Alpha Centauri (Star)

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Alpha Centauri A
Spectral G2 V
Type Main Sequence
Multiple Trinary
Luminosity 1.519 LSol LSol
Lum Rank 21st
Absolute magnitude +4.38
Mag Terra
0.01
Temperature 5,790K° K
Mass 1.100 MSol MSol
Companion Alpha Centauri B & Alpha Centauri C


Alpha Centauri B
Spectral K1 V
Type Main Sequence
Luminosity 0.50 LSol LSol
Absolute magnitude +5.71
Mag Terra
1.33
Temperature K° K
Mass MSol MSol


Alpha Centauri C (Proxima Centauri)
Spectral M5 Ve
Type Main Sequence
Luminosity LSol LSol
Absolute magnitude
Temperature K° K
Mass MSol MSol


Alpha Centauri (also called Rigil Kentaurus or Toliman) is a multiple star system consisting of a main binary system (Alpha Centauri A and Alpha Centauri B) as well as a third red dwarf companion, Alpha Centauri C or "Proxima Centauri" lying at about 5000AU (~0.1ly) distant from the main pair.

Description (Specifications)

During the pair's 79.91-year orbit about a common center,[14] the distance between them varies from about that between Pluto and the Sun to that between Saturn and the Sun. Proxima is at the slightly smaller distance of 1.29 parsecs or 4.24 light years from the Sun, making it the closest star to the Sun, even though it is not visible to the naked eye. The separation of Proxima from Alpha Centauri AB is about 0.06 parsecs, 0.2 light years or 15,000 astronomical units (AU),[15] equivalent to 500 times the size of Neptune's orbit.

Alpha Centauri A is the principal member, or primary, of the binary system, being slightly larger and more luminous than the Sun. It is a solar-like main-sequence star with a similar yellowish color,[17] whose stellar classification is spectral type G2 V. From the determined mutual orbital parameters, Alpha Centauri A is about 10% more massive than the Sun, with a radius about 23% larger. The projected rotational velocity ( v·sin i ) of this star is 2.7 ± 0.7 km·s−1, resulting in an estimated rotational period of 22 days,[18] which gives it a slightly faster rotational period than the Sun's 25 days. When considered among the individual brightest stars in the sky (excluding the Sun), Alpha Centauri A is the fourth brightest at an apparent visual magnitude of +0.01, being fractionally fainter than Arcturus at an apparent visual magnitude of −0.04.

Alpha Centauri B is the companion star, or secondary, of the binary system, and is slightly smaller and less luminous than the Sun. It is a main-sequence star of spectral type K1 V, making it more an orange color than the primary star.[17] Alpha Centauri B is about 90% the mass of the Sun and 14% smaller in radius. The projected rotational velocity ( v·sin i ) is 1.1 ± 0.8 km·s−1, resulting in an estimated rotational period of 41 days. (An earlier, 1995 estimate gave a similar rotation period of 36.8 days.)[19] Although it has a lower luminosity than component A, star B emits more energy in the X-ray band. The light curve of B varies on a short time scale and there has been at least one observed flare.[20] Alpha Centauri B at an apparent visual magnitude of 1.33 would be twenty-first in brightness if it could be seen independently of Alpha Centauri A.

Alpha Centauri C, also known as Proxima Centauri, is of spectral class M6 Ve, a small main-sequence star (Type V) with emission lines. Its B−V color index is +1.82 and its mass is about 0.123 solar masses (M☉), or 129 Jupiter masses.

With the orbital period of 79.91 years,[14] the A and B components of this binary star can approach each other to 11.2 astronomical units, equivalent to 1.67 billion km or about the mean distance between the Sun and Saturn, or may recede as far as 35.6 AU (5.3 billion km—approximately the distance from the Sun to Pluto).[14][51] This is a consequence of the binary's moderate orbital eccentricity e = 0.5179.[14] From the orbital elements, the total mass of both stars is about 2.0 M☉[52]—or twice that of the Sun.[51] The average individual stellar masses are 1.09 M☉ and 0.90 M☉, respectively,[53] though slightly higher masses have been quoted in recent years, such as 1.14 M☉ and 0.92 M☉,[54] or totalling 2.06 M☉. Alpha Centauri A and B have absolute magnitudes of +4.38 and +5.71, respectively. Stellar evolution theory implies both stars are slightly older than the Sun at 5 to 6 billion years, as derived by both mass and their spectral characteristics.[34][53]

Viewed from Earth, the apparent orbit of this binary star means that its separation and position angle (P.A.) are in continuous change throughout its projected orbit. Observed stellar positions in 2010 are separated by 6.74 arcsec through the P.A. of 245.7°, reducing to 6.04 arcsec through 251.8° in 2011.[14] The closest approach in the future will be in February 2016, at 4.0 arcsec through 300°.[14][55] The observed maximum separation of these stars is about 22 arcsec, while the minimum distance is 1.7 arcsec.[56] The widest separation occurred during February 1976 and the next will be in January 2056.[14]

In the true orbit, closest approach or periastron was in August 1955, and next in May 2035. Furthest orbital separation at apastron last occurred in May 1995 and the next will be in 2075. The apparent distance between the two stars is rapidly decreasing, at least until 2019.[14] Proxima Centauri Main article: Proxima Centauri

The much fainter red dwarf Proxima Centauri, or simply Proxima, is about 15,000 AU away from Alpha Centauri AB.[21][34][42] This is equivalent to 0.24 light years or 2.2 trillion kilometres—about 5% the distance between Alpha Centauri AB and the Sun. Proxima is likely gravitationally bound to Alpha Centauri AB, orbiting it with a period between 100,000 and 500,000 years.[34] However, it is also possible that Proxima is not gravitationally bound and thus moving along a hyperbolic trajectory[57] with respect to Alpha Centauri AB.[21]:72 The main evidence for a bound orbit is that Proxima's association with Alpha Centauri AB is unlikely to be coincidental, because they share approximately the same motion through space.[34] Theoretically, Proxima could leave the system after several million years.[58] It is not yet certain whether Proxima and Alpha Centauri are truly gravitationally bound.[59]

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

  • The Alpha Centauri star system is approximately 1.35pc from Terra, and is located in a Coretrailing/Trailing direction from Terra at bearing 315.7o Galactic Longitude and -00.7o South Galactic Latitude.

History & Background (Dossier)

Alpha Centauri A is the primary star of the world Prometheus/Sol (Solomani Rim 2027), one of Terra's earliest colonies.

References & Contributors (Sources)

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  • External Link: SIMBAD @ University of Strasbourg: [2]
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