Alpha Centauri

This article is about α Centauri, the multi-star system in the constellation Centaurus; not to be confused with a Centauri, A Centauri, or Centaurus A. "Alpha Centauri AB" redirects here; not to be confused with Alpha Centauri Ab or Alpha Centauri b. For other uses, see Alpha Centauri (disambiguation).

Alpha Centauri AB

Alpha Centauri AB (left) forms a triple star system with Proxima Centauri (below, south of, α Centauri AB). (See labelled version)
Observation data Epoch J2000.0      Equinox ICRS
Constellation	Centaurus
α Centauri A (Rigil Kentaurus)
Right ascension	14h 39m 36.49400s
Declination	−60° 50′ 02.3737″
Apparent magnitude (V)	+0.01
α Centauri B (Toliman)
Right ascension	14h 39m 35.06311s
Declination	−60° 50′ 15.0992″
Apparent magnitude (V)	+1.33
Characteristics
A
Spectral type	G2V
B−V colour index	+0.71
B
Spectral type	K1V
B−V colour index	+0.88
Astrometry
A
Radial velocity (Rv)	−21.4±0.76 km/s
Proper motion (μ)	RA: −3679.25 mas/yr Dec.: +473.67 mas/yr
Parallax (π)	750.81±0.38 mas
Distance	4.344 ± 0.002 ly (1.3319 ± 0.0007 pc)
Absolute magnitude (MV)	4.38
B
Radial velocity (Rv)	−18.6±1.64 km/s
Proper motion (μ)	RA: −3614.39 mas/yr Dec.: +802.98 mas/yr
Parallax (π)	750.81±0.38 mas
Distance	4.344 ± 0.002 ly (1.3319 ± 0.0007 pc)
Absolute magnitude (MV)	5.71
Orbit
Primary	A
Name	B
Period (P)	79.762±0.019 yr
Semi-major axis (a)	17.493±0.0096" (23.299 AU)
Eccentricity (e)	0.51947±0.00015
Inclination (i)	79.243±0.0089°
Longitude of the node (Ω)	205.073±0.025°
Periastron epoch (T)	1875.66±0.012
Argument of periastron (ω) (secondary)	231.519±0.027°
Details
α Centauri A
Mass	1.0788±0.0029 M☉
Radius	1.2175±0.0055 R☉
Luminosity	1.5059±0.0019 L☉
Surface gravity (log g)	4.30 cgs
Temperature	5,804±13 K
Metallicity [Fe/H]	0.20±0.01 dex
Rotation	28.3±0.5 d
Rotational velocity (v sin i)	2.7±0.7 km/s
Age	7.2–7.8 Gyr
α Centauri B
Mass	0.9092±0.0025 M☉
Radius	0.8591±0.0036 R☉
Luminosity	0.4981±0.0007 L☉
Surface gravity (log g)	4.37 cgs
Temperature	5,207±12 K
Metallicity [Fe/H]	0.24±0.01 dex
Rotation	36.7±0.3 d
Rotational velocity (v sin i)	1.1±0.8 km/s
Age	7.2–7.8 Gyr
Other designations
α Cen, CD−60°5293, CPD−60°5483, FK5 538, GC 19728, GJ 559, SAO 252838, CCDM J14396-6050
α Cen A: Rigil Kentaurus, Rigil Kent, α1 Cen, HD 128620, HIP 71683, HR 5459, LHS 50, PLX 3309
α Cen B: Toliman, α2 Cen, HD 128621, HIP 71681, HR 5460, LHS 51
Database references
SIMBAD	AB
	A
	B
Exoplanet Archive	data
ARICNS	data

Alpha Centauri (α Centauri, α Cen, or Alpha Cen) is a star system in the southern constellation of Centaurus. It consists of three stars: Rigil Kentaurus (α Centauri A), Toliman (α Centauri B), and Proxima Centauri (α Centauri C). Proxima Centauri is the closest star to the Sun at 4.2465 light-years (ly), which is 1.3020 parsecs (pc), while Alpha Centauri A and B are the nearest stars visible to the naked eye.

Rigil Kentaurus and Toliman are Sun-like stars (class G and K, respectively) that together form the binary star system α Centauri AB. To the naked eye, these two main components appear to be a single star with an apparent magnitude of −0.27. It is the brightest star in the constellation and the third-brightest in the night sky, outshone by only Sirius and Canopus. α Centauri AB are the nearest binary stars to the Sun at a distance of 4.344 ly (1.33 pc).

Rigil Kentaurus has 1.1 times the mass (M_☉) and 1.5 times the luminosity of the Sun (L_☉), while Toliman is smaller and cooler, at 0.9 M_☉ and less than 0.5 L_☉. The pair orbit around a common centre with an orbital period of 79 years. Their elliptical orbit is eccentric, so that the distance between A and B varies from 35.6 astronomical units (AU), or about the distance between Pluto and the Sun, to 11.2 AU, or about the distance between Saturn and the Sun.

Proxima Centauri is a small faint red dwarf (class M). Though not visible to the naked eye, Proxima Centauri is the closest star to the Sun at a distance of 4.24 ly (1.30 pc), slightly closer than α Centauri AB. The distance between Proxima Centauri and α Centauri AB is about 13,000 AU (0.21 ly), equivalent to about 430 times the radius of Neptune's orbit.

Proxima Centauri has two confirmed planets—Proxima b and Proxima d. The former is an Earth-sized planet in the habitable zone (though it is unlikely to be habitable) while the latter is a sub-Earth which orbits very closely to the star. A possible but disputed third planet, Proxima c, is a mini-Neptune 1.5 astronomical units away. Rigil Kentaurus may have a Saturn-mass planet (Alpha Centauri Ab) in the habitable zone, though it is not yet known with certainty to be planetary in nature. Toliman has no known planets; the only candidate planet, Alpha Centauri Bb, was disproven in 2015.