Alpha Centauri

Alpha Centauri is a star of the southern sky, and is not visible to observers at a latitude greater than about 25° north (roughly the latitude of Florida, Egypt or Taiwan). For those in the southern hemisphere, though, it is not hard to locate: it sits in the centre of the Milky Way, about fifteen degrees east of Crux. In fact, a line through the cross-piece of the Southern Cross (from the stars Imai to Mimosa) points the way to Alpha Centauri.

Alpha Centauri A is a star very similar to our own Sun, and so the presence of Earth-like planets within its system is by no means impossible, though none have yet been definitively identified. In this hypothetical illustration, orange Alpha Centauri B is visible in the background, more than eleven AU away.

Alpha Centauri - famous as the nearest visible star to the Sun - represents the foot of the Centaur, and falls near the southern and eastern borders of that constellation, on the fringes of Circinus.

While most of the sky's more famous stars are known by their proper names (Polaris, Sirius, Betelgeuse and so on), Alpha Centauri is unique among these in being better known by its Bayer designation, rather than one of the proper names for stars in the system, Rigil (or Rigel) Kentaurus or Toliman. Of these, Rigil Kentaurus means 'The Centaur's Foot', and is easy to understand as the name of a star in the southern parts of Centaurus, the constellation of the Centaur. Toliman is a little more obscure: it means 'The Vine-shoot', and refers to the ancient tradition that the Centaur held a rod entwined with vines in his hand. These traditional names are now formally assigned to different component stars within the Alpha Centauri system: the International Astronomical Union has designated Rigil Kentaurus as Alpha Centauri A, and Toliman as Alpha Centauri B.

Its magnitude of -0.01 makes this the fourth brightest star in the sky, but only by a tiny fraction: it is just 0.04 magnitudes brighter than Vega in the northern constellation of Lyra. In fact, Vega is intrinsically a much more luminous star than Alpha Centauri, but this is balanced by the fact that it is more than five times further away from Earth, so the two stars appear artificially similar in the sky. Vega is not alone in being farther away than Alpha Centauri: there are no stars closer to the Sun than those in this system.

Seen from a hypothetical planet orbiting Alpha Centauri A, the star would be recognisably similar to our own Sun. Less familiar, though, would be its orange companion in the sky.

The Alpha Centauri system seems to have formed somewhat earlier than the Sun and Solar System ('somewhat' in this context is probably of the order of a thousand million years or so). At its heart, two remarkably Sun-like stars orbit their common centre of gravity roughly once every eighty years. Like our Sun, both stars are relatively stable in nature and, again like our Sun, they are comparatively rich in more complex elements. Alpha Centauri B is an orange star, rather cooler than the Sun, and slightly less massive. By comparison, yellow Alpha Centauri A is close to a twin of our own star. Its spectral classification (G2V) is identical, though the star itself is just a little more massive, and hence a little more luminous, than the Sun.

With these two stars, particularly Alpha Centauri A, being so similar to the Sun, it's natural to wonder whether they might host a system of planets, perhaps even terrestrial planets comparable to our own Earth, as shown in the speculative illustrations on this page. No terrestrial planets have so far been confirmed in orbit around either Alpha Centauri A or B, although a candidate object possibly representing a small gas giant may potentially exist in orbit around Alpha Centauri A. (An earlier proposed planet around Alpha Centauri B has subsequently been shown not to exist.) The third member star of the system, Alpha Centauri C or Proxima, has three known planets in orbit.

Beyond the A and B stars, the third component in the system is designated Alpha Centauri C. This is a tiny, faint red dwarf orbiting the two main stars at an immense distance: roughly one fifth of a light year. This is Proxima, a star with a faint reddish glow so feeble that it would hardly be visible even from the core the Alpha Centauri system. It seems to be a late-comer to that system, perhaps a wanderer pulled into orbit by the two older stars, but it has one important distinguishing feature: it is the single closest star to our own Solar System. For that reason, though it is properly an outlier of the Alpha Centauri system, Proxima is given its own entry on this site.