, one concern was that the torrents of charged particles from the star's flares couldstrip the atmosphere off any nearby planet. However, if the planet had a strong magnetic field, the fieldwould deflect the particles from the atmosphere; even the slow rotation of a tidally locked dwarf planet thatspins once for every time it orbits its star would be enough to generate a magnetic field, as long as part ofthe planet's interior remained molten.[71]

Other scientists, especially proponents of the Rare Earth hypothesis,[72] disagree that red dwarfs can sustainlife. The tide­locked rotation may result in a relatively weak planetary magnetic moment, leading to strongatmospheric erosion by coronal mass ejections from Proxima Centauri.[26]

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The Sun as seen from the AlphaCentauri system, using Celestia

Interstellar travel

Proxima Centauri has been suggested as a possible first destinationfor interstellar travel.[27] The star is in motion toward Earth at a rateof 22.4 km/s. Ηowever, after 26,700 years, when it will come asclose as 3.11 light­years, it will begin to move farther away.[59] Ifnon­nuclear propulsion were used, a voyage of a spacecraft to aplanet orbiting Proxima Centauri would probably require thousandsof years.[73] For example, Voyager 1, which is now travelling17.043 km/s (38,120 mph) relative to the Sun, would reach Proximain 73,775 years, were the spacecraft traveling in the direction of thatstar. A slow­moving probe would have only several tens ofthousands of years to catch Proxima Centauri near its closestapproach, and could end up watching it recede into the distance.[74]Nuclear pulse propulsion might enable such interstellar travel with a trip timescale of a century, beginningwithin the next century, inspiring several studies such as Project Orion, Project Daedalus, and ProjectLongshot.[74]

From Proxima Centauri, the Sun would appear as a bright 0.4­magnitude star in the constellationCassiopeia.[75]

See also

Orders of magnitude (length)Proxima Centauri in fiction

References

Explanatory notes

1. Proxima is pronounced /ˈprɒksɪmә/. Centauri may be pronounced /sɛnˈtɔriː/ or /sɛnˈtɔraɪ/.2. The density (ρ) is given by the mass divided by the volume. Relative to the Sun, therefore, the density is:

=

= 0.123 · 0.145−3 · 1.41 × 103 kg/m3

= 40.3 · 1.41 × 103 kg/m3

= 5.68 × 104 kg/m3

where is the average solar density. See:

Munsell, Kirk; Smith, Harman; Davis, Phil; Harvey, Samantha (June 11, 2008). "Sun: Facts & Figures".Solar System Exploration. NASA. Retrieved July 12, 2008.Bergman, Marcel W.; Clark, T. Alan; Wilson, William J. F. (2007). Observing Projects Using StarryNight Enthusiast (8th ed.). Macmillan. pp. 220–221. ISBN 1­4292­0074­X.

3. This is actually an upper limit on the quantity m sin i, where i is the angle between the orbit normal and the line

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of sight, in a circular orbit. If the planetary orbits are close to face­on as observed from Earth, or in an eccentricorbit, more massive planets could have evaded detection by the radial velocity method.

4. For a star south of the zenith, the angle to the zenith is equal to the Latitude minus the Declination. The star ishidden from sight 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: Campbell, William Wallace (1899). The Elements of Practical Astronomy. London: Macmillan. pp. 109–110. Retrieved August 12, 2008.

Citations

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3. Torres, C. A. O.; et al. (December 2006). "Search for associations containing young stars (SACY). I. Sample andsearching method". Astronomy and Astrophysics 460 (3): 695–708. arXiv:astro­ph/0609258.Bibcode:2006A&A...460..695T. doi:10.1051/0004­6361:20065602.

4. Cutri, R. M.; Skrutskie, M. F.; Van Dyk, S.; Beichman, C. A.; Carpenter, J. M.; Chester, T.; Cambresy, L.;Evans, T.; Fowler, J.; Gizis, J.; Howard, E.; Huchra, J.; Jarrett, T.; Kopan, E. L.; Kirkpatrick, J. D.; Light, R.M.; Marsh, K. A.; McCallon, H.; Schneider, S.; Stiening, R.; Sykes, M.; Weinberg, M.; Wheaton, W. A.;Wheelock, S.; Zacarias, N. (2003). "VizieR Online Data Catalog: 2MASS All­Sky Catalog of Point Sources(Cutri+ 2003)". VizieR On­line Data Catalog: II/246. Originally published in: 2003yCat.2246....0C 2246: 0.Bibcode:2003yCat.2246....0C.

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8. See Table 1, Doyle, J. G.; Butler, C. J. (1990). "Optical and infrared photometry of dwarf M and K stars".Astronomy and Astrophysics 235: 335–339. Bibcode:1990A&A...235..335D. and p. 57, Peebles, P. J. E. (1993).Principles of Physical Cosmology. Princeton, New Jersey: Princeton University Press. ISBN 0­691­01933­9.

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10. Benedict, G. F., McArthur, B., et al. (1998). "Photometry of Proxima Centauri and Barnard's Star Using HubbleSpace Telescope Fine Guidance Sensor 3: A Search for Periodic Variations". The Astronomical Journal 116 (1):429–439. arXiv:astro­ph/9806276. Bibcode:1998AJ....116..429B. doi:10.1086/300420.

11. Kervella, Pierre; Thevenin, Frederic (March 15, 2003). "A Family Portrait of the Alpha Centauri System: VLTInterferometer Studies the Nearest Stars". ESO. Retrieved July 9, 2007.

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24. Watanabe, Susan (October 18, 2006). "Planet­Finding by Numbers". NASA JPL. Retrieved July 9, 2007.25. Tarter, Jill C., et al. (2007). "A Reappraisal of The Habitability of Planets around M Dwarf Stars". Astrobiology

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75. The coordinates of the Sun would be diametrically opposite Proxima, at α=02h 29m 42.9487s,δ=+62° 40′ 46.141″. The absolute magnitude Mv of the Sun is 4.83, so at a parallax π of 0.77199 the apparentmagnitude m is given by 4.83 − 5(log10(0.77199) + 1) = 0.40. See: Tayler, Roger John (1994). The Stars: TheirStructure and Evolution. Cambridge University Press. p. 16. ISBN 0­521­45885­4.

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External links

"Proxima Centauri: The Closest Star". NASA. AstronomyPicture of the Day. July 15, 2002. Retrieved June 25, 2008.

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Categories: Alpha Centauri Centaurus (constellation) Flare stars M­type main­sequence starsStars with proper names Hipparcos objects Gliese and GJ objectsObjects named with variable star designations Astronomical objects discovered in 1915

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