The Alpha Centauri system is said to be the closest star system to our sun. On our sky’s dome, we see this multiple system as a single star – the third-brightest star visible from Earth.
Alpha Centauri is part of a double, or triple, star system. The two main components are Alpha Centauri A and Alpha Centauri B. The third star, a red dwarf called Proxima Centauri, is thought to be about 4.22 light-years distant and is actually our sun’s closest neighbor among the stars. Is it part of the Alpha Centauri system? The actual status of Proxima as a system member is unclear. It might simply be passing nearby but not part of the system, or it might be gravitationally bound. Still, we say – and others say – that Alpha Centauri is the closest star to our solar system, with the assumption that Proxima is a true part of the Alpha Centauri system.
If you looked through a small telescope at the Alpha Centauri system, you’d see the two main stars, but you wouldn’t see Proxima Centauri. It’s too faint and appears too far (4 diameters of the full moon) away to be easily recognized at part of the system.
Taken by itself, Alpha Centauri A is the fourth-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 is the third-brightest. These stars are an average of 4.3 light-years away.
Yellow Alpha Centauri A is the same stellar type as the Sun (G2), although a bit larger. It looks bright in our sky because of its nearness to Earth. Just a few degrees away, the star Hadar (a separate star sometimes called Beta Centauri, not to be confused with Alpha Centauri B) appears dimmer in our sky than Alpha Centauri. But in fact, Hadar is much farther away at 525 light-years. So you see that Alpha Centauri is not a fabulously brilliant star, as stars go.
Alpha Centauri’s surface temperature is a few degrees Kelvin less than our 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 our sun, with a spectral type of K2. With lower temperature (about 5,300 K) and only half the luminosity of the sun, B would shine as 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 A.U., with one A.U. being one Earth-sun distance.
Faint red Proxima Centauri – at only 3,100 K and 500 times less bright than our sun – is nearly a fifth of a light year from Alpha Centauri A and B. This great distance is what calls into question its status as part of a triple star system.
How to see it
Unluckily for us in the Northern Hemisphere, Alpha Centauri is located very far to the south on the sky’s dome. Most 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.
Northern Hemisphere dwellers might glimpse Alpha Centauri at roughly 1 a.m. (local Daylight savings time) in early May. That is when the star system would be highest above the southern horizon. By early July, the system reaches its highest point to the south at nightfall.
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.
For northern observers, there really are no good pointer stars to Alpha Centauri. When the bright star Arcturus is high overhead, Alpha Centauri might low in your southern sky, assuming you are south of 29 degrees N. latitude.
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 for this star, 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, a motion of Earth called precession – which causes the identity of the Pole Star to change over time – caused Alpha Centauri to appear higher in the sky as seen from the Northern Hemisphere than it does now. 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.
Alpha Centauri’s position is RA: 14h 39m 41s, dec: -60° 50′ 07″