Alpha Centauri

Template:Infobox Double Star/Maintenance/Single Coordinates

Alpha Centauri [ˈalfa t͡sɛnˈtaʊ̯ʀi] (α Centauri, abbreviated α Cen, but also called Rigil Kentaurus, Rigilkent, Toliman or Bungula) is a binary star system in the constellation of Centaur in the southern sky, about 4.34 light-years away. It forms a hierarchical triple star system together with the orbiting red dwarf Proxima Centauri (about 4.2465 Lj from the Sun), which is 0.22 Lj away from Alpha Centauri. Alpha Centauri consists of the brighter yellow star Alpha Centauri A and the orange Alpha Centauri B currently 6″ apart. Along with the Sun, it is located in what is known as the Local Flake. Just 4.4° to the west is another 1st magnitude star, Beta Centauri.

As a telescopic double star (separable only in a telescope) Alpha Centauri is the brightest object in the constellation and the third brightest star in the night sky with an apparent total brightness of -0.27 mag. The brighter Alpha Centauri A alone has an apparent magnitude of -0.01 mag, making it the fourth brightest star in the sky.

Location in the starry sky

Alpha Centauri and Beta Centauri, 4.4° distant, and the three brightest stars from the Southern Cross constellation, which lies west of Centaur, together form the clearest cluster of 1st magnitude stars within a hand span in the entire night sky.

The line through Alpha and Beta Centauri points to the Southern Cross constellation. The "pointers" were so named to distinguish in a simple way between the Southern Cross and the eastern asterism (collection of stars mistaken for a constellation) often confused with it, the "False Cross" (the constellation Sail of the Ship or Vela). The "False Cross" includes the stars visible to the naked eye ε Car, Turais, κ Vel and δ Vel.

Alpha and Beta Centauri are too far south to be seen from mid-northern latitudes (e.g. Europe). From 33° south latitude, the two stars are circumpolar and thus always remain above the horizon.

Alpha Centauri is the brightest star in the constellation Centaurus (upper left). In the treetop lies the constellation Crux.

Physical properties

Alpha Centauri A and B, as a pair of stars formed together, are about 6.5 ± 0.3 billion years old. Both are ordinary main-sequence stars and are thus in a stable phase of hydrogen burning (fusion of hydrogen to helium). Because Alpha Centauri A is more massive than Alpha Centauri B, it remains in the main sequence for a shorter time before becoming a red giant. This means that Alpha Centauri A, in contrast to the smaller and therefore longer-lived Alpha Centauri B, is already more than halfway through its life. Proxima Centauri, on the other hand, is only about 4.85 billion years old.

Over Alpha Centauri A and B, which together are often called α Cen AB, detailed observations of the surface oscillations are available, from which asteroseismology can draw conclusions about the internal structure of the stars. Combining this with traditional observational methods yields more precise values about the stars' properties than would be possible using the individual methods.

Comparison of the element distribution in mass percent
Name	Hydrogen	Helium	heavy elements
α Centauri A	71,5	25,8	2,74
α Centauri B	69,4	27,7	2,89
Sun	73,3	24,5	1,81

Alpha Centauri A

Like the Sun, Alpha Centauri A is a yellow dwarf of spectral type G2 V. Thus, like the Sun, it belongs to the hotter G stars (within spectral class G the numerical designation ranges from 0 (hottest) to 9 (coolest) star). Luminosity class V indicates that it is one of the main sequence stars. With an apparent magnitude of 0.00 it is the fourth brightest star in the night sky after Sirius (-1.46 mag), Canopus (-0.72 mag) and Arcturus (-0.05 mag) before Vega (0.03 mag).

Since Alpha Centauri A is of the same spectral type and has similar dimensions to the Sun, it is considered the closest "solar twin" to Earth (but this does not mean that they formed together). Its surface temperature is about 5800 K. With 1.22 times the diameter of the Sun, it is larger than Alpha Centauri B. It has 1.1 solar masses and emits 1.52 times as much radiation as the Sun. Its chemical composition is very similar to that of the Sun. However, the proportion of heavy elements (elements with an atomic number greater than helium are called metals in astrophysics) is almost 70 % higher (the metallicity is [Fe/H]_A = 0.22 ± 0.05). Its habitable zone lies between 1.2 and 1.3 astronomical units (AU).

Alpha Centauri B

Alpha Centauri B belongs to spectral type K1 with luminosity class V. Compared to the brighter star Alpha Centauri A it has a brightness of only 1.33 mag and is therefore number 21 in the list of the brightest stars in the sky. It has 0.93 solar masses and has a diameter 0.86 times that of the Sun. Its composition is also similar to that of the Sun. However, its heavy element content is a good 70% higher (metallicity is [Fe/H]_B = 0.24 ± 0.05). A rotation period of 41 days was found. For comparison: The sun rotates once around its own axis in about 25 days.

With a surface temperature of about 5300 K, it is only slightly cooler than the sun. However, because of the lower temperature and the smaller surface it reaches only 50 % of the solar radiation power. Thus the brightness of the orange-yellow radiating K1-V star Alpha Centauri B is only one third of the larger star Alpha Centauri A. The habitable zone lies at a distance of 0.73 to 0.74 AU.

Although less bright than Alpha Centauri A, Alpha Centauri B emits more energy in the X-ray region of the spectrum. The light curve of B varies in short time intervals and at least one flare has been observed.

Comparison of important star parameters
Name	Diameter [million km]	Radius [RSonne]	Mass [MSun]	Luminosity [LSun]	Spectral class
α Centauri A	1,70	1,22	1,1	1,52	G2 V
α Centauri B	1,20	0,86	0,93	0,50	K1 V
Sun	1,39	1	1	1	G2 V