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Alpha Centauri (α Cen), also known as Rigil Kent () or Toliman, is the closest star system to the Solar System at . It consists of three stars, the pair Alpha Centauri A and Alpha Centauri B and a small and faint red dwarf, Alpha Centauri C—better known as Proxima Centauri—that is probably (but not certainly) gravitationally bound to the other two. To the unaided eye, the two main components appear as a single object of an apparent visual magnitude of −0.27, forming the brightest star in the southern constellation Centaurus and the third-brightest star in the night sky, only outshone by Sirius and Canopus. Alpha Centauri A (α Cen A) has 110% of the mass and 151.9% the luminosity of the Sun, and Alpha Centauri B (α Cen B) is smaller and cooler, at 90.7% of the Sun's mass and 44.5% of its luminosity. During the pair's 79.91-year orbit about a common center, 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), equivalent to 500 times the size of Neptune's orbit. == Nature and components == Alpha Centauri is the name given to what appears as a single star to the naked eye and the brightest star in the southern constellation of Centaurus. At −0.27 apparent visual magnitude (calculated from A and B magnitudes), it is fainter only than Sirius and Canopus. The next-brightest star in the night sky is Arcturus. Alpha Centauri is a multiple-star system, with its two main stars being Alpha Centauri A and Alpha Centauri B , usually defined to identify them as the different components of the binary . A third companion—Proxima Centauri (or Proxima or )—has a distance much greater than the observed separation between stars A and B and is probably gravitationally associated with the AB system. As viewed from Earth, it is located at an angular separation of 2.2° from the two main stars. If it were bright enough to be seen without a telescope, Proxima Centauri would appear to the naked eye as a star separate from . Alpha Centauri AB and Proxima Centauri form a ''visual'' double star. Direct evidence that Proxima Centauri has an elliptical orbit typical of binary stars has yet to be found. Together all three components make a triple star system, referred to by double-star observers as the triple star (or multiple star), . 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, 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 of this star is , resulting in an estimated rotational period of 22 days, 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.〔 Alpha Centauri B is about 90% the mass of the Sun and 14% smaller in radius. The projected rotational velocity is , resulting in an estimated rotational period of 41 days. (An earlier, 1995 estimate gave a similar rotation period of 36.8 days.) 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. 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 (), or 129 Jupiter masses. Together, the bright visible components of the binary star system are called Alpha Centauri AB . This "AB" designation denotes the apparent gravitational centre of the main binary system relative to other companion star(s) in any multiple star system. "AB-C" refers to the orbit of Proxima around the central binary, being the distance between the centre of gravity and the outlying companion. Some older references use the confusing and now discontinued designation of A×B. Because the distance between the Sun and Alpha Centauri AB does not differ significantly from either 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 α Cen system being similar in age to, or slightly older than, the Sun, with typical ages quoted between 4.5 and 7 billion years (Gyr). Asteroseismic analyses that incorporate the tight observational constraints on the stellar parameters for α 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 ± 0.3 Gyr. Age estimates for stars A and B based on chromospheric activity (Calcium H & K emission) yield 4.4–6.5 Gyr, whereas gyrochronology yields 5.0 ± 0.3 Gyr.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Alpha Centauri」の詳細全文を読む スポンサード リンク
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