Alpha Centauri
Alpha Centauri (α Centauri, Alpha Cen, or α Cen) is a triple star system in the southern constellation of Centaurus. It consists of three stars: Rigil Kentaurus (Alpha Centauri A), Toliman (B) and Proxima Centauri (C). Proxima Centauri is the closest star to the Sun at 4.2465 light-years (1.3020 pc).
Alpha Centauri AB (left) forms a triple star system with Proxima Centauri, circled in red. The bright star system to the right is Beta Centauri. | |
| Observation data Epoch J2000.0 Equinox J2000.0 | |
|---|---|
| Constellation | Centaurus |
| Alpha Centauri A | |
| Right ascension | 14h 39m 36.49400s |
| Declination | −60° 50′ 02.3737″ |
| Apparent magnitude (V) | +0.01 |
| Alpha Centauri B | |
| Right ascension | 14h 39m 35.06311s |
| Declination | −60° 50′ 15.0992″ |
| Apparent magnitude (V) | +1.33 |
| Characteristics | |
| A | |
| Spectral type | G2V |
| U−B color index | +0.24 |
| B−V color index | +0.71 |
| B | |
| Spectral type | K1V |
| U−B color index | +0.68 |
| B−V color 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 |
| Companion | B |
| Period (P) | 79.762±0.019 yr |
| Semi-major axis (a) | 17.493±0.0096″ |
| 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 | |
| Alpha 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 | 5,790 K |
| Metallicity [Fe/H] | <0.20 dex |
| Rotation | 28.3±0.5 d |
| Rotational velocity (v sin i) | 2.7±0.7 km/s |
| Age | 4.85 Gyr |
| Alpha 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 | 5,260 K |
| Metallicity [Fe/H] | 0.23 dex |
| Rotation | 36.7±0.3 d |
| Rotational velocity (v sin i) | 1.1±0.8 km/s |
| Age | 5.3±0.3 Gyr |
| Other designations | |
| α Cen A: Rigil Kentaurus, Rigil Kent, α1 Centauri, HR 5459, HD 128620, GCTP 3309.00, LHS 50, SAO 252838, HIP 71683 | |
| α Cen B: Toliman, α2 Centauri, HR 5460, HD 128621, LHS 51, HIP 71681 | |
| Database references | |
| SIMBAD | AB |
| A | |
| B | |
| Exoplanet Archive | data |
| ARICNS | data |
Alpha Centauri A and B are Sun-like stars (Class G and K, respectively) that together form the binary star system Alpha 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.
Alpha Centauri A has 1.1 times the mass and 1.5 times the luminosity of the Sun, while Alpha Centauri B is smaller and cooler, at 0.9 solar mass and less than 0.5 solar luminosity. 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.
Alpha Centauri C, or 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 Alpha Centauri AB. Currently, the distance between Proxima Centauri and Alpha 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, an Earth-sized planet in the habitable zone discovered in 2016, and Proxima d, a candidate sub-Earth which orbits very closely to the star, announced in 2022. The existence of Proxima c, a mini-Neptune 1.5 AU away discovered in 2019, is controversial. Alpha Centauri A may have a Neptune-sized planet in the habitable zone, though it is not yet known with certainty to be planetary in nature and could be an artifact of the discovery mechanism. Alpha Centauri B has no known planets: planet Bb, purportedly discovered in 2012, was later disproven, and no other planet has yet been confirmed.