Alpha Centauri (α Centauri, abbreviated α Cen or Alpha Cen) is a star system located 4.37 light years away from Earth in the constellation Centaurus. It is the closest star system and the closest planetary system to Earth. The system is a triple star system which contains three stars: α Centauri A (officially "Rigil Kentaurus"), α Centauri B (officially Toliman), and α Centauri C (officially Proxima Centauri).
Alpha Centauri's A and B are both Sun-like stars of Class G and Class K, and together, they form a binary star system called Alpha Centauri AB. To the naked eye, these two components appear to be a single star with an apparent magnitude of -0.27, the brightest star in the southern constellation of Centaurus and the third-brightest in the night sky, outshone only by Sirius and Canopus.
Alpha Centauri A is very similar to the Sun, and has 1.1 times the mass and 1.519 times the luminosity of the Sun. Alpha Centauri B is smaller and cooler - having around 0.907 times the mass and 0.445 times the Sun's luminosity. The pair orbit around a common centre with an orbital period of 79.91 years. Their elliptical orbit is eccentric, so the distance between the two can vary between 35.6 AU (about the distance between Pluto and the Sun), to about 11.2 AU (about the distance between the Sun and Saturn).
Alpha Centauri C, commonly called "Proxima Centauri", is a small faint red dwarf Class M star. It is not visible to the naked eye. Proxima Centauri is the closest star to the Sun, at a distance of around 4.24 light years (1.30 parsecs), slightly closer than Alpha Centauri AB. The distance between Proxima Centauri and Alpha Centauri AB is about 13,000 AU, or 0.21 light years, about 430 times that of Neptune's orbit from the Sun.
No extrasolar planets have been directly confirmed to orbit Alpha Centauri A, though, as of 2019, a Neptune-sized named Candidate 1 has been discovered possibly orbiting it, though it is not confirmed. Alpha Centauri B has no known planets; a planet in 2012 named Bb was purpoted to exist, but was found to be an artifact. Proxima Centauri has two confirmed planets: Proxima Centauri b, an Earth-sized exoplanet in the habitable zone, and Proxima Centauri c, an uninhabitable super-Earth orbiting about 1.5 AU from the star.
Nomenclature[]
"α Centauri" (Alpha Centauri) is the system's designation, given by Johann Bayer in 1603. It bears the traditional name Rigil Kentaurus, which is a Latinisation of the Arabic name رِجْل القِنْطورُس Rijl al-Qinṭūrus, meaning 'the Foot of the Centaur'.
An alternative name found in European sources, Toliman, is an approximation of the Arabic الظَّلِيمَان aẓ-Ẓalīmān (in older transcription, aṭ-Ṭhalīmān), meaning 'the (two male) Ostriches', an appellation Zakariya al-Qazwini had applied to Lambda and Mu Sagittarii, also in the southern hemisphere.
A third name that has been applied is Bungula, of obscure origin.
Alpha Centauri C was discovered in 1915 by Robert T. A. Innes, who suggested that it should be named Proxima Centaurus, from Latin "the nearest star of Centaurus". The name "Proxima Centauri" later became more widely used, and is listed by the IAU as the approved proper name.
Stellar system[]
Alpha Centauri is a triple star system, with its two main stars, Alpha Centauri A and Alpha Centauri B, being a binary component. The AB designation, or older A×B, denotes the mass centre of a main binary system relative to companion star(s) in a multiple star system. AB-C refers to the component of Proxima Centauri in relation to its central binary, being the distance between the center of mass and the outlying companion. Because the distance between Proxima (C) and either of Alpha Centauri A or B is similar, the AB binary system is sometimes treated as a single gravitational object.
Orbital properties[]
The A and B components of Alpha Centauri have an orbital period of about 79.91 years. The orbital is moderately eccentric, and their closet approach (or periastron) is 11.2 AU (1.68 billion km), or about the distance between the Sun and Saturn; while the furthest separation (or apastron) is 35.6 AU (5.33 billion km), or about the distance between the Sun and Pluto.
The last time Alpha Centauri AB reached apastron was in May 1995; the next occurrence will be in 2075. The last time they reached periastron was in August 1955 and the next will occur in May 2035.
From Earth, the apparent orbit of A and B means that their separation and position angle (PA) are in continuous change throughout their projected orbit. Observed stellar positions in 2019 are separated by 4.92 arcsec through the PA of 337.1°, increasing to 5.49 arcsec through 345.3° in 2020. The closest recent approach was in February 2016, at 4.0 arcsec through the PA of 300°. The observed maximum separation of these stars is about 22 arcsec, while the minimum distance is 1.7 arcsec. The widest separation occurred during February 1976, and the next will be in January 2056.
Alpha Centauri C is, currently, around 13,000 AU (or 0.21 light years) away from Alpha Centauri.
Physical properties[]
The Alpha Centauri system appears to be very similar in age to the Sun; it is likely slightly older. The estimated age of the system is approximately 4.85 billion years. From the orbital elements, the total mass of Alpha Centauri AB is around 2.0 Solar mass - or twice the Sun's mass. The average individual stellar masses are 1.09 Solar mass, and 0.90 Solar mass, respectively. Alpha Centauri A and B have absolute magnitudes of +4.38 and +5.71, respectively.
Alpha Centauri A[]
Alpha Centauri A is the principal member (or primary) of the binary system. It is a Sun-like main-sequence star of spectral type G2 V. The star is about 10 percent more massive than the Sun, with a radius about 22 percent larger. When considered among the individual brightest stars in the sky (excluding the Sun), it is the fourth brightest at an apparent magnitude of -0.01, being slightly fainter than Arcturus at an apparent magnitude of -0.05.
Alpha Centauri B[]
Alpha Centauri B is the secondary member of the binary system. It is a main-sequence star of spectral type K1 V, making it have a slightly more orange color than Alpha Centauri A. It has around 90 percent the mass of the Sun and is 14 percent smaller. Although it is slightly less luminous than A, it emits more energy in the X-ray band. Its light curve varies on a short time scale, and there has been at least one observed flare. It is more magnetically active than Alpha Centauri A, showing a cycle of 8.2 years compared to 11 years for the Sun, and about half the minimum-to-peak variation in coronal luminosity of the Sun. Alpha Centauri B has an apparent magnitude of +1.35, slightly dimmer than Mimosa.
Alpha Centauri C (Proxima Centauri)[]
- Main article: Proxima Centauri
Alpha Centauri C, or Proxima Centauri, is the third member of the system. It is a small, main-sequence red dwarf of spectral class M6 Ve. It has an absolute magnitude of +15.60 which is over 20,000 times fainter than the Sun, and is not visible from Earth. Its mass is calculated to be 0.1221. It is the closest star to the Sun.
Observation[]
Location of Alpha Centauri in Centaurus
To the naked eye, Alpha Centauri AB appears as a single star, the brightest star in the southern constellation of Centaurus. Their apparent angular separation varies over about 80 years between 2 and 22 arcsec (the naked eye has a resolution of about 60 arsec), but through much of the orbit, both are easily resolved in binoculars or small telescopes. At -0.27 apparent magnitude (combined for A and B magnitudes), Alpha Centauri is fainter only than Sirius and Canopus. It is the outer star of The Pointers or The Southern Pointers, so called because the line through Beta Centauri (Hadar/Agena), some 4.5° west, points to the constellation Crux - the Southern Cross. The Pointers easily distinguish the true Southern Cross from the fainter asterism known as the False Cross.
South of about 29° S latitude, Alpha Centauri is circumpolar and never sets below the horizon. North of about 29° N latitude, Alpha Centauri never rises. Alpha Centauri lies close to the southern horizon when viewed from the 29° N latitude to the equator (close to Hermosillo, Chihuahua City in Mexico, Galveston, Texas, Ocala, Florida and Lanzarote, the Canary Islands of Spain), but only for a short time around its culmination. The star culminates each year at local midnight on 24 April and at local 9 p.m. on 8 June.
As seen from Earth, Proxima Centauri is 2.2° southwest from Alpha Centauri AB, about four times the angular diameter of the Moon. Proxima Centauri appears as a deep-red star of a typical apparent magnitude of 11.1 in a sparsely populated star field, requiring moderately sized telescopes to be seen. Listed as V645 Cen in the General Catalogue of Variable Stars Version 4.2, this UV Ceti-type flare star can unexpectedly brighten rapidly by as much as 0.6 magnitudes at visual wavelengths, then fade after only a few minutes. Some amateur and professional astronomers regularly monitor for outbursts using either optical or radio telescopes. In August 2015, the largest recorded flares of the star occurred, with the star becoming 8.3 times brighter than normal on 13 August, in the B band (blue light region).
Alpha Centauri is inside the G-cloud, and its nearest known system is the binary brown dwarf system Luhman 16 at 3.6 light years (1.1 parsecs).