Alpha Centauri, a double star system with the binary designation Alpha Centauri AB, is the brightest visible object in the southern constellation Centaurus. Its component stars are Alpha Centauri A (the primary—somewhat larger and brighter than the Sun) and Alpha Centauri B (the secondary—slightly smaller and dimmer). These stars are of spectral classes G2V (as is the Sun) and K1V, respectively; in the former case there is an obvious model and potential for planets capable of supporting complex biospheres, and in the latter, as it turns out, an even stronger probability of a stable habitable zone that is well suited for life. (Proxima Centauri—a late-discovered red dwarf, and the closest known star to the Solar System) appears to be gravitationally bound to the AB system although at a considerable distance. The collection of three stars together is called Alpha Centauri AB-C.