How to see it

The Alpha Centauri system is the closest star system to our sun. On our sky’s dome, we see this mutliple system as a single star – the fourth brightest star visible from Earth.

Unluckily for us in the northern hemisphere, Alpha Centauri is mainly visible to those in Earth’s southern hemisphere. The large majority of North Americans never see it. The cut-off latitude is about 29 degrees north, and anyone north of that is out of luck. In the U.S. the line passes near Houston and Orlando, but even from the Florida Keys, the star never rises more than a few degrees above the southern horizon. Things are a little better in Hawaii and Puerto Rico, where it can get 10 or 11 degrees high.

Meanwhile, in Australia and much of the southern hemisphere, Alpha Centauri is circumpolar, meaning that it never sets. Also called Rigel Kentaurus or Rigel Kent, this neighboring star is probably the most famous star that almost no one in the northern hemisphere has ever seen.

Best viewing, low to the southern horizon, comes at roughly 1 a.m. (local Daylight savings time) in early May. By early July it culminates (reaches its highest point) to the south at nightfall. It also can be seen briefly in the predawn spring skies. For northern observers, there really are no good pointer stars. When the bright star Arcturus is high overhead, look for Alpha Centauri low in your southern sky. Observers in the tropical and subtropical regions of the northern hemisphere can find Alpha Centauri by first identifying the distinctive Southern Cross. A short line drawn through the crossbar (Delta and Beta Crucis) eastward first comes to Hadar (Beta Centauri), then Alpha Centauri.

History and Myth

Alpha Centauri is the brightest star in the southern constellation Centaurus, the Centaur. Two alternative names, Toliman and Bungula, are rarely used any more. The derivations are somewhat questionable, but Toliman may be from the Arabic for “ostriches” and Bungula apparently derives from Latin meaning “hoof”.

Thousands of years ago, Earth’s precession caused Alpha Centauri to appear higher in the sky, but it was still far to the south and often difficult to see. Classical myth-makers didn’t spend much time with this constellation, although it was thought to represent an uncharacteristically wise centaur that figured in the mythology of Heracles and Jason. The centaur was accidentally wounded by Heracles, and placed into the sky after death by Zeus.

Alpha Centauri itself marked the right front hoof of the Centaur, although little is known of its mythological significance, if any. Ancient Egyptians revered it, and may have built temples aligned to its rising point. In southern China it was part of a star group known as the South Gate.

Science

Alpha Centauri is part of a double (maybe triple) star system. Taken by itself, Alpha Centauri A is the 4th brightest star seen from Earth, just slightly outshone by Arcturus. However, the combined light of Alpha Centauri A and B is slightly greater than Arcturus, so in that sense it could be considered the third brightest. They are an average of 4.37 light-years away. A third star, a red dwarf called Proxima Centauri, is thought to be about 4.22 light-years distant, but is considerably smaller and fainter than the Sun. The double nature of Alpha Centauri is clearly revealed in a small telescope, although Proxima Centauri is too faint and appears too far (4 diameters of the full moon) away to be easily recognized at part of the system. The actual status of Proxima as a system member is unclear. It may simple be passing nearby but not part of the system.

Yellow Alpha Centauri A is the same stellar type as the Sun (G2), although a bit larger. At visual magnitude it is -0.01, the fourth brightest star in the sky. Alpha Centauri’s apparent brightness is due to its nearness to Earth. Just a few degrees away, Hadar appears dimmer than Alpha Centauri, but in fact is much farther away (525 light years). If it were placed at Alpha Centauri’s distance, it would outshine it by 14,000 times, and would appear from earth about as bright as a first quarter moon.

Alpha Centauri’s surface temperature is a few degrees Kelvin less than the sun (that is, about 5770 k), but its greater diameter (about 25% more than the sun) and the overall larger surface area gives it a luminosity nearly 1.6 times that of our star. The smaller member of the system, orangish Alpha Centauri B, is slightly smaller than the sun, and has spectral type of K2. With lower temperature (about 5300 K) and only half the luminosity of the sun, B shines at magnitude 1.35, making it the 21st brightest star in all the heavens by itself.

These two brighter components of the system orbit a common center of gravity once every 80 years. The orbit is notably elliptical, with average distance between the two stars of about 11 AU, or a little more than the distance between the sun and Saturn. Faint red Proxima Centauri, at only 3100 K and 500 times less bright than the sun, is nearly a fifth of a light year from the other two stars. This great distance is what calls into question its status as part of a triple star system.

Alpha Centauri’s position is RA: 14h 39m 41s, dec: -60° 50′ 07″