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Alpha Centauri

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PDF generated using the open source mwlib toolkit. See http://code.pediapress.com/ for more information.PDF generated at: Mon, 14 Apr 2014 11:36:11 UTC

Alpha Centauri

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ContentsArticles

Alpha Centauri 1Proxima Centauri 16Binary star 23

ReferencesArticle Sources and Contributors 35Image Sources, Licenses and Contributors 36

Article LicensesLicense 37

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Alpha Centauri 1

Alpha Centauri

Alpha Centauri A/B

The position of Alpha Centauri A and Alpha Centauri BObservation data

Epoch J2000.0      Equinox J2000.0

Constellation Centaurus

Alpha Centauri A

Right ascension 14h 39m 36.4951s

Declination –60° 50′ 02.308″

Apparent magnitude (V) −0.01

Alpha Centauri B

Right ascension 14h 39m 35.0803s

Declination –60° 50′ 13.761″

Apparent magnitude (V) +1.33

Characteristics

Spectral type G2 V

U−B color index +0.23

B−V color index +0.69

Characteristics

Spectral type K1 V

U−B color index +0.63

B−V color index +0.90

Astrometry

Radial velocity (Rv) −21.6 km/s

Proper motion (μ) RA: −3678.19 mas/yrDec.: 481.84 mas/yr

Parallax (π) 747.1 ± 1.2[1] mas

Distance 4.366 ± 0.007 ly(1.339 ± 0.002 pc)

Absolute magnitude (MV

) 4.38 / 5.71

Details

Alpha Centauri A

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Mass 1.100 M☉

Radius 1.227 R☉

Luminosity 1.519 L☉

Surface gravity (log g) 4.30 cgs

Temperature 5790 K

Metallicity 151% Sun

Rotation ~22.5 ± 5.9 days

Age 6 ± 1 Gyr

Alpha Centauri B

Mass 0.907 M☉

Radius 0.865 R☉

Luminosity 0.500 L☉

Surface gravity (log g) 4.37 cgs

Temperature 5260 K

Metallicity 160% Sun

Rotation 47 days

Orbit

Companion Alpha Centauri AB

Period (P) 79.91 ± 0.011 yr

Semi-major axis (a) 17.57 ± 0.022"

Eccentricity (e) 0.5179 ± 0.00076

Inclination (i) 79.205 ± 0.041°

Longitude of the node (Ω) 204.85 ± 0.084°

Periastron epoch (T) 1875.66 ± 0.012

Argument of periastron (ω)(secondary)

231.65 ± 0.076°

Other designations

Rigil Kentaurus, Rigil Kent, Toliman, Bungula, FK5 538, CP(D)−60°5483, GC 19728, CCDM J14396-6050α Cen A

α1 Centauri, GJ 559 A, HR 5459, HD 128620, GCTP 3309.00, LHS 50, SAO 252838, HIP 71683α Cen B

α2 Centauri, GJ 559 B, HR 5460, HD 128621, LHS 51, HIP 71681α Cen C (= Proxima Cen)LHS 49, HIP 70890

Database references

SIMBAD data [2]

Exoplanet Archive data [3]

ARICNS data [4]

Extrasolar PlanetsEncyclopaedia

data [5]

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Location of Alpha Centauri in Centaurus (right-click on starmap to enlarge)Alpha Centauri (α Centauri, α Cen; also known as Rigil Kent /ˈraɪdʒəlHelp:IPA for English#Keyˈkɛnt/—seeNames) is the brightest star in the southern constellation of Centaurus, and the third brightest star in the nightsky.[6][7] The Alpha Centauri system is located 1.34 parsecs or 4.37 light years from the Sun, making it the closeststar system to the Solar System. Although it appears to the unaided eye as a single object, Alpha Centauri is actuallya binary star system (designated Alpha Centauri AB or α Cen AB) whose combined visual magnitude of −0.27makes it the third brightest star (other than the Sun) seen from Earth after the −1.46 magnitude Sirius and the −0.72magnitude Canopus.Its component stars are named Alpha Centauri A (α Cen A), with 110% of the mass and 151.9% the luminosity ofthe Sun, and Alpha Centauri B (α Cen B), at 90.7% of the Sun's mass and 44.5% of its luminosity. During the pair's79.91-year orbit about a common center, the distance between them varies from about that between Pluto and theSun to that between Saturn and the Sun.A third star, known as Proxima Centauri, Proxima, or Alpha Centauri C (α Cen C), is probably gravitationallyassociated with Alpha Centauri AB. Proxima is at the slightly smaller distance of 1.29 parsecs or 4.24 light yearsfrom the Sun, making it the closest star to the Sun, even though it is not visible to the naked eye. The separation ofProxima from Alpha Centauri AB is about 0.06 parsecs, 0.2 light years or 13,000 astronomical units (AU);equivalent to 400 times the size of Neptune's orbit.The system may also contain at least one planet, the Earth-sized Alpha Centauri Bb, which if confirmed will be theclosest known exoplanet to Earth. The planet has a mass at least 113% of Earth's and orbits Alpha Centauri B with aperiod of 3.236 days. Orbiting at a distance of 6 million kilometers from the star, 4% of the distance of the Earth tothe Sun and a tenth of the distance between Mercury and the Sun, the planet has an estimated surface temperature of1500 K (roughly 1200 °C), too hot to be habitable.[8] More recently, on June 10, 2013, scientists reported that theearlier claims of an Earth-like exoplanet orbiting Alpha Centauri B may not be supported.

Nature and components

Mobile notation diagram of the system

"Alpha Centauri" is the name given to what appears as a single star tothe naked eye and the brightest star in the southern constellation ofCentaurus. At −0.27v visual magnitude, it is fainter only than Siriusand Canopus. The next brightest star in the night sky is Arcturus.Actually a multiple star system, its two main stars are AlphaCentauri A (α Cen A) and Alpha Centauri B (α Cen B), usuallydefined to identify them as the different components of the binary αCen AB. A third companion—Proxima Centauri (or Proxima or α CenC)—has a distance much greater than the observed separation betweenstars A and B and is probably gravitationally associated with the ABsystem. As viewed from Earth, it is located at an angular separation of

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2.2° from the two main stars. If it were bright enough to be seen without a telescope, Proxima Centauri would appearto the naked eye as a star separate from α Cen AB. Alpha Centauri AB and Proxima Centauri form a visual doublestar. Direct evidence that Proxima Centauri has an elliptical orbit typical of binary stars has yet to be found. Togetherall three components make a triple star system, referred to by double-star observers as the triple star (or multiplestar), α Cen AB-C.

Artist’s impression of the planet around Alpha Centauri B

View of Alpha Centauri from the Digitized Sky Survey 2

Alpha Centauri A is the principal member,or primary, of the binary system, beingslightly larger and more luminous than theSun. It is a solar-like main-sequence starwith a similar yellowish color, whose stellarclassification is spectral type G2 V. Fromthe determined mutual orbital parameters,Alpha Centauri A is about 10% moremassive than the Sun, with a radius about23% larger. The projected rotationalvelocity ( v·sin i ) of this star is 2.7 ± 0.7km·s−1, resulting in an estimated rotationalperiod of 22 days, which gives it a slightlyfaster rotational period than the Sun's 25days. When considered among theindividual brightest stars in the sky(excluding the Sun), Alpha Centauri A is thefourth brightest at −0.01 magnitude, beingfractionally fainter than Arcturus at −0.04vmagnitude.

Alpha Centauri B is the companion star, orsecondary, of the binary system, and isslightly smaller and less luminous than theSun. It is a main-sequence star is of spectraltype K1 V, making it more an orange colorthan the primary star. Alpha Centauri B isabout 90% the mass of the Sun and 14%smaller in radius. The projected rotationalvelocity ( v·sin i ) is 1.1 ± 0.8 km·s−1,resulting in an estimated rotational period of41 days. (An earlier, 1995 estimate gave asimilar rotation period of 36.8 days.)Although it has a lower luminosity thancomponent A, star B emits more energy inthe X-ray band. The light curve of B varies on a short time scale and there has been at least one observed flare.Alpha Centauri B at 1.33v magnitude would be twenty-first in brightness if it could be seen independently of AlphaCentauri A.

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Component sizes and colors. Shows the relative sizes and colors of stars in theAlpha Centauri system and compares them with those of the Sun.

Alpha Centauri C, also known as ProximaCentauri, is of spectral class M5 Ve orM5 VIe, suggesting this is either a smallmain-sequence star (Type V) or subdwarf(VI) with emission lines. Its B−V colorindex is +1.90 and its mass is about0.123 M☉,[9] or 129 Jupiter masses.

Together, the bright visible components ofthe binary star system are called AlphaCentauri AB (α Cen AB). This "AB"designation denotes the apparent gravitational centre of the main binary system relative to other companion star(s) inany multiple star system. "AB-C" refers to the orbit of Proxima around the central binary, being the distance betweenthe centre of gravity and the outlying companion. Some older references use the confusing and now discontinueddesignation of A×B. Since the distance between the Sun and Alpha Centauri AB does not differ significantly fromeither star, gravitationally this binary system is considered as if it were one object.

Asteroseismic studies, chromospheric activity, and stellar rotation (gyrochronology), are all consistent with the αCen system being similar in age to, or slightly older than, the Sun, with typical ages quoted between 4.5 and 7 billionyears (Gyr). Asteroseismic analyses that incorporate the tight observational constraints on the stellar parameters forα Cen A and/or B have yielded age estimates of 4.85 ± 0.5 Gyr, 5.0 ± 0.5 Gyr, 5.2–7.1 Gyr, 6.4 Gyr, and 6.52 ± 0.3Gyr. Age estimates for stars A and B based on chromospheric activity (Calcium H & K emission) yield 4.4–6.5 Gyr,while gyrochronology yields 5.0 ± 0.3 Gyr.

ObservationThe Alpha Centauri AB binary is too close to be resolved by the naked eye, because the angular separation variesbetween 2 and 22 arcsec, but through much of the orbit, both are easily resolved in binoculars or small 5 cm (2 in)telescopes.In the southern hemisphere, Alpha Centauri forms the outer star of The Pointers or The Southern Pointers, so calledbecause the line through Beta Centauri (Hadar/Agena), some 4.5° west, points directly to the constellation Crux —the Southern Cross. The Pointers easily distinguish the true Southern Cross from the fainter asterism known as theFalse Cross.South of about 29° S latitude, Alpha Centauri is circumpolar and never sets below the horizon.[10] Both stars,including Crux, are too far south to be visible for mid-latitude northern observers. Below about 29° N latitude to theequator (roughly Hermosillo, Chihuahua in Mexico, Galveston, Texas, Ocala, Florida and Lanzarote, the CanaryIslands of Spain) during the northern summer, Alpha Centauri lies close to the southern horizon. The star culminateseach year at midnight on 24 April or 9 p.m. on 8 June.As seen from Earth, Proxima Centauri lies 2.2° southwest from Alpha Centauri AB. This is about four times theangular diameter of the Full Moon, and almost exactly half the distance between Alpha Centauri AB and BetaCentauri. Proxima usually appears as a deep-red star of 13.1v visual magnitude in a poorly populated star field,requiring moderately sized telescopes to see. Listed as V645 Cen in the General Catalogue of Variable Stars(G.C.V.S.) Version 4.2, this UV Ceti-type flare star can unexpectedly brighten rapidly to about 11.0v or 11.09Vmagnitude. Some amateur and professional astronomers regularly monitor for outbursts using either optical or radiotelescopes.

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Observational historyEnglish explorer Robert Hues brought Alpha Centauri to the attention of European observers in his 1592 workTractatus de Globis, along with Canopus and Achernar, noting "Now, therefore, there are but three Stars of the firstmagnitude that I could perceive in all those parts which are never seene here in England. The first of these is thatbright Star in the sterne of Argo which they call Canobus. The second is in the end of Eridanus. The third is in theright foote of the Centaure."[11]

The binary nature of Alpha Centauri AB was first recognized in December 1689 by astronomer and Jesuit priest JeanRichaud. The finding was made incidentally while observing a passing comet from his station in Puducherry. AlphaCentauri was only the second binary star system to be discovered, preceded only by Alpha Crucis. By 1752, Frenchastronomer Abbé Nicolas Louis de Lacaille made astrometric positional measurements using a meridian circle whileJohn Herschel, in 1834, made the first micrometrical observations. Since the early 20th century, measures have beenmade with photographic plates.By 1926, South African astronomer William Stephen Finsen calculated the approximate orbit elements close to thosenow accepted for this system.[12] All future positions are now sufficiently accurate for visual observers to determinethe relative places of the stars from a binary star ephemeris. Others, like the Belgian astronomer D. Pourbaix (2002),have regularly refined the precision of any new published orbital elements.

Alpha Centauri A and B resolved over the limb of Saturn, as seen byCassini–Huygens

The two bright stars are (left) Alpha Centauri and (right) Beta Centauri. Thefaint red star in the center of the red circle is Proxima Centauri. Taken with

Canon 85mm f/1.8 lens with 11 frames stacked, each frame exposed 30seconds.

Alpha Centauri is the closest star system to theSolar System. It lies about 4.37 light-years indistance, or about 41.5 trillion kilometres, 25.8trillion miles or 277,600 AU. Scottishastronomer Thomas Henderson made theoriginal discovery from many exactingobservations of the trigonometric parallaxes ofthe AB system between April 1832 and May1833. He withheld the results because hesuspected they were too large to be true, buteventually published in 1839 after FriedrichWilhelm Bessel released his own accuratelydetermined parallax for 61 Cygni in 1838.[13]

For this reason, Alpha Centauri is considered asthe second star to have its distance measuredbecause it was not formally recognized first.Alpha Centauri is currently inside the G-cloud,and the nearest known system to it is Luhman 16at 3.6 light years.

Scottish astronomer Robert Innes discoveredProxima Centauri in 1915 by blinkingphotographic plates taken at different timesduring a dedicated proper motion survey. Thisshowed the large proper motion and parallax ofthe star was similar in both size and direction tothose of Alpha Centauri AB, suggestingimmediately it was part of the system andslightly closer to us than Alpha Centauri AB. Lying 4.24 light-years away, Proxima Centauri is the nearest star to the

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Sun. All current derived distances for the three stars are from the parallaxes obtained from the Hipparcos star catalog(HIP).

Binary system

Apparent and true orbits of Alpha Centauri. The A component is held stationaryand 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 trueorbit is the shape of the orbit viewed perpendicular to the plane of the orbital

motion. According to the radial velocity vs. time the radial separation of A and Balong the line of sight had reached a maximum in 2007 with B being behind A.Since the orbit is divided here into 80 points, each step refers to a timestep of

approx. 0.99888 years or 364.84 days.

With the orbital period of 79.91 years, the Aand B components of this binary star canapproach each other to 11.2 astronomicalunits, equivalent to 1.67 billion km or aboutthe mean distance between the Sun andSaturn, or may recede as far as 35.6 AU (5.3billion km—approximately the distancefrom the Sun to Pluto).[14] This is aconsequence of the binary's moderate orbitaleccentricity e = 0.5179. From the orbitalelements, the total mass of both stars isabout 2.0 M☉

[15]—or twice that of the Sun.The average individual stellar masses are1.09 M☉ and 0.90 M☉, respectively, thoughslightly higher masses have been quoted inrecent years, such as 1.14 M☉ and 0.92 M☉,or totalling 2.06 M☉. Alpha Centauri A andB have absolute magnitudes of +4.38 and+5.71, respectively. Stellar evolution theoryimplies both stars are slightly older than theSun at 5 to 6 billion years, as derived byboth mass and their spectral characteristics.

Viewed from Earth, the apparent orbit ofthis binary star means that the separationand position angle (P.A.) are in continuouschange throughout the projected orbit. Observed stellar positions in 2010 are separated by 6.74 arcsec through theP.A. of 245.7°, reducing to 6.04 arcsec through 251.8° in 2011. Next closest approach will be in February 2016, at4.0 arcsec through 300°. Observed maximum separation of these stars is about 22 arcsec, while the minimumdistance is 1.7 arcsec.[16] Widest separation occurred during February 1976 and the next will be in January 2056.

In the true orbit, closest approach or periastron was in August 1955, and next in May 2035. Furthest orbitalseparation at apastron last occurred in May 1995 and the next will be in 2075. The apparent distance between the twostars is presently rapidly decreasing, at least until 2019.

Companion: Proxima CentauriThe much fainter red dwarf star named Proxima Centauri, or simply Proxima, is about 15,000 AU away from Alpha Centauri AB. This is equivalent to 0.24 light years or 2.2 trillion kilometres—about 5% the distance between the Sun and Alpha Centauri AB. Proxima is likely gravitationally bound to Alpha Centauri AB, orbiting it with a period between 100,000 and 500,000 years. However, it is also possible that Proxima is not gravitationally bound and thus moving along a hyperbolic trajectory with respect to Alpha Centauri AB. The main evidence for a bound orbit is that Proxima's association with Alpha Centauri AB is unlikely to be accidental, since they share approximately the same motion through space. Theoretically, Proxima could leave the system after several million years. It is not yet certain

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whether Proxima and Alpha are truly gravitationally bound.Proxima is an M5.5 V spectral class red dwarf with an absolute magnitude of +15.53, which is only a small fractionof the Sun's luminosity. By mass, Proxima is presently calculated as 0.123 ± 0.06 M☉ (rounded to 0.12 M☉) or aboutone-eighth that of the Sun.

High-proper-motion starAll components of Alpha Centauri display significant proper motions against the background sky, similar to the firstmagnitude stars Sirius and Arcturus. Over the centuries, this causes the apparent stellar positions to slowly change.Such motions define the high-proper-motion stars. These stellar motions were unknown to ancient astronomers.Most assumed that all stars were immortal and permanently fixed on the celestial sphere, as stated in the works of thephilosopher Aristotle.Edmond Halley in 1718 found that some stars had significantly moved from their ancient astrometric positions.[17]

For example, the bright star Arcturus (α Boo) in the constellation of Boötes showed an almost 0.5° difference in1800 years,[18] as did the brightest star, Sirius, in Canis Major (α CMa). Halley's positional comparison wasPtolemy's catalogue of stars contained in the Almagest whose original data included portions from an earlier catalogby Hipparchos during the 1st century BCE.[19][20] Halley's proper motions were mostly for northern stars, so thesouthern star Alpha Centauri was not determined until the early 19th century.Scottish-born observer Thomas James Henderson in the 1830s at the Royal Observatory at the Cape of Good Hopediscovered the true distance to Alpha Centauri. He soon realised this system displayed an unusually high propermotion,[21] and therefore its observed true velocity through space should be much larger. In this case, the apparentstellar motion was found using Abbé Nicolas Louis de Lacaille's astrometric observations of 1751–1752, by theobserved differences between the two measured positions in different epochs. Using the Hipparcos Star Catalogue(HIP) data, the mean individual proper motions are −3678 mas/yr or −3.678 arcsec per year in right ascension and+481.84 mas/yr or 0.48184 arcsec per year in declination.[22][23] As proper motions are cumulative, the motion ofAlpha Centauri is about 6.1 arcmin each century, and 61.3 arcmin or 1.02° each millennium. These motions areabout one-fifth and twice, respectively, the diameter of the full moon. Using spectroscopy the mean radial velocityhas been determined to be 25.1 ± 0.3 km/s towards the Solar System.[24]

As the stars of Alpha Centauri approach us, the measured proper motion and trigonometric parallax slowly increase.Changes are also observed in the size of the semi-major axis of the orbital ellipse, increasing by 0.03 arcsec percentury. This change slightly shortens the observed orbital period of Alpha Centauri AB by some 0.006 years percentury. This small effect is gradually decreasing until the star system is at its closest to us, and is then reversed asthe distance increases again. Consequently, the observed position angles of the stars are subject to changes in theorbital elements over time, as first determined by W. H. van den Bos in 1926.[25] Some slight differences of about0.5% in the measured proper motions are caused by Alpha Centauri AB's orbital motion.Based on these observed proper motions and radial velocities, Alpha Centauri will continue to gradually brighten,passing just north of the Southern Cross or Crux, before moving northwest and up towards the celestial equator andaway from the galactic plane. By about 29,700 AD, in the present-day constellation of Hydra, Alpha Centauri will be1.00 pc or 3.26 ly away. Then it will reach the stationary radial velocity (RVel) of 0.0 km/s and the maximumapparent magnitude of −0.86V (which is comparable to present-day magnitude of Canopus). However, even duringthe time of this nearest approach, the apparent magnitude of Alpha Centauri will still not surpass that of Sirius(which will brighten incrementally over the next 60,000 years, and will continue to be the brightest star as seen fromEarth for the next 210,000 years).[26]

The Alpha Centauri system will then begin to move away from the Solar System, showing a positive radial velocity. Due to visual perspective, about 100,000 years from now, these stars will reach a final vanishing point and slowly disappear among the countless stars of the Milky Way. Here this once bright yellow star will fall below naked-eye visibility somewhere in the faint present day southern constellation of Telescopium (this unusual location results

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from the fact that Alpha Centauri's orbit around the galactic centre is highly tilted with respect to the plane of theMilky Way galaxy).

Apparent movementIn about 4000 years, the proper motion of Alpha Centauri will mean that from the point of view of Earth it willappear close enough to Beta Centauri to form an optical double star. Beta Centauri is in reality far more distant thanAlpha Centauri.

Apparent motion of Alpha Centauri relative to Beta Centauri

Planets

Until the 1990s, technologies did not exist that coulddetect planets outside the Solar System.

The Alpha Centauri B system

Companion(in order from star)

Mass Semimajoraxis(AU)

Orbital period(days)

Eccentricity Inclination Radius

b 1.13 ± 0.09 M⊕ 0.04 3.2357 ± 0.0008 — — —

Alpha Centauri BbOn 16 October 2012, researchers, mainly from the Observatory of Geneva and from the Centre for Astrophysics ofthe University of Porto, announced that an Earth-mass planet had been detected in orbit around Alpha Centauri Busing the radial velocity technique.[27] Over three years of observations had been needed for the difficult analysis.The planet has a minimum mass of 1.13 times Earth's mass. It is not in the habitable zone, orbiting very close to thehost star at just 0.04 AU and completing one orbit every 3.236 days. Its surface temperature is estimated to be 1200°C (about 1500 K), far too hot for liquid water and also above the melting temperatures of many silicate magmas.For comparison, the surface temperature of Venus, the hottest planet in the Solar System, is 462 °C (735 K).

Possibility of additional planetsThe discovery of planets orbiting other star systems, including similar binary systems (Gamma Cephei), raises thepossibility that additional planets may exist in the Alpha Centauri system. Such planets could orbit Alpha Centauri Aor Alpha Centauri B individually, or be on large orbits around the binary Alpha Centauri AB. Since both theprincipal stars are fairly similar to the Sun (for example, in age and metallicity), astronomers have been especiallyinterested in making detailed searches for planets in the Alpha Centauri system. Several established planet-huntingteams have used various radial velocity or star transit methods in their searches around these two bright stars. All theobservational studies have so far failed to find any evidence for brown dwarfs or gas giant planets.

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In 2009, computer simulations (then unaware of the close-in planet Bb) showed that a planet might have been able toform near the inner edge of Alpha Centauri B's habitable zone, which extends from 0.5 to 0.9 AU from the star.Certain special assumptions, such as considering that Alpha Centauri A and B may have initially formed with awider separation and later moved closer to each other (as might be possible if they formed in a dense star cluster)would permit an accretion-friendly environment farther from the star. Bodies around A would be able to orbit atslightly farther distances due to A's stronger gravity. In addition, the lack of any brown dwarfs or gas giants in closeorbits around A or B make the likelihood of terrestrial planets greater than otherwise. Theoretical studies on thedetectability via radial velocity analysis have shown that a dedicated campaign of high-cadence observations with a1–m class telescope can reliably detect a hypothetical planet of 1.8 Earth masses in the habitable zone of B withinthree years.Radial velocity measurements of Alpha Centauri B with HARPS spectrograph ruled out planets of more than 4 Earthmasses to the distance of the habitable zone of the star (orbital period P = 200 days).Alpha Centauri is envisioned as the first target for unmanned interstellar exploration. Crossing the huge distancebetween the Sun and Alpha Centauri using current spacecraft technologies would take several millennia, though thepossibility of solar sail or nuclear pulse propulsion technology could cut this down to a matter of decades.

Theoretical planetsEarly computer-generated models of planetary formation predicted the existence of terrestrial planets around bothAlpha Centauri A and B,[28][29] but most recent numerical investigations have shown that the gravitational pull of thecompanion star renders the accretion of planets very difficult. Despite these difficulties, given the similarities to theSun in spectral types, star type, age and probable stability of the orbits, it has been suggested that this stellar systemcould hold one of the best possibilities for harbouring extraterrestrial life on a potential planet.Some astronomers speculated that any possible terrestrial planets in the Alpha Centauri system may be bone dry orlack significant atmospheres. In the Solar System both Jupiter and Saturn were probably crucial in perturbing cometsinto the inner Solar System. Here the comets provided the inner planets with their own source of water and variousother ices but Proxima Centauri may have influenced the planetary disk as the Alpha Centauri system was formingenriching the area round Alpha Centauri A and B with volatile materials. This would be discounted if, for example,Alpha Centauri B happened to have gas giants orbiting Alpha Centauri A (or conversely, Alpha Centauri A forAlpha Centauri B), or if the stars B and A themselves were able to successfully perturb comets into each other's innersystem as Jupiter and Saturn presumably have done in the Solar System. Because icy bodies probably also reside inOort clouds of other planetary systems, when they are influenced gravitationally by either the gas giants ordisruptions by passing nearby stars many of these icy bodies then travel starwards. There is no direct evidence yet ofthe existence of such an Oort cloud around Alpha Centauri AB, and theoretically this may have been totallydestroyed during the system's formation.To be in the star's habitable zone, any suspected Earth-like planet around Alpha Centauri A would have to be placedabout 1.25 AU away – about halfway between the distances of Earth's orbit and Mars's orbit in the Solar System – soas to have similar planetary temperatures and conditions for liquid water to exist. For the slightly less luminous andcooler Alpha Centauri B, the habitable zone would lie closer at about 0.7 AU (100 million km), approximately thedistance that Venus is from the Sun.With the goal of finding evidence of such planets, both Proxima Centauri and Alpha Centauri AB were among thelisted "Tier 1" target stars for NASA's Space Interferometry Mission (SIM). Detecting planets as small as threeEarth-masses or smaller within two astronomical units of a "Tier 1" target would have been possible with this newinstrument.[30] The SIM mission, however, was cancelled due to financial issues in 2010.

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View from this system

Looking toward the Sun from Alpha Centauri in Celestia

Looking toward the sky around Orion from Alpha Centauri with Sirius nearBetelgeuse and the Sun between Perseus and Cassiopeia generated by Celestia

Viewed from near the Alpha Centaurisystem, the sky would appear very much asit does for earthbound observers, except thatCentaurus would be missing its brighteststar. The Sun would be a yellow +0.5 visualmagnitude star in eastern Cassiopeia at theantipodal point of Alpha Centauri's currentRA and Dec. at 02h 39m 35s +60° 50′(2000). This place is close to the 3.4magnitude star ε Cassiopeiae. An interstellaror alien observer would find the \/\/ ofCassiopeia had become a /\/\/ shape [31]

nearly in front of the Heart Nebula inCassiopeia. Sirius lies less than a degreefrom Betelgeuse in the otherwiseunmodified Orion and is with −1.2 a littlefainter than from Earth but still the brighteststar in the Alpha Centauri sky. Procyon isalso displaced into the middle of Gemini,outshining Pollux, while both Vega andAltair are shifted northwestward relative toDeneb (which barely moves, due to its greatdistance)- giving the Summer Triangle amore equilateral appearance.

From Proxima itself, Alpha Centauri ABwould appear like two close bright starswith the combined magnitude of −6.8.Depending on the binary's orbital position,the bright stars would appear noticeablydivisible to the naked eye, or occasionally,but briefly, as single unresolved star. Basedon the calculated absolute magnitudes, thevisual magnitudes of Alpha Centauri A andB would be −6.5 and −5.2, respectively.[32]

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View from a hypothetical planet

Artist's rendition of the view from a hypothetical airless planet orbiting AlphaCentauri A

An observer on a hypothetical planetorbiting around either Alpha Centauri A orAlpha Centauri B would see the other star ofthe binary system as an intensely brightobject in the night sky, showing a small butdiscernible disk.For example, some theoretical Earth-likeplanet orbiting about 1.25 AU from AlphaCentauri A (so that the star appears roughlyas bright as the Sun viewed from the Earth)would see Alpha Centauri B orbit the entiresky once roughly every one year and threemonths (or 1.3(4) a), the planet's own orbitalperiod. Added to this would be the changingapparent position of Alpha Centauri Bduring its long eighty-year elliptical orbitwith respect to Alpha Centauri A(comparable in speed to Uranus here). Depending on the position on its orbit, Alpha Centauri B would vary inapparent magnitude between −18.2 (dimmest) and −21.0 (brightest). These visual magnitudes are much dimmer thanthe currently observed −26.7 magnitude for the Sun as viewed from the Earth. The difference of 5.7 to 8.6magnitudes means Alpha Centauri B would appear, on a linear scale, 2500 to 190 times dimmer than AlphaCentauri A (or the Sun viewed from the Earth), but also 190 to 2500 times brighter than the −12.5 magnitude fullMoon as seen from the Earth.

Also, if another similar Earth-like planet orbited at 0.71 AU from Alpha Centauri B (so that in turn Alpha Centauri Bappeared as bright as the Sun seen from the Earth), this hypothetical planet would receive slightly more light fromthe more luminous Alpha Centauri A, which would shine 4.7 to 7.3 magnitudes dimmer than Alpha Centauri B (orthe Sun seen from the Earth), ranging in apparent magnitude between −19.4 (dimmest) and −22.1 (brightest). ThusAlpha Centauri A would appear between 830 and 70 times dimmer than the Sun but some 580 to 6900 times brighterthan the full Moon. During such planet's orbital period of 0.6(3) a, an observer on the planet would see this intenselybright companion star circle the sky just as we see with the Solar System's planets. Furthermore, Alpha Centauri Asidereal period of approximately eighty years means that this star would move through the local ecliptic as slowly asUranus with its eighty-four year period, but as the orbit of Alpha Centauri A is more elliptical, its apparentmagnitude will be far more variable. Although intensely bright to the eye, the overall illumination would notsignificantly affect climate nor influence normal plant photosynthesis.An observer on the hypothetical planet would notice a change in orientation to VLBI reference points commensuratewith the binary orbit periodicity plus or minus any local effects such as precession or nutation.Assuming this hypothetical planet had a low orbital inclination with respect to the mutual orbit of Alpha Centauri Aand B, then the secondary star would start beside the primary at 'stellar' conjunction. Half the period later, at 'stellar'opposition, both stars would be opposite each other in the sky. Then, for about half the planetary year the appearanceof the night sky would be a darker blue – similar to the sky during totality at any total solar eclipse. Humans couldeasily walk around and clearly see the surrounding terrain, and reading a book would be quite possible without anyartificial light. After another half period in the stellar orbit, the stars would complete their orbital cycle and return tothe next stellar conjunction, and the familiar Earth-like day and night cycle would return.

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NamesThe colloquial name of Alpha Centauri is Rigel Kent or Rigil Kent,[33] short for Rigil/Rigel Kentaurus,[34][35] theromanization of the Arabic name رجل القنطورس Rijl Qanṭūris, from the phrase Rijl al-Qanṭūris "the foot of theCentaur".[36] This is sometimes further abbreviated to Rigel, though that is ambiguous with Beta Orionis, which isalso called Rigel. Although the short form Rigel Kent is common in English, the stars are most often referred to bytheir Bayer designation Alpha Centauri.

A medieval name is Toliman, whose etymology may be Arabic الظلمان al-Ẓulmān "the ostriches". During the 19thcentury, the northern amateur popularist Elijah H. Burritt used the now-obscure name Bungula,[37] possibly coinedfrom "β" and the Latin ungula ("hoof"). Together, Alpha and Beta Centauri form the "Southern Pointers" or "ThePointers", as they point towards the Southern Cross, the asterism of the constellation of Crux.

In Chinese, 南 門 Nán Mén, meaning Southern Gate, refers to an asterism consisting of α Centauri and ε Centauri.Consequently, α Centauri itself is known as 南 門 二 Nán Mén Èr, the Second Star of the Southern Gate.[38]

To the Australian aboriginal Boorong peopleWikipedia:Avoid weasel words of northwestern Victoria, Alpha andBeta Centauri are Bermbermgle, two brothers noted for their courage and destructiveness, who speared and killedTchingal "The Emu" (the Coalsack Nebula). The form in Wotjobaluk is Bram-bram-bult.

Use in modern fiction

Distances of the nearest stars from 20,000 years ago until 80,000 years in thefuture. .

Alpha Centauri's relative proximity makes itin some ways the logical choice as "firstport of call". Speculative fiction aboutinterstellar travel often predicts eventualhuman exploration, and even the discoveryand colonization of planetary systems.These themes are common to many worksof science fiction and video games.

Notes[1][1] See Table 3.[2] http:/ / simbad. u-strasbg. fr/ simbad/

sim-id?Ident=alpha+ centauri[3] http:/ / exoplanetarchive. ipac. caltech. edu/

cgi-bin/ DisplayOverview/nph-DisplayOverview?objname=alpha+ centauri

[4] http:/ / wwwadd. zah. uni-heidelberg. de/datenbanken/ aricns/ cnspages/ 4c01151. htm

[5] http:/ / exoplanet. eu/ star. php?st=alf+ cen[6] http:/ / interstellar. jpl. nasa. gov/ interstellar/ probe/ introduction/ neighborhood. html, Our Local Galactic Neighborhood, NASA[7] http:/ / www. centauri-dreams. org/ ?p=14203, Into the Interstellar Void, Centauri Dreams[8] "The exoplanet next door: Earth-sized world discovered in nearby α Centauri star system". (http:/ / www. nature. com/ news/

the-exoplanet-next-door-1. 11605) Eric Hand, Nature, October 16, 2012. Accessed October 16, 2012.[9] — some of the data is located under "Measurements".[10] This is calculated for a fixed latitude by knowing the star's declination (δ) using the formulae (90°+ δ). Alpha Centauri's declination is −60°

50′, so the latitude where the star is circumpolar will be south of −29° 10′S or 29°. Similarly, the place where Alpha Centauri never rises fornorthern observers is north of the latitude (90°+ δ) N or +29°N.

[11][11] Knobel, p. 416.[12] Aitken, R.G., "The Binary Stars", Dover, 1961, pp. 236–237.[13] Pannekoek, A., "A Short History of Astronomy", Dover, 1989, pp. 345–6[14] Aitken, R.G., "The Binary Stars", Dover, 1961, p. 236.[15] UNIQ-math-0-13b91fa059fa7d74-QINU , see formula

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Alpha Centauri 14

[16] Aitken, R.G., "The Binary Stars", Dover, 1961, p. 235.[17] Berry, A., "A History of Astronomy", Dover, 1989, pp. 357–358[18][18] Pannekoek, A., "A Short History of Astronomy", Dover, 1989[19] Newton R.R., "The Crime of Claudius Ptolemy", T. Baltimore: Johns Hopkins University Press, (1977)[20][20] Pannekoek, A., "A Short History of Astronomy", Dover, 1989, p. 157[21][21] Pannekoek, A., "A Short History of Astronomy", Dover, 1989, p. 333[22] European Space Agency: The Hipparcos and Tycho Catalogues Search facility(2008) (http:/ / www. rssd. esa. int/ index.

php?project=HIPPARCOS& page=hipsearch)[23] Proper motions are expressed in smaller angular units than arcsec, being measured in milli-arcsec (mas.) or one-thousandth of an arcsec. A

negative value for proper motion in RA indicates the sky motion is east to west, in declination north to south.[24] HD 128620/1 (http:/ / webviz. u-strasbg. fr/ viz-bin/ VizieR-5?-out. add=. & -source=V/ 117A/ newcat. dat& recno=9988), database entry,

The Geneva-Copenhagen Survey of Solar neighbourhood, J. Holmberg et al., 2007, CDS ID V/117A (http:/ / vizier. u-strasbg. fr/ viz-bin/Cat?V/ 117A). Accessed on line 19 November 2008

[25] Calculated as; θ − θo = μα × sin α × (t − to ), where; α = right ascension (in degrees), μα is the common proper motion (cpm.) expressed indegrees, and θ and θo are the current position angle and calculated position angle at the different epochs.

[26] Sky and Telescope, April 1998 (p60), based on computations from HIPPARCOS data.[27] Planetary Habitability Laboratory, UPR Arecibo: A Planetary System Around Our Nearest Star is Emerging (http:/ / phl. upr. edu/

press-releases/ aplanetarysystemaroundourneareststarisemerging)[28] Javiera Guedes, Terrestrial Planet Formation Around Alpha Cen B (http:/ / www. ucolick. org/ ~javiera/ alphacen. shtml)[29][29] see Lissauer and Quintana in references below[30] " Planet Hunting by Numbers (http:/ / www. jpl. nasa. gov/ news/ features. cfm?feature=1209)", (Press Release), NASA, Stars and Galaxies,

Jet Propulsion Laboratory, 18 October 2006. Retrieved 24 April 2007.[31] The coordinates of the Sun would be diametrically opposite Alpha Centauri AB, at α=, δ=[32] Computed; using in solar terms: 1.1 M☉ and 0.92 M☉, luminosities 1.57 and 0.51 L*/L☉, Sun magnitude −26.73(v), 11.2 to 35.6 AU orbit;

The minimum luminosity adds planet's orbital radius to A–B distance (max) (conjunction). Max. luminosity subtracts the planet's orbitalradius to A–B distance (min) (opposition).

[33] Kunitzsch P., & Smart, T., A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations, Cambride, SkyPub. Corp., 2006, p. 27

[34] Bailey, F., "The Catalogues of Ptolemy, Ulugh Beigh, Tycho Brahe, Halley, and Hevelius", Memoirs of Royal Astronomical Society, vol.XIII, London, 1843.

[35] Spellings include Rigjl Kentaurus, Hyde T., "Ulugh Beighi Tabulae Stellarum Fixarum", Tabulae Long. ac Lat. Stellarum Fixarum exObservatione Ulugh Beighi, Oxford, 1665, p. 142., Hyde T., "In Ulugh Beighi Tabulae Stellarum Fixarum Commentarii", op. cit., p. 67.,Portuguese Riguel Kentaurus da Silva Oliveira, R., "Crux Australis: o Cruzeiro do Sul" (http:/ / www. asterdomus. com. br/Artigo_crux_australis. htm), Artigos: Planetario Movel Inflavel AsterDomus.

[36] Davis Jr., G. A., "The Pronunciations, Derivations, and Meanings of a Selected List of Star Names," (http:/ / adsabs. harvard. edu/ abs/1944PA. . . . . 52. . . . 8D)Popular Astronomy, Vol. LII, No. 3, Oct. 1944, p. 16.

[37] Burritt, E. H., Atlas, Designed to Illustrate the Geography of the Heavens, (New Edition), New York, F. J. Huntington and Co., 1835, pl.VII.

[38] [ AEEA (Activities of Exhibition and Education in Astronomy) 天 文 教 育 資 訊 網 2006 年 6 月 27 日]

References

External links• SIMBAD observational data (http:/ / simbad. u-strasbg. fr/ sim-id. pl?protocol=html& Ident=alpha+ centauri)• Sixth Catalogue of Orbits of Visual Binary Stars U.S.N.O. (http:/ / ad. usno. navy. mil/ wds/ orb6. html)• The Imperial Star – Alpha Centauri (http:/ / www. southastrodel. com/ PageAlphaCen001. htm)• Alpha Centauri – A Voyage to Alpha Centauri (http:/ / www. southastrodel. com/ PageAlphaCen006. htm)• Immediate History of Alpha Centauri (http:/ / www. southastrodel. com/ PageAlphaCen006. htm)• eSky : Alpha Centauri (http:/ / www. glyphweb. com/ esky/ stars/ alphacentauri. html)

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Hypothetical planets or exploration• "A Family Portrait of the Alpha Centauri System" (http:/ / www. spaceref. com/ news/ viewpr. html?pid=11016).

SpaceRef.com. Retrieved 21 March 2003.• Alpha Centauri System (http:/ / jumk. de/ astronomie/ near-stars/ alpha-centauri. shtml)• O Sistema Alpha Centauri (Portuguese) (http:/ / www. uranometrianova. pro. br/ astronomia/ AA002/ alphacen.

htm)• Alpha Centauri – Associação de Astronomia (Portuguese) (http:/ / www. alpha-centauri. pt)• http:/ / www. space. com/ scienceastronomy/ 080307-another-earth. html• Thompson, Andrea (2008-03-07). "Nearest Star System Might Harbor Earth Twin" (http:/ / www. space. com/

scienceastronomy/ 080307-another-earth. html). SPACE.com. Archived (http:/ / web. archive. org/ web/20080602011008/ http:/ / www. space. com/ scienceastronomy/ 080307-another-earth. html) from the original on2 June 2008. Retrieved 2008-07-17.

Coordinates: 14h 39m 36.4951s, −60° 50′ 02.308″ (http:/ / www. wikisky. org/ ?ra=14. 660137527778& de=-60.833974444444& zoom=2& show_grid=1& show_constellation_lines=1& show_constellation_boundaries=1&show_const_names=1& show_galaxies=1& img_source=IMG_all)

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Proxima CentauriCoordinates: 14h 29m 42.9487s, −62° 40′ 46.141″ [1]

Proxima Centauri

Proxima Centauri (center inset) as seen by HubbleObservation data

Epoch J2000.0      Equinox J2000.0 (ICRS)

Constellation Centaurus

Pronunciation /ˈprɒksɪməHelp:IPA for English#Keysɛnˈtɔriː/[2]

Right ascension 14h 29m 42.9487s

Declination −62° 40′ 46.141″

Apparent magnitude (V) 11.05

Characteristics

Spectral type M5.5 Ve

Apparent magnitude (J) 5.35 ± 0.02

U−B color index 1.43

B−V color index 1.90

Variable type Flare star

Astrometry

Radial velocity (Rv) −21.7 ± 1.8 km/s

Proper motion (μ) RA: −3775.40 mas/yrDec.: 769.33 mas/yr

Parallax (π) 768.7 ± 0.3 mas

Distance 4.243 ± 0.002 ly(1.3009 ± 0.0005 pc)

Absolute magnitude (MV

) 15.49

Details

Mass 0.123 ± 0.006 M☉

Radius 0.141 ± 0.007 R☉

Luminosity (bolometric) 0.0017 L☉

Surface gravity (log g) 5.20 ± 0.23 cgs

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Temperature 3,042 ± 117 K

Metallicity [Fe/H] 0.21 dex

Rotation 83.5 days

Rotational velocity (v sin i) 2.7 ± 0.3 km/s

Age 4.85 Gyr

Other designations

Alpha Centauri C, CCDM J14396-6050C, GCTP 3278.00, GJ 551, HIP 70890, LFT 1110, LHS 49, LPM 526, LTT 5721,NLTT 37460, V645 Centauri

Database references

SIMBAD data [3]

Proxima Centauri (Latin proxima, meaning "next to" or "nearest to") is a red dwarf about 4.24 light-years from theSun, inside the G-cloud, in the constellation of Centaurus. It was discovered in 1915 by Scottish astronomer RobertInnes, the Director of the Union Observatory in South Africa, and is the nearest known star to the Sun, although it istoo faint to be seen with the naked eye (apparent magnitude 11.05). Its distance to the second- and third-nearest stars,which form the bright binary Alpha Centauri, is 0.237 ± 0.011 ly (15,000 ± 700 AU). Proxima Centauri is very likelypart of a triple star system with Alpha Centauri A and B.Because of the proximity of this star, its distance from the Sun and angular diameter can be measured directly, fromwhich it can be determined that its diameter is about one-seventh of that of the Sun. Proxima Centauri's mass is aboutan eighth of the Sun's, and its average density is about 40 times that of the Sun. Although it has a very low averageluminosity, Proxima is a flare star that undergoes random dramatic increases in brightness because of magneticactivity. The star's magnetic field is created by convection throughout the stellar body, and the resulting flare activitygenerates a total X-ray emission similar to that produced by the Sun. The mixing of the fuel at Proxima Centauri'score through convection and the star's relatively low energy-production rate suggest that it will be a main-sequencestar for another four trillion years, or nearly 300 times the current age of the universe.Searches for companions orbiting Proxima Centauri have been unsuccessful, ruling out the presence of brown dwarfsand supermassive planets. Precision radial velocity surveys have also ruled out the presence of super-Earths withinthe star's habitable zone.[4] The detection of smaller objects will require the use of new instruments, such as theproposed James Webb Space Telescope. Because Proxima Centauri is a red dwarf and a flare star, whether a planetorbiting this star could support life is disputed. Nevertheless, because of the star's proximity to Earth, it has beenproposed as a destination for interstellar travel.

ObservationIn 1915, Scottish astronomer Robert Innes, Director of the Union Observatory in Johannesburg, South Africa,discovered a star that had the same proper motion as Alpha Centauri. He suggested it be named Proxima Centauri. In1917, at the Royal Observatory at the Cape of Good Hope, the Dutch astronomer Joan Voûte measured the star'strigonometric parallax at 0.755 ± 0.028″ and determined that Proxima Centauri was approximately the same distancefrom the Sun as Alpha Centauri. It was also found to be the lowest-luminosity star known at the time. An equallyaccurate parallax determination of Proxima Centauri was made by American astronomer Harold L. Alden in 1928,who confirmed Innes's view that this star is closer, with a parallax of 0.783 ± 0.005″.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 in magnitude on about 8% of the images, making it the most active flare star then known. 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

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Centauri. Further observations of flare activity were made with the EXOSAT and ROSAT satellites, and the X-rayemissions of smaller, solar-like flares were observed by the Japanese ASCA satellite in 1995. Proxima Centauri hassince been the subject of study by most X-ray observatories, including XMM-Newton and Chandra.Because of Proxima Centauri's southern declination, it can only be viewed south of latitude 27° N.[5] Red dwarfssuch as Proxima Centauri are far too faint to be seen with the naked eye. Even from Alpha Centauri A or B, Proximawould only be seen as a fifth magnitude star. It has an apparent visual magnitude of 11, so a telescope with anaperture of at least 8 cm (3.1 in.) is needed to observe this star even under ideal viewing conditions—under clear,dark skies with Proxima Centauri well above the horizon.

CharacteristicsProxima Centauri is classified as a red dwarf because it belongs to the main sequence on the Hertzsprung–Russelldiagram and is of spectral class M5.5. It is further classified as a "late M-dwarf star", meaning that at M5.5, it falls tothe low-mass extreme of M-type stars. This star's absolute visual magnitude, or its visual magnitude as viewed froma distance of 10 parsecs, is 15.5. Its total luminosity over all wavelengths is 0.17% that of the Sun,[] although whenobserved in the wavelengths of visible light the eye is most sensitive to, it is only 0.0056% as luminous as the Sun.[6]

More than 85% of its radiated power is at infrared wavelengths.[7]

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

The two bright stars are (left) Alpha Centauri and (right) Beta Centauri. The faint red star in the center of the redcircle is Proxima Centauri. Taken with Canon 85mm f/1.8 lens with 11 frames stacked, each frame exposed 30seconds.In 2002, optical interferometry with the Very Large Telescope (VLTI) found that the angular diameter of Proxima Centauri was 1.02 ± 0.08 milliarcsec. Because its distance is known, the actual diameter of Proxima Centauri can be

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calculated to be about 1/7 that of the Sun, or 1.5 times that of Jupiter. The star's estimated mass is only 12.3% of asolar mass, or 129 Jupiter masses. The mean density of a main-sequence star increases with decreasing mass, andProxima Centauri is no exception: it has a mean density of 56.8 × 103 kg/m3 (56.8 g/cm3), compared with the Sun'smean density of 1.411 × 103 kg/m3 (1.411 g/cm3).[]

Because of its low mass, the interior of the star is completely convective, causing energy to be transferred to theexterior by the physical movement of plasma rather than through radiative processes. This convection means that thehelium ash left over from the thermonuclear fusion of hydrogen does not accumulate at the core, but is insteadcirculated throughout the star. Unlike the Sun, which will only burn through about 10% of its total hydrogen supplybefore leaving the main sequence, Proxima Centauri will consume nearly all of its fuel before the fusion of hydrogencomes to an end.Convection is associated with the generation and persistence of a magnetic field. The magnetic energy from this fieldis released at the surface through stellar flares that briefly increase the overall luminosity of the star. These flares cangrow as large as the star and reach temperatures measured as high as 27 million K—hot enough to radiate X-rays.Indeed, the quiescent X-ray luminosity of this star, approximately (4–16) × 1026 erg/s ((4–16) × 1019 W), is roughlyequal to that of the much larger Sun. The peak X-ray luminosity of the largest flares can reach 1028 erg/s (1021 W.)The chromosphere of this star is active, and its spectrum displays a strong emission line of singly ionized magnesiumat a wavelength of 280 nm. About 88% of the surface of Proxima Centauri may be active, a percentage that is muchhigher 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 million K, compared to the 2 million K ofthe Sun's corona. However, the overall activity level of this star is considered low compared to other M-class dwarfs,which is consistent with the star's estimated age of 4.85 × 109 years, since the activity level of a red dwarf isexpected to steadily wane over billions of years as its stellar rotation rate decreases. The activity level also appears tovary with a period of roughly 442 days, which is shorter than the solar cycle of 11 years.Proxima Centauri has a relatively weak stellar wind, resulting in no more than 20% of the Sun's mass loss rate fromthe solar wind. Because the star is much smaller than the Sun, however, the mass loss per unit surface area fromProxima Centauri may be eight times that from the solar surface.A red dwarf with the mass of Proxima Centauri will remain on the main sequence for about four trillion years. As theproportion of helium increases because of hydrogen fusion, the star will become smaller and hotter, graduallytransforming 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 hydrogenfuel 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.

Distance and motionBased on the parallax of 768.7 ± 0.3 milliarcseconds, measured using the Hipparcos astrometry satellite, and moreprecisely with the Fine Guidance Sensors on the Hubble Space Telescope, Proxima Centauri is about 4.24 light yearsfrom the Sun, or 270,000 times more distant than the Earth is from the Sun. From Earth's vantage point, Proxima isseparated by 2.18° from Alpha Centauri, or four times the angular diameter of the full Moon. Proxima also has arelatively large proper motion—moving 3.85 arcseconds per year across the sky. It has a radial velocity toward theSun of 21.7 km/s.[9]

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Distances of the nearest stars from 20,000 years ago until 80,000 years in the future.Proxima Centauri is in yellow

Among the known stars, ProximaCentauri has been the closest star to theSun for about 32,000 years and will beso for about another 33,000 years, afterwhich the closest star to the Sun willbe Ross 248. In 2001, J.García-Sánchez et al. predicted thatProxima will make its closest approachto the Sun, coming within 3.11 lightyears of the latter, in approximately26,700 years. A 2010 study by V. V.Bobylev predicted a closest approachdistance of 2.90 ly in about27,400 years. Proxima Centauri isorbiting through the Milky Way at adistance from the Galactic Center thatvaries from 8.3 to 9.5 kpc, with anorbital eccentricity of 0.07.

Ever since the discovery of Proxima it has been suspected to be a true companion of the Alpha Centauri binary starsystem. At a distance to Alpha Centauri of just 0.21 ly (15,000 ± 700 astronomical units [AU]), Proxima Centaurimay be in orbit around Alpha Centauri, with an orbital period of the order of 500,000 years or more. For this reason,Proxima is sometimes referred to as Alpha Centauri C. Modern estimates, taking into account the small separationbetween and relative velocity of the stars, suggest that the chance of the observed alignment being a coincidence isroughly one in a million. Data from the Hipparcos satellite, combined with ground-based observations, is consistentwith the hypothesis that the three stars are truly a bound system. If so, Proxima would currently be near apastron, thefarthest point in its orbit from the Alpha Centauri system. Such a triple Proxima Cen-alpha Cen A/B system can formnaturally through a low-mass star being dynamically captured by a more massive binary of 1.5–2 solar masseswithin their embedded star cluster before the cluster disperses. More accurate measurement of the radial velocity isneeded to confirm this hypothesis.

If Proxima was bound to the Alpha Centauri system during its formation, the stars would be likely to share the sameelemental composition. The gravitational influence of Proxima may also have stirred up the Alpha Centauriprotoplanetary disks. This would have increased the delivery of volatiles such as water to the dry inner regions. Anyterrestrial planets in the system may have been enriched by this material.Six single stars, two binary star systems, and a triple star share a common motion through space with ProximaCentauri and the Alpha Centauri system. The space velocities of these stars are all within 10 km/s of AlphaCentauri's peculiar motion. Thus, they may form a moving group of stars, which would indicate a common point oforigin, such as in a star cluster. If it is determined that Proxima Centauri is not gravitationally bound to AlphaCentauri, then such a moving group would help explain their relatively close proximity.Though Proxima Centauri is the nearest bona fide star, it is still possible that one or more as-yet undetectedsub-stellar brown dwarfs may lie closer.

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Possible companions

RV-derived Upper MassLimits of Companion

Orbitalperiod(days)

Separation(AU)

MaximumMass

(× 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, some displacement of the star would occur over the course of eachorbit. If the orbital plane of the planet is not perpendicular to the line of sight from the Earth, then this displacementwould cause periodic changes in the radial velocity of Proxima Centauri. The fact that multiple measurements of thestar's radial velocity have detected no such shifts has lowered the maximum mass that a possible companion toProxima Centauri could possess. The activity level of the star adds noise to the radial velocity measurements,limiting future prospects for detection of a companion using this method.In 1998, an examination of Proxima Centauri using the Faint Object Spectrograph on board the Hubble SpaceTelescope appeared to show evidence of a companion orbiting at a distance of about 0.5 AU. However, a subsequentsearch using the Wide Field Planetary Camera 2 failed to locate any companions. Proxima Centauri, along withAlpha Centauri A and B, was among the "Tier 1" target stars for NASA's now-canceled Space InterferometryMission (SIM), which would theoretically have been able to detect planets as small as three Earth-masses within twoAU of a "Tier 1" target star.

Artist's concept of a red dwarf

Habitable zone

The TV documentary Alien Worlds hypothesized that a life-sustainingplanet could exist in orbit around Proxima Centauri or other reddwarfs. Such a planet would lie within the habitable zone of ProximaCentauri, about 0.023–0.054 AU from the star, and would have anorbital period of 3.6–14 days. A planet orbiting within this zone willexperience tidal locking to the star, so that Proxima Centauri moveslittle in the planet's sky, and most of the surface experiences either dayor night perpetually. However, the presence of an atmosphere couldserve to redistribute the energy from the star-lit side to the far side of

the planet.

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 Basri of theUniversity of California, Berkeley, even mentioned that "no one [has] found any showstoppers to habitability." Forexample, one concern was that the torrents of charged particles from the star's flares could strip the atmosphere offany nearby planet. However, if the planet had a strong magnetic field, the field would deflect the particles from theatmosphere; even the slow rotation of a tidally locked dwarf planet that spins once for every time it orbits its starwould be enough to generate a magnetic field, as long as part of the planet's interior remained molten.Other scientists, especially proponents of the Rare Earth hypothesis, disagree that red dwarfs can sustain life. Thetide-locked rotation may result in a relatively weak planetary magnetic moment, leading to strong atmosphericerosion by coronal mass ejections from Proxima Centauri.

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Interstellar travel

The Sun as seen from the Alpha Centauri system,using Celestia

Proxima Centauri has been suggested as a possible first destination forinterstellar travel. The star is in motion toward Earth at a rate of21.7 km/s. Ηowever, after 26,700 years, when it will come as close as3.11 light-years, it will begin to move farther away. If non-nuclearpropulsion were used, a voyage of a spacecraft to a planet orbitingProxima Centauri would probably require thousands of years. Forexample, Voyager 1, which is now travelling 17.043 km/s(38,120 mph) relative to the Sun, would reach Proxima in 73,775years, were the spacecraft traveling in the direction of that star. Aslow-moving probe would have only several tens of thousands of yearsto catch Proxima Centauri near its closest approach, and could end upwatching it recede into the distance. Nuclear pulse propulsion mightenable such interstellar travel with a trip timescale of a century, beginning within the next century, inspiring severalstudies such as Project Orion, Project Daedalus, and Project Longshot.

From Proxima Centauri, the Sun would appear as a bright 0.4-magnitude star in the constellation Cassiopeia.[8]

References

Explanatory notes[1] http:/ / www. wikisky. org/ ?ra=14. 495263527778& de=-62. 679483611111& zoom=2& show_grid=1& show_constellation_lines=1&

show_constellation_boundaries=1& show_const_names=1& show_galaxies=1& img_source=IMG_all[2][2] Proxima is pronounced . Centauri may be pronounced or .[3] http:/ / simbad. u-strasbg. fr/ simbad/ sim-id?Ident=V645+ Cen[4] 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.[5][5] 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 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:

[6][6] See p. 8.[7][7] See p. 357.[8] The coordinates of the Sun would be diametrically opposite Proxima, at α=, δ=. The absolute magnitude Mv of the Sun is 4.83, so at a

parallax π of 0.77199 the apparent magnitude m is given by 4.83 − 5(log10(0.77199) + 1) = 0.40. See:

Citations

External links• "Proxima Centauri: The Closest Star" (http:/ / antwrp. gsfc. nasa. gov/ apod/ ap020715. html). NASA. Astronomy

Picture of the Day. 2002-07-15. Retrieved 2008-06-25.• "Proxima Centauri: The Nearest Star to the Sun" (http:/ / chandra. harvard. edu/ photo/ 2004/ proxima/ ). Chandra

X-ray Observatory. Astronomy Picture of the Day. 2008-07-01. Retrieved 2008-07-01.• James, Andrew (2008-03-11). "A Voyage to Alpha Centauri" (http:/ / www. southastrodel. com/

PageAlphaCen006. htm). The Imperial Star - Alpha Centauri. Southern Astronomical Delights. Retrieved2008-08-05.

• "Alpha Centauri 3" (http:/ / www. solstation. com/ stars/ alp-cent3. htm). SolStation. Retrieved 2008-08-05.• "O Sistema Alpha Centauri" (http:/ / www. uranometrianova. pro. br/ astronomia/ AA002/ alphacen. htm).

Astronomia & Astrofísica (in Portuguese). Retrieved 2008-06-25.

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• Wikisky image (http:/ / www. wikisky. org/ ?ra=14. 495264& de=-62. 67948000000001& zoom=8&show_grid=1& show_constellation_lines=1& show_constellation_boundaries=1& show_const_names=0&show_galaxies=1& show_box=1& box_ra=14. 495264& box_de=-62. 67948& box_width=50& box_height=50&img_source=DSS2) of Proxima Centauri

Binary star

Artist's impression of the evolution of a hot high-mass binary star.

Hubble image of the Sirius binary system, inwhich Sirius B can be clearly distinguished

(lower left)

A binary star is a star systemconsisting of two stars orbiting aroundtheir common center of mass. Themore massive star is called theprimary and the other is itscompanion star, comes /ˈkoʊmiːz/,or secondary. Systems of two, three,four, or even more stars are calledmultiple star systems. These systems,especially when more distant, oftenappear to the unaided eye as a singlepoint of light, and are then revealed asdouble (or more) by other means.Research over the last two centuriessuggests that half or more of visiblestars are part of multiple starsystems.[1]

The term double star is often usedsynonymously with binary star;however, double star can also meanoptical double star. Optical doublesare so called because the two starsappear close together in the sky as seenfrom the Earth; they are almost on thesame line of sight. Nevertheless, their"doubleness" depends only on thisoptical effect; the stars themselves aredistant from one another and share nophysical connection. A double star canbe revealed as optical by means of differences in their parallax measurements, proper motions, or radial velocities.Even so, in most cases of known double stars, it is not known whether they are optical doubles or they are doublesphysically bound through gravitation into a multiple star system.

Binary star systems are very important in astrophysics because calculations of their orbits allow the masses of theircomponent stars to be directly determined, which in turn allows other stellar parameters, such as radius and density,to be indirectly estimated. This also determines an empirical mass-luminosity relationship (MLR) from which themasses of single stars can be estimated.

Binary stars are often detected optically, in which case they are called visual binaries. Many visual binaries have long orbital periods of several centuries or millennia and therefore have orbits which are uncertain or poorly known.

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They may also be detected by indirect techniques, such as spectroscopy (spectroscopic binaries) or astrometry(astrometric binaries). If a binary star happens to orbit in a plane along our line of sight, its components will eclipseand transit each other; these pairs are called eclipsing binaries, or, as they are detected by their changes in brightnessduring eclipses and transits, photometric binaries.If components in binary star systems are close enough they can gravitationally distort their mutual outer stellaratmospheres. In some cases, these close binary systems can exchange mass, which may bring their evolution tostages that single stars cannot attain. Examples of binaries are Sirius and Cygnus X-1 (Cygnus X-1 being a wellknown black hole). Binary stars are also common as the nuclei of many planetary nebulae, and are the progenitors ofboth novae and type Ia supernovae.

DiscoveryThe term binary was first used in this context by Sir William Herschel in 1802, when he wrote:

"If, on the contrary, two stars should really be situated very near each other, and at the same time so farinsulated as not to be materially affected by the attractions of neighbouring stars, they will then composea separate system, and remain united by the bond of their own mutual gravitation towards each other.This should be called a real double star; and any two stars that are thus mutually connected, form thebinary sidereal system which we are now to consider."

By the modern definition, the term binary star is generally restricted to pairs of stars which revolve around acommon centre of mass. Binary stars which can be resolved with a telescope or interferometric methods are knownas visual binaries. For most of the known visual binary stars one whole revolution has not been observed yet, theyare observed to have travelled along a curved path or a partial arc.

This figure shows a system with two stars

The more general term double star is used for pairs of stars which areseen to be close together in the sky.[] This distinction is rarely made inlanguages other than English. Double stars may be binary systems ormay be merely two stars that appear to be close together in the sky buthave vastly different true distances from the Sun. The latter are termedoptical doubles or optical pairs.

Since the invention of the telescope, many pairs of double stars have been found. Early examples include Mizar andAcrux. Mizar, in the Big Dipper (Ursa Major), was observed to be double by Giovanni Battista Riccioli in 1650[2][3]

(and probably earlier by Benedetto Castelli and Galileo).[4] The bright southern star Acrux, in the Southern Cross,was discovered to be double by Father Fontenay in 1685.

John Michell was the first to suggest that double stars might be physically attached to each other when he argued in1767 that the probability that a double star was due to a chance alignment was small.[5][6] William Herschel beganobserving double stars in 1779 and soon thereafter published catalogs of about 700 double stars. By 1803, he hadobserved changes in the relative positions in a number of double stars over the course of 25 years, and concluded thatthey must be binary systems;[7] the first orbit of a binary star, however, was not computed until 1827, when FélixSavary computed the orbit of Xi Ursae Majoris.[8] Since this time, many more double stars have been catalogued andmeasured. The Washington Double Star Catalog, a database of visual double stars compiled by the United StatesNaval Observatory, contains over 100,000 pairs of double stars,[9] including optical doubles as well as binary stars.Orbits are known for only a few thousand of these double stars,[10] and most have not been ascertained to be eithertrue binaries or optical double stars.[11] This can be determined by observing the relative motion of the pairs. If themotion is part of an orbit, or if the stars have similar radial velocities and the difference in their proper motions issmall compared to their common proper motion, the pair is probably physical. One of the tasks that remains forvisual observers of double stars is to obtain sufficient observations to prove or disprove gravitational connection.

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Classifications

Methods of observationBinary stars are classified into four types according to the way in which they are observed: visually, by observation;spectroscopically, by periodic changes in spectral lines; photometrically, by changes in brightness caused by aneclipse; or astrometrically, by measuring a deviation in a star's position caused by an unseen companion. Any binarystar can belong to several of these classes; for example, several spectroscopic binaries are also eclipsing binaries.

Visual binaries

A visual binary star is a binary star for which the angular separation between the two components is great enough topermit them to be observed as a double star in a telescope, or even high-powered binoculars. The angular resolutionof the telescope is an important factor in the detection of visual binaries, and as better angular resolutions are appliedto binary star observations increasing number of visual binaries will be detected. The relative brightness of the twostars is also an important factor, as glare from a bright star may make it difficult to detect the presence of a faintercomponent.The brighter star of a visual binary is the primary star, and the dimmer is considered the secondary. In somepublications (especially older ones), a faint secondary is called the comes (plural comites; companion). If the starsare the same brightness, the discoverer designation for the primary is customarily accepted.[12]

The position angle of the secondary with respect to the primary is measured, together with the angular distancebetween the two stars. The time of observation is also recorded. After a sufficient number of observations arerecorded over a period of time, they are plotted in polar coordinates with the primary star at the origin, and the mostprobable ellipse is drawn through these points such that the Keplerian law of areas is satisfied. This ellipse is knownas the apparent ellipse, and is the projection of the actual elliptical orbit of the secondary with respect to the primaryon the plane of the sky. From this projected ellipse the complete elements of the orbit may be computed, where thesemi-major axis can only be expressed in angular units unless the stellar parallax, and hence the distance, of thesystem is known.

Spectroscopic binaries

Sometimes, the only evidence of a binary star comes from the Doppler effect on its emitted light. In these cases, thebinary consists of a pair of stars where the spectral lines in the light emitted from each star shifts first toward theblue, then toward the red, as each moves first toward us, and then away from us, during its motion about theircommon center of mass, with the period of their common orbit.In these systems, the separation between the stars is usually very small, and the orbital velocity very high. Unless theplane of the orbit happens to be perpendicular to the line of sight, the orbital velocities will have components in theline of sight and the observed radial velocity of the system will vary periodically. Since radial velocity can bemeasured with a spectrometer by observing the Doppler shift of the stars' spectral lines, the binaries detected in thismanner are known as spectroscopic binaries. Most of these cannot be resolved as a visual binary, even withtelescopes of the highest existing resolving power.In some spectroscopic binaries, spectral lines from both stars are visible and the lines are alternately double andsingle. Such a system is known as a double-lined spectroscopic binary (often denoted "SB2"). In other systems, thespectrum of only one of the stars is seen and the lines in the spectrum shift periodically towards the blue, thentowards red and back again. Such stars are known as single-lined spectroscopic binaries ("SB1").The orbit of a spectroscopic binary is determined by making a long series of observations of the radial velocity of one or both components of the system. The observations are plotted against time, and from the resulting curve a period is determined. If the orbit is circular then the curve will be a sine curve. If the orbit is elliptical, the shape of the curve will depend on the eccentricity of the ellipse and the orientation of the major axis with reference to the line

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of sight.It is impossible to determine individually the semi-major axis a and the inclination of the orbit plane i. However, theproduct of the semi-major axis and the sine of the inclination (i.e. a sin i) may be determined directly in linear units(e.g. kilometres). If either a or i can be determined by other means, as in the case of eclipsing binaries, a completesolution for the orbit can be found.Binary stars that are both visual and spectroscopic binaries are rare, and are a precious source of valuableinformation when found. Visual binary stars often have large true separations, with periods measured in decades tocenturies; consequently, they usually have orbital speeds too small to be measured spectroscopically. Conversely,spectroscopic binary stars move fast in their orbits because they are close together, usually too close to be detected asvisual binaries. Binaries that are both visual and spectroscopic thus must be relatively close to Earth.

Eclipsing binaries

Algol B orbits Algol A. This animation wasassembled from 55 images of the CHARAinterferometer in the near-infrared H-band,

sorted according to orbital phase.

An eclipsing binary star is a binary star in which the orbit plane of thetwo stars lies so nearly in the line of sight of the observer that thecomponents undergo mutual eclipses. In the case where the binary isalso a spectroscopic binary and the parallax of the system is known, thebinary is quite valuable for stellar analysis. Algol is the best-knownexample of an eclipsing binary.

In the last decade, measurement of extragalactic eclipsing binaries'fundamental parameters has become possible with 8 meter classtelescopes. This makes it feasible to use them to directly measure thedistances to external galaxies, a process that is more accurate than usingstandard candles. Recently, they have been used to give direct distanceestimates to the LMC, SMC, Andromeda Galaxy and TriangulumGalaxy. Eclipsing binaries offer a direct method to gauge the distance togalaxies to a new improved 5% level of accuracy.

Eclipsing binaries are variable stars, not because the light of theindividual components vary but because of the eclipses. The light curve of an eclipsing binary is characterized byperiods of practically constant light, with periodic drops in intensity. If one of the stars is larger than the other, onewill be obscured by a total eclipse while the other will be obscured by an annular eclipse.

The period of the orbit of an eclipsing binary may be determined from a study of the light curve, and the relativesizes of the individual stars can be determined in terms of the radius of the orbit by observing how quickly thebrightness changes as the disc of the near star slides over the disc of the distant star. If it is also a spectroscopicbinary the orbital elements can also be determined, and the mass of the stars can be determined relatively easily,which means that the relative densities of the stars can be determined in this case.

Astrometric binaries

Astronomers have discovered some stars that seemingly orbit around an empty space. Astrometric binaries arerelatively nearby stars which can be seen to wobble around a point in space, with no visible companion. The samemathematics used for ordinary binaries can be applied to infer the mass of the missing companion. The companioncould be very dim, so that it is currently undetectable or masked by the glare of its primary, or it could be an objectthat emits little or no electromagnetic radiation, for example a neutron star.The visible star's position is carefully measured and detected to vary, due to the gravitational influence from its counterpart. The position of the star is repeatedly measured relative to more distant stars, and then checked for periodic shifts in position. Typically this type of measurement can only be performed on nearby stars, such as those

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within 10 parsecs. Nearby stars often have a relatively high proper motion, so astrometric binaries will appear tofollow a wobbly path across the sky.If the companion is sufficiently massive to cause an observable shift in position of the star, then its presence can bededuced. From precise astrometric measurements of the movement of the visible star over a sufficiently long periodof time, information about the mass of the companion and its orbital period can be determined. Even though thecompanion is not visible, the characteristics of the system can be determined from the observations using Kepler'slaws.This method of detecting binaries is also used to locate extrasolar planets orbiting a star. However, the requirementsto perform this measurement are very exacting, due to the great difference in the mass ratio, and the typically longperiod of the planet's orbit. Detection of position shifts of a star is a very exacting science, and it is difficult toachieve the necessary precision. Space telescopes can avoid the blurring effect of the Earth's atmosphere, resulting inmore precise resolution.

Configuration of the system

Artist's conception of a cataclysmic variablesystem

Another classification is based on the distance of the stars, relative totheir sizes:Detached binaries are binary stars where each component is within itsRoche lobe, i.e. the area where the gravitational pull of the star itself islarger than that of the other component. The stars have no major effecton each other, and essentially evolve separately. Most binaries belongto this class.

Semidetached binary stars are binary stars where one of thecomponents fills the binary star's Roche lobe and the other does not.Gas from the surface of the Roche-lobe-filling component (donor) istransferred to the other, accreting star. The mass transfer dominates the evolution of the system. In many cases, theinflowing gas forms an accretion disc around the accretor.

A contact binary is a type of binary star in which both components of the binary fill their Roche lobes. Theuppermost part of the stellar atmospheres forms a common envelope that surrounds both stars. As the friction of theenvelope brakes the orbital motion, the stars may eventually merge.

Cataclysmic variables and X-ray binariesWhen a binary system contains a compact object such as a white dwarf, neutron star or black hole, gas from the other(donor) star can accrete onto the compact object. This releases gravitational potential energy, causing the gas tobecome hotter and emit radiation. Cataclysmic variable stars, where the compact object is a white dwarf, areexamples of such systems. In X-ray binaries, the compact object can be either a neutron star or a black hole. Thesebinaries are classified as low-mass or high-mass according to the mass of the donor star. High-mass X-ray binariescontain a young, early type, high-mass donor star which transfers mass by its stellar wind, while low-mass X-raybinaries are semidetached binaries in which gas from a late-type donor star overflows the Roche lobe and fallstowards the neutron star or black hole.[13] Probably the best known example of an X-ray binary at present is thehigh-mass X-ray binary Cygnus X-1. In Cygnus X-1, the mass of the unseen companion is believed to be about ninetimes that of our sun, far exceeding the Tolman–Oppenheimer–Volkoff limit for the maximum theoretical mass of aneutron star. It is therefore believed to be a black hole; it was the first object for which this was widely believed.[14]

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Orbital periodOrbital periods can be less than an hour (for AM CVn stars), or a few days (components of Beta Lyrae), but alsohundreds of thousands of years (Proxima Centauri around Alpha Centauri AB).

Designations

A and BThe components of binary stars are denoted by the suffixes A and B appended to the system's designation, A denotingthe primary and B the secondary. The suffix AB may be used to denote the pair (for example, the binary star αCentauri AB consists of the stars α Centauri A and α Centauri B.) Additional letters, such as C, D, etc., may be usedfor systems with more than two stars. In cases where the binary star has a Bayer designation and is widely separated,it is possible that the members of the pair will be designated with superscripts; an example is Zeta Reticuli, whosecomponents are ζ1 Reticuli and ζ2 Reticuli.

Discoverer designationsDouble stars are also designated by an abbreviation giving the discoverer together with an index number.[15] αCentauri, for example, was found to be double by Father Richaud in 1689, and so is designated RHD 1.[16] Thesediscoverer codes can be found in the Washington Double Star Catalog.[17]

Hot and coldThe components of a binary star system may be designated by their relative temperatures as the hot companion andcool companion.Examples:• Antares (Alpha Scorpii) is a red supergiant star in a binary system with a hotter blue main sequence star Antares

B. Antares B can therefore be termed a hot companion of the cool supergiant.[18]

• Symbiotic stars are binary star systems composed of a late-type giant star and a hotter companion object. Sincethe nature of the companion is not well-established in all cases, it may be termed a "hot companion".

• The luminous blue variable Eta Carinae has recently been determined to be a binary star system. The secondaryappears to have a higher temperature than the primary and has therefore been described as being the "hotcompanion" star. It may be a Wolf–Rayet star.

• R Aquarii shows a spectrum which simultaneously displays both a cool and hot signature. This combination is theresult of a cool red supergiant accompanied by a smaller, hotter companion. Matter flows from the supergiant tothe smaller, denser companion.

• NASA's Kepler mission has discovered examples of eclipsing binary stars where the secondary is the hottercomponent. KOI-74b is a 12,000 K white dwarf companion of KOI-74 (KIC 6889235 [19]), a 9,400 K earlyA-type main sequence star. KOI-81b is a 13,000 K white dwarf companion of KOI-81 (KIC 8823868 [20]), a10,000 K late B-type main sequence star.

Evolution

FormationWhile it is not impossible that some binaries might be created through gravitational capture between two single stars, given the very low likelihood of such an event (three objects are actually required, as conservation of energy rules out a single gravitating body capturing another) and the high number of binaries, this cannot be the primary formation process. Also, the observation of binaries consisting of pre main sequence stars, supports the theory that

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binaries are already formed during star formation. Fragmentation of the molecular cloud during the formation ofprotostars is an acceptable explanation for the formation of a binary or multiple star system.The outcome of the three-body problem, where the three stars are of comparable mass, is that eventually one of thethree stars will be ejected from the system and, assuming no significant further perturbations, the remaining two willform a stable binary system.

Mass transfer and accretionAs a main-sequence star increases in size during its evolution, it may at some point exceed its Roche lobe, meaningthat some of its matter ventures into a region where the gravitational pull of its companion star is larger than its own.The result is that matter will transfer from one star to another through a process known as Roche Lobe overflow(RLOF), either being absorbed by direct impact or through an accretion disc. The mathematical point through whichthis transfer happens is called the first Lagrangian point.[21] It is not uncommon that the accretion disc is thebrightest (and thus sometimes the only visible) element of a binary star.If a star grows outside of its Roche lobe too fast for all abundant matter to be transferred to the other component, it isalso possible that matter will leave the system through other Lagrange points or as stellar wind, thus beingeffectively lost to both components.[22] Since the evolution of a star is determined by its mass, the process influencesthe evolution of both companions, and creates stages that cannot be attained by single stars.Studies of the eclipsing ternary Algol led to the Algol paradox in the theory of stellar evolution: althoughcomponents of a binary star form at the same time, and massive stars evolve much faster than the less massive ones,it was observed that the more massive component Algol A is still in the main sequence, while the less massive AlgolB is a subgiant star at a later evolutionary stage. The paradox can be solved by mass transfer: when the more massivestar became a subgiant, it filled its Roche lobe, and most of the mass was transferred to the other star, which is still inthe main sequence. In some binaries similar to Algol, a gas flow can actually be seen.

Runaways and novaeIt is also possible for widely separated binaries to lose gravitational contact with each other during their lifetime, as aresult of external perturbations. The components will then move on to evolve as single stars. A close encounterbetween two binary systems can also result in the gravitational disruption of both systems, with some of the starsbeing ejected at high velocities, leading to runaway stars.If a white dwarf has a close companion star that overflows its Roche lobe, the white dwarf will steadily accrete gasesfrom the star's outer atmosphere. These are compacted on the white dwarf's surface by its intense gravity,compressed and heated to very high temperatures as additional material is drawn in. The white dwarf consists ofdegenerate matter, and so is largely unresponsive to heat, while the accreted hydrogen is not. Hydrogen fusion canoccur in a stable manner on the surface through the CNO cycle, causing the enormous amount of energy liberated bythis process to blow the remaining gases away from the white dwarf's surface. The result is an extremely brightoutburst of light, known as a nova.In extreme cases this event can cause the white dwarf to exceed the Chandrasekhar limit and trigger a supernova thatdestroys the entire star, and is another possible cause for runaways. An example of such an event is the supernovaSN 1572, which was observed by Tycho Brahe. The Hubble Space Telescope recently took a picture of the remnantsof this event.

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Astrophysics

A simulated example of a binary star, where twobodies with similar mass orbit around a common

barycenter in elliptic orbits

Binaries provide the best method for astronomers to determine themass of a distant star. The gravitational pull between them causes themto orbit around their common center of mass. From the orbital patternof a visual binary, or the time variation of the spectrum of aspectroscopic binary, the mass of its stars can be determined. In thisway, the relation between a star's appearance (temperature and radius)and its mass can be found, which allows for the determination of themass of non-binaries.Because a large proportion of stars exist in binary systems, binaries areparticularly important to our understanding of the processes by whichstars form. In particular, the period and masses of the binary tell us about the amount of angular momentum in thesystem. Because this is a conserved quantity in physics, binaries give us important clues about the conditions underwhich the stars were formed.

Calculating the center of mass in binary starsIn a simple binary case, r1, the distance from the center of the first star to the center of mass, is given by:

where:a is the distance between the two stellar centers andm1 and m2 are the masses of the two stars.

If a is taken to be the semi-major axis of the orbit of one body around the other, then r1 will be the semimajor axis ofthe first body's orbit around the center of mass or barycenter, and r2 = a – r1 will be the semimajor axis of the secondbody's orbit. When the center of mass is located within the more massive body, that body will appear to wobblerather than following a discernible orbit.

Center of mass animationsImages are representative, not simulated. The position of the red cross indicates the center of mass of the system.

(a.) Two bodies of similar mass orbiting arounda common center of mass, or barycenter.

(b.) Two bodies with a difference in massorbiting around a common barycenter, like theCharon-Pluto system

(c.) Two bodies with a major difference in massorbiting around a common barycenter (similar tothe Earth–Moon system)

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(d.) Two bodies with an extreme difference inmass orbiting around a common barycenter(similar to the Sun–Earth system)

(e.) Two bodies with similar mass orbiting in an ellipse around a common barycenter.

Research findingsIt is estimated that approximately 1/3 of the star systems in the Milky Way are binary or multiple, with the remaining2/3 consisting of single stars.[23]

There is a direct correlation between the period of revolution of a binary star and the eccentricity of its orbit, withsystems of short period having smaller eccentricity. Binary stars may be found with any conceivable separation,from pairs orbiting so closely that they are practically in contact with each other, to pairs so distantly separated thattheir connection is indicated only by their common proper motion through space. Among gravitationally boundbinary star systems, there exists a so-called log normal distribution of periods, with the majority of these systemsorbiting with a period of about 100 years. This is supporting evidence for the theory that binary systems are formedduring star formation.In pairs where the two stars are of equal brightness, they are also of the same spectral type. In systems where thebrightnesses are different, the fainter star is bluer if the brighter star is a giant star, and redder if the brighter starbelongs to the main sequence.

Artist's impression of the sight from a(hypothetical) moon of planet HD 188753 Ab

(upper left), which orbits a triple star system. Thebrightest companion is just below the horizon.

The mass of a star can be directly determined only from itsgravitational attraction. Apart from the Sun and stars which act asgravitational lenses, this can be done only in binary and multiple starsystems, making the binary stars an important class of stars. In the caseof a visual binary star, after the orbit and the stellar parallax of thesystem has been determined, the combined mass of the two stars maybe obtained by a direct application of the Keplerian harmonic law.

Unfortunately, it is impossible to obtain the complete orbit of aspectroscopic binary unless it is also a visual or an eclipsing binary, sofrom these objects only a determination of the joint product of massand the sine of the angle of inclination relative to the line of sight ispossible. In the case of eclipsing binaries which are also spectroscopicbinaries, it is possible to find a complete solution for the specifications(mass, density, size, luminosity, and approximate shape) of both members of the system.

Planets

Science fiction has often featured planets of binary or ternary stars as a setting, for example George Lucas' Tatooine from Star Wars, Greg Farshtey's Spherus Magna from Bionicle, and one notable story, "Nightfall", even takes this to a six-star system. In reality, some orbital ranges are impossible for dynamical reasons (the planet would be expelled from its orbit relatively quickly, being either ejected from the system altogether or transferred to a more inner or outer orbital range), whilst other orbits present serious challenges for eventual biospheres because of likely extreme

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variations in surface temperature during different parts of the orbit. Planets that orbit just one star in a binary pair aresaid to have "S-type" orbits, whereas those that orbit around both stars have "P-type" or "circumbinary" orbits. It isestimated that 50–60% of binary stars are capable of supporting habitable terrestrial planets within stable orbitalranges.Simulations have shown that the presence of a binary companion can actually improve the rate of planet formationwithin stable orbital zones by "stirring up" the protoplanetary disk, increasing the accretion rate of the protoplanetswithin.Detecting planets in multiple star systems introduces additional technical difficulties, which may be why they areonly rarely found. Examples include the white dwarf-pulsar binary PSR B1620-26, the subgiant-red dwarf binaryGamma Cephei, and the white dwarf-red dwarf binary NN Serpentis. More planets around binaries are listed in:[Muterspaugh; Lane; Kulkarni; Maciej Konacki; Burke; Colavita; Shao; Hartkopf et al. (2010). "The PHASESDifferential Astrometry Data Archive. V. Candidate Substellar Companions to Binary Systems". arXiv:1010.4048[24] [astro-ph.SR [25]].].A study of fourteen previously known planetary systems found three of these systems to be binary systems. Allplanets were found to be in S-type orbits around the primary star. In these three cases the secondary star was muchdimmer than the primary and so was not previously detected. This discovery resulted in a recalculation of parametersfor both the planet and the primary star.

Examples

The two visibly distinguishable components ofAlbireo

The large distance between the components, as well as their differencein color, make Albireo one of the easiest observable visual binaries.The brightest member, which is the third brightest star in theconstellation Cygnus, is actually a close binary itself. Also in theCygnus constellation is Cygnus X-1, an X-ray source considered to bea black hole. It is a high-mass X-ray binary, with the opticalcounterpart being a variable star.[26] Sirius is another binary and thebrightest star in the night time sky, with a visual apparent magnitude of−1.46. It is located in the constellation Canis Major. In 1844 FriedrichBessel deduced that Sirius was a binary. In 1862 Alvan Graham Clarkdiscovered the companion (Sirius B; the visible star is Sirius A). In1915 astronomers at the Mount Wilson Observatory determined thatSirius B was a white dwarf, the first to be discovered. In 2005, usingthe Hubble Space Telescope, astronomers determined Sirius B to be12,000 km (7,456 mi) in diameter, with a mass that is 98% of the Sun.

An example of an eclipsing binary is Epsilon Aurigae in the constellation Auriga. The visible component belongs tothe spectral class F0, the other (eclipsing) component is not visible. The last such eclipse occurred from 2009–2011,and it is hoped that the extensive observations that will likely be carried out may yield further insights into the natureof this system. Another eclipsing binary is Beta Lyrae, which is a semi-detached binary star system in theconstellation of Lyra.

Other interesting binaries include 61 Cygni (a binary in the constellation Cygnus, composed of two K class (orange)main sequence stars, 61 Cygni A and 61 Cygni B, which is known for its large proper motion), Procyon (thebrightest star in the constellation Canis Minor and the eighth brightest star in the night time sky, which is a binaryconsisting of the main star with a faint white dwarf companion), SS Lacertae (an eclipsing binary which stoppedeclipsing), V907 Sco (an eclipsing binary which stopped, restarted, then stopped again) and BG Geminorum (aneclipsing binary which is thought to contain a black hole with a K0 star in orbit around it).

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Multiple star examplesSystems with more than two stars are termed multiple stars. Algol is the most noted ternary (long thought to be abinary), located in the constellation Perseus. Two components of the system eclipse each other, the variation in theintensity of Algol first being recorded in 1670 by Geminiano Montanari. The name Algol means "demon star" (fromArabic: الغول al-ghūl), which was probably given due to its peculiar behavior. Another visible ternary is AlphaCentauri, in the southern constellation of Centaurus, which contains the fourth brightest star in the night sky, with anapparent visual magnitude of −0.01. This system also underscores the fact that binaries need not be discounted in thesearch for habitable planets. Alpha Centauri A and B have an 11 AU distance at closest approach, and both shouldhave stable habitable zones.There are also examples of systems beyond ternaries: Castor is a sextuple star system, which is the second brighteststar in the constellation Gemini and one of the brightest stars in the nighttime sky. Astronomically, Castor wasdiscovered to be a visual binary in 1719. Each of the components of Castor is itself a spectroscopic binary. Castoralso has a faint and widely separated companion, which is also a spectroscopic binary. The Alcor–Mizar visualbinary in Ursa Majoris also consists of six stars, four comprising Mizar and two comprising Alcor.

Notes and references[1] Filippenko, Alex, Understanding the Universe (of The Great Courses on DVD), Lecture 46, time 1:17, The Teaching Company, Chantilly,

VA, USA, 2007[2] The Binary Stars, Robert Grant Aitken, New York: Dover, 1964, p. 1.[3] Vol. 1, part 1, p. 422, Almagestum Novum (http:/ / leo. astronomy. cz/ mizar/ riccioli. htm), Giovanni Battista Riccioli, Bononiae: Ex

typographia haeredis Victorij Benatij, 1651.[4] A New View of Mizar (http:/ / leo. astronomy. cz/ mizar/ article. htm), Leos Ondra, accessed on line May 26, 2007.[5] pp. 10–11, Observing and Measuring Double Stars, Bob Argyle, ed., London: Springer, 2004, ISBN 1-85233-558-0.[6] pp. 249–250, An Inquiry into the Probable Parallax, and Magnitude of the Fixed Stars, from the Quantity of Light Which They Afford us, and

the Particular Circumstances of Their Situation (http:/ / www. jstor. org/ stable/ 105952), John Michell,Philosophical Transactions(1683–1775) 57 (1767), pp. 234–264.

[7] Account of the Changes That Have Happened, during the Last Twenty-Five Years, in the Relative Situation of Double-Stars; With anInvestigation of the Cause to Which They Are Owing (http:/ / www. jstor. org/ stable/ 107080), William Herschel, Philosophical Transactionsof the Royal Society of London 93 (1803), pp. 339–382.

[8] p. 291, French astronomers, visual double stars and the double stars working group of the Société Astronomique de France, E. Soulié, TheThird Pacific Rim Conference on Recent Development of Binary Star Research, proceedings of a conference sponsored by Chiang MaiUniversity, Thai Astronomical Society and the University of Nebraska-Lincoln held in Chiang Mai, Thailand, 26 October-1 November 1995,ASP Conference Series 130 (1997), ed. Kam-Ching Leung, pp. 291–294, .

[9] "Introduction and Growth of the WDS", The Washington Double Star Catalog (http:/ / ad. usno. navy. mil/ wds/ wdstext. html#intro), BrianD. Mason, Gary L. Wycoff, and William I. Hartkopf, Astrometry Department, United States Naval Observatory, accessed on line August 20,2008.

[10] Sixth Catalog of Orbits of Visual Binary Stars (http:/ / ad. usno. navy. mil/ wds/ orb6. html), William I. Hartkopf and Brian D. Mason,United States Naval Observatory, accessed on line August 20, 2008.

[11] The Washington Double Star Catalog (http:/ / ad. usno. navy. mil/ wds/ ), Brian D. Mason, Gary L. Wycoff, and William I. Hartkopf, UnitedStates Naval Observatory. Accessed on line December 20, 2008.

[12] The Binary Stars, Robert Grant Aitken, New York: Dover, 1964, p. 41.[13] Neutron Star X-ray binaries (http:/ / www. mporzio. astro. it/ ~gianluca/ phdthesis/ node11. html), A Systematic Search of New X-ray

Pulsators in ROSAT Fields, Gian Luca Israel, Ph. D. thesis, Trieste, October 1996.[14] Black Holes (http:/ / imagine. gsfc. nasa. gov/ docs/ science/ know_l2/ black_holes. html), Imagine the Universe!, NASA. Accessed on line

August 22, 2008.[15] pp. 307–308, Observing and Measuring Double Stars, Bob Argyle, ed., London: Springer, 2004, ISBN 1-85233-558-0.[16] Entry 14396-6050, discoverer code RHD 1AB, The Washington Double Star Catalog (http:/ / ad. usno. navy. mil/ wds/ Webtextfiles/

wdsnewframe3. html), United States Naval Observatory. Accessed on line August 20, 2008.[17] References and discoverer codes, The Washington Double Star Catalog (http:/ / ad. usno. navy. mil/ wds/ Webtextfiles/ wdsnewframe.

html), United States Naval Observatory. Accessed on line August 20, 2008.[18] (http:/ / simbad. u-strasbg. fr/ simbad/ sim-id?Ident=* alf Sco B) – see essential notes: "Hot companion to Antares at 2.9arcsec; estimated

period: 678yr."[19] http:/ / archive. stsci. edu/ kepler/ kic10/ search. php?kic_kepler_id=6889235& action=Search[20] http:/ / archive. stsci. edu/ kepler/ kic10/ search. php?kic_kepler_id=8823868& action=Search

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[21] " Contact Binary Star Envelopes (http:/ / demonstrations. wolfram. com/ ContactBinaryStarEnvelopes/ )" by Jeff Bryant, WolframDemonstrations Project.

[22] " Mass Transfer in Binary Star Systems (http:/ / demonstrations. wolfram. com/ MassTransferInBinaryStarSystems/ )" by Jeff Bryant withWaylena McCully, Wolfram Demonstrations Project.

[23] Most Milky Way Stars Are Single (http:/ / www. cfa. harvard. edu/ press/ pr0611. html), Harvard-Smithsonian Center for Astrophysics[24] http:/ / arxiv. org/ abs/ 1010. 4048[25] http:/ / arxiv. org/ archive/ astro-ph. SR[26] See sources at Cygnus X-1

External links• The Double Star Library (http:/ / ad. usno. navy. mil/ wds/ dsl. html), at the U.S. Naval Observatory• ianridpath.com: List of the best visual binaries (http:/ / www. ianridpath. com/ binaries. htm), for amateurs, with

orbital elements• Pictures of binaries at Hubblesite.org (http:/ / hubblesite. org/ newscenter/ newsdesk/ archive/ releases/ category/

star/ multiple star systems/ )• Chandra X-ray Observatory (http:/ / chandra. harvard. edu/ xray_sources/ binary_stars. html)• Binary Stars (http:/ / www. dmoz. org/ Science/ Astronomy/ Stars/ Binary_Stars) at DMOZ• An extensive simulation for the Algol system by North Carolina State University (http:/ / wonka. physics. ncsu.

edu/ Astro/ Research/ Algol/ )• Selected visual double stars and their relative position as a function of time (http:/ / astroclub. tau. ac. il/ ephem/

VisualDoubleStars/ )• Artistic representations of binary stars by Mark A. Garlick (http:/ / www. space-art. co. uk/ gallery.

php?gallery=Stars2)• Orbits and Velocity Curves of Spectroscopic Binaries, J. Miller Barr (1908) (http:/ / www. datasync. com/ ~rsf1/

barr1908. htm)• Eclipsing Binaries in the 21st Century—Opportunities for Amateur Astronomers (http:/ / www. aavso. org/

ejaavso401467)

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Article Sources and ContributorsAlpha Centauri  Source: http://en.wikipedia.org/w/index.php?oldid=603290000  Contributors: 68Kustom, AKYF, ALK, Aarghdvaark, Abtract, Acalamari, Acoma Magic, Adam78, AdultSwim,Aecis, Ajstov, Ajuk, Aldaron, Aldebaran66, Alexander Mikhalenko, Alexikoua, Alexwcovington, Alfakim, Alfio, Algebraist, All Is One, AlphaZelda, Alvaro, Andycjp, Aqwis, Aranel, ArcticKangaroo, Argo Navis, Arianewiki1, Ariochiv, Armetrek, Arsia Mons, Arthena, Artman40, Ashley Pomeroy, Ashmoo, Asiaticus, AssegaiAli, Astrobiologist, Ataleh, Attilios, B.d.mills, B00P,BRW, Babbage, Bad Astronomer, BartBenjamin, Basalisk, Baseball437, Bay Flam, Bazonka, Bellerophon5685, Ben Arnold, Benbest, Bender235, Benhocking, Bgwhite, Bilconixon, BlartVersenwald III, Bluecollarchessplayer, Bob A, Bob rulz, Bongwarrior, Brockert, Bronger, Brother Officer, Bryan Derksen, Burningdwarf, C17GMaster, Cadiomals, Calvin 1998,CambridgeBayWeather, Casliber, Catolishis, Catskul, Changeup, Chaos syndrome, Chase 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Gene Nygaard, George100, Ghewgill, Ghpink, Giler, Ginsuloft, Gob Lofa, Goldom, Gr8opinionater, Graham87, Granttimmerman, GrowlyGenet, Grutter, Grzes,Guardianrule, Gunnar Larsson, Guppyfinsoup, Gurubrahma, Hadrian89, Hairmetal4ever, Hairy Dude, Hans Dunkelberg, Hans-Peter Scholz, Haoie, Happyguy49, Headbomb,Heisenbergthechemist, Hekerui, Helicoptor, Henrykus, Hgrosser, Hibernian, Hidan, HisSpaceResearch, Hughey, HumphreyW, Hunnjazal, IanM, Icairns, Icalanise, Ilvon, Imzogelmo, Inomyabcs,Insineratehymn, Isoginchaku, J.delanoy, JCHall, JRM, JaGa, Jake Nelson, JasonAQuest, Jaxl, Jedishive, Jeff G., Jeronimo, Jessopher, JohnDBuell, Jomanted, JorisvS, Joseph Solis in Australia,Just James, JustAGal, Jwrosenzweig, Jyril, KRRK, Kbthompson, Kencomer, Ketiltrout, Kevin Nelson, Kheider, Kingdon, Kintetsubuffalo, Kitch, Klingon83, Koavf, Kotra, KuboF, Kukini,Kuralyov, Kuru, KuwarOnline, Kwamikagami, LOL, Lambiam, LeCire, Lenoxus, Lightmouse, Livajo, Lolm8, LordRahl, Lugia2453, LukeSurl, M.O.X, 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Proxima Centauri  Source: http://en.wikipedia.org/w/index.php?oldid=602616862  Contributors: 77Mike77, @pple, ARC Gritt, AThinkingScientist, Adeptzare3, Ahoerstemeier, Alcazar84,AleksanderV, Alfio, Amorymeltzer, An Siarach, Andre Engels, Andrewhayes, Angie Y., Angilbas, Antandrus, Apandada1, Aranel, Arianewiki1, Arsia Mons, Art LaPella, Ashill, B00P, Balcer,Bazonka, Bender235, Bhorrock, Bob rulz, Bradeos Graphon, Brutal Chicken, Calle Cool, Capester, Captdoc, Casliber, Chaos syndrome, Chermundy, Chris the speller, Chrislintott, ChronicMist,Clammybells, Cliff12345, Cody escadron delta, CommonsDelinker, Complex01, Crisco 1492, Curps, DARTH SIDIOUS 2, Dabomb87, DadaNeem, Davandron, Dave3457, Deb, Deflective,DenisMoskowitz, Diannaa, Dolerite, Drunken Pirate, Earthandmoon, Efe, El C, Elwad345, Enirac Sum, Epbr123, Epicnessawsome123, EricWesBrown, Feedmecereal, Feinoha, FelineAvenger,Fotaun, Fricasso, Fui in terra aliena, GabrielVelasquez, Gazjo, Gilliam, Ginger Conspiracy, GrayFox92, Greenfeast, Greg Allen, Grondemar, Gsklee, Gurch, Gwsk55970, HJ Mitchell, HansDunkelberg, Harley peters, Headbomb, Henrykus, Herbee, Hgrosser, Hiberniantears, Hunnjazal, Hydrargyrum, Icalanise, Icek, Intelati, InternetMeme, Interstellar Man, IwikUwik, J.L.Main,JH-man, Jake Nelson, Jeder1, Jesse0986, Jimfbleak, Jirt, Jmencisom, Joanjoc, John254, JohnI, JorisvS, Josquius, Jurgis.Rudkus, Just plain Bill, Jyril, Jzlcdh, KDS4444, Kaimiddleton, Keraunos,Keta, Kheider, Knutux, Kobrabones, Kogge, Kotra, Kwamikagami, Kyng, Largehole, Lawman, Lolm8, M1ss1ontomars2k4, MIKHEIL, MPF, MZMcBride, Marauder40, Mark Foskey, MarkusPössel, Martarius, Mav, Michael Hardy, Michael Snow, Michael.chlistalla, Michaelbusch, Mickeymousechen, Midgrid, Mike Rosoft, Mirek256, Mlindroo, Modster, Mollwollfumble, Monedula,Mopcwiki, Moxy, Mr. Billion, Murgh, MusicScienceGuy, Myrrhlin, N-true, NatureA16, NeilTarrant, NellieBly, NewEnglandYankee, Newone, Nexus2006, Ng.j, Nickkid5, Nihiltres, Novangelis,NuclearVacuum, Nxavar, PRRfan, Patrick, Patrick1982, Pedro.feio, Philip Trueman, Pietrow, Piledhigheranddeeper, Piperh, Pizza Puzzle, PlanetStar, Potatoswatter, Praemonitus, ProximaCentauri, RJHall, RandomCritic, RedSpruce, RedWolf, Renum, Rich Farmbrough, Richard Arthur Norton (1958- ), Richard001, RingtailedFox, Rjp0i, Rjwilmsi, Robert Brockway, RobertG,Roberto Mura, Robma, Rogermw, Rominandreu, Rothorpe, Rpyle731, Rursus, Ruslik0, Rusted AutoParts, Sadalsuud, Sardonicone, Savemaxim, Screw the arsehoel, Scwlong, Sdsds, SeanQuixote, Shadowmask, Shambolic Entity, Skatebiker, Skud0rz, Smartech, Smeira, Smkolins, Solar-Wind, Sonicology, Spacepotato, Splarka, SqueakBox, Starmurderer, Stephan Schulz,Steve2011, Sunshine4921, SuperHamster, Tanmoy Panigrahi, Tbhotch, Telescopi, The Anome, The Herald, The Thing That Should Not Be, Threeafterthree, Too Old, Treisijs, Urhixidur, VagueRant, Van der Hoorn, Vegardw, Voidxor, Vrenator, Vyacheslav84, Vyznev Xnebara, Wavelength, Well-rested, Whitetigah, Whkoh, WikipedianProlific, Willardmcg, William Avery, Wjhonson,Wknight94, WolfmanSF, Xanzzibar, Zaslav, 192 anonymous edits

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Image Sources, Licenses and ContributorsFile:Alpha centauri.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Alpha_centauri.jpg  License: unknown  Contributors: ESOFile:Position Alpha Cen.png  Source: http://en.wikipedia.org/w/index.php?title=File:Position_Alpha_Cen.png  License: Public Domain  Contributors: User:ZwergelsternFile:Mobile diagram of Alpha Centauri system.png  Source: http://en.wikipedia.org/w/index.php?title=File:Mobile_diagram_of_Alpha_Centauri_system.png  License: Creative Commons Zero Contributors: EvenGreenerFishFile:Artist’s impression of the planet around Alpha Centauri B (Annotated).jpg  Source:http://en.wikipedia.org/w/index.php?title=File:Artist’s_impression_of_the_planet_around_Alpha_Centauri_B_(Annotated).jpg  License: unknown  Contributors: Denniss, Hekerui, IgniX,Quantanew, Stas1995, 2 anonymous editsFile:The bright star Alpha Centauri and its surroundings.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:The_bright_star_Alpha_Centauri_and_its_surroundings.jpg  License:unknown  Contributors: Courcelles, JorisvS, Stas1995File:Alpha Centauri relative sizes.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Alpha_Centauri_relative_sizes.svg  License: Creative Commons Attribution-ShareAlike 3.0Unported  Contributors: Original MetaPost program by David Benbennick Program rendered as SVG by QefFile:Alpha Centauri AB over limb of Saturn PIA10406.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Alpha_Centauri_AB_over_limb_of_Saturn_PIA10406.jpg  License: PublicDomain  Contributors: NASA/JPL/Space Science Institute Original uploader was Kwamikagami at en.wikipediaFile:Alpha, Beta and Proxima Centauri.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Alpha,_Beta_and_Proxima_Centauri.jpg  License: GNU Free Documentation License Contributors: SkatebikerFile:Orbit Alpha Centauri AB arcsec.png  Source: http://en.wikipedia.org/w/index.php?title=File:Orbit_Alpha_Centauri_AB_arcsec.png  License: Creative Commons Attribution-Sharealike3.0  Contributors: SiriusBImage:Motion-of-Alpha-Cen.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Motion-of-Alpha-Cen.jpg  License: Public Domain  Contributors: DynV, Juiced lemon, Maksim,Ranveig, と あ る 白 い 猫

File:Sol View from AlpCenA.png  Source: http://en.wikipedia.org/w/index.php?title=File:Sol_View_from_AlpCenA.png  License: GNU General Public License  Contributors: Calle Cool,Ckatz, Dandaman32, DynV, LERK, Misza13, Paddu, RandyKaelber, 2 anonymous editsFile:Sky-from-alpha-centauri.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Sky-from-alpha-centauri.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors:SkatebikerFile:Planet-alphacen1.png  Source: http://en.wikipedia.org/w/index.php?title=File:Planet-alphacen1.png  License: Creative Commons Attribution-Sharealike 3.0  Contributors: The plagueFile:Near-stars-past-future-en.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Near-stars-past-future-en.svg  License: Creative Commons Attribution-Sharealike 3.0  Contributors:User:FrancescoAFile:Celestia.png  Source: http://en.wikipedia.org/w/index.php?title=File:Celestia.png  License: GNU General Public License  Contributors: 555, Bourrichon, Cody escadron delta,ComputerHotline, CyberSkull, Czeror, Gildemax, Go for it!, Nnemo, Rocket000, Rursus, Tony Wills, XBrain130File:New shot of Proxima Centauri, our nearest neighbour.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:New_shot_of_Proxima_Centauri,_our_nearest_neighbour.jpg  License:unknown  Contributors: Jmencisom, Stas1995file:Alpha centauri size.png  Source: http://en.wikipedia.org/w/index.php?title=File:Alpha_centauri_size.png  License: Creative Commons Attribution-Sharealike 3.0  Contributors: RJHallfile:Alpha, Beta and Proxima Centauri.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Alpha,_Beta_and_Proxima_Centauri.jpg  License: GNU Free Documentation License Contributors: SkatebikerFile:RedDwarfNASA-hue-shifted.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:RedDwarfNASA-hue-shifted.jpg  License: Public Domain  Contributors:File:RedDwarfNASA.jpg: NASA/Walt FeimerDerivative: Original uploader was Klamasa at en.wikipediaFile:Sol View from AlpCenA1.png  Source: http://en.wikipedia.org/w/index.php?title=File:Sol_View_from_AlpCenA1.png  License: GNU Free Documentation License  Contributors:FrancescoAFile:Artist's impression of the evolution of a hot high-mass binary star.ogv  Source:http://en.wikipedia.org/w/index.php?title=File:Artist's_impression_of_the_evolution_of_a_hot_high-mass_binary_star.ogv  License: unknown  Contributors: ESO/L. Calçada/M. Kornmesser/S.E.de MinkImage:Sirius A and B Hubble photo.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Sirius_A_and_B_Hubble_photo.jpg  License: Creative Commons Attribution 3.0  Contributors:NASA, ESA, H. Bond (STScI), and M. Barstow (University of Leicester)File:Wiktionary-logo-en.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Wiktionary-logo-en.svg  License: Public Domain  Contributors: Vectorized by , based on original logotossed together by Brion VibberImage:Gwiazda podwójna zaćmieniowa schemat.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Gwiazda_podwójna_zaćmieniowa_schemat.svg  License: GNU FreeDocumentation License  Contributors: MesserWolandFile:Algol AB movie imaged with the CHARA interferometer - labeled.gif  Source:http://en.wikipedia.org/w/index.php?title=File:Algol_AB_movie_imaged_with_the_CHARA_interferometer_-_labeled.gif  License: Creative Commons Attribution-Sharealike 3.0  Contributors:User:Stigmatella aurantiacaImage:Accretion Disk Binary System.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Accretion_Disk_Binary_System.jpg  License: Public Domain  Contributors: 84user, IlmariKaronen, 3 anonymous editsImage:orbit5.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Orbit5.gif  License: Public Domain  Contributors: User:ZhattImage:orbit1.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Orbit1.gif  License: Public Domain  Contributors: User:ZhattImage:orbit2.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Orbit2.gif  License: Public Domain  Contributors: User:ZhattImage:orbit3.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Orbit3.gif  License: Public Domain  Contributors: User:ZhattImage:orbit4.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Orbit4.gif  License: Public Domain  Contributors: User:ZhattImage:Triple-star sunset.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Triple-star_sunset.jpg  License: Public Domain  Contributors: NASA/JPL-Caltech Original uploader wasSnoopY at en.wikipediaImage:Albireo.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Albireo.jpg  License: GNU Free Documentation License  Contributors: ComputerHotline, Dbenbenn, Maxim Razin

Page 39: Alpha Centauri

License 37

LicenseCreative Commons Attribution-Share Alike 3.0//creativecommons.org/licenses/by-sa/3.0/