What is Alpha Centauri?

Alpha Centauri is the closest star system to the Earth aside from our own. It is a trio of stars: Alpha Centauri, Beta Centauri, and Proxima Centauri. The first two are often referred to as Centauri A and Centauri B while the last is Centauri C. Alpha Centauri is a yellowish-white dwarf star with about 10% greater mass than our Sun. Beta Centauri is slightly smaller, at about 90% solar mass, and produces less heat, giving off an orangeish-yellow-white glow. Proxima Centauri is a small red dwarf, orbiting the other two at a great distance. This red dwarf is the closest star to us aside from the Sun.

The Centauri system gets its name from its location in the constellation Centaurus, the centaur. The entire system is 4.2 - 4.4 light years away from the Earth. A starship traveling at 10% the speed of light, perhaps powered by a nuclear pulse engine, would reach Alpha Centauri in only about 50 years. It is plausible that the system has its own planets, though none have been detected yet.

Alpha Centauri and Beta Centauri have an eccentric orbit around each other, coming as close as 11 AU (astronomical units, or Earth-Sun distances) and separating as far as 35 AU. This is comparable to the distance between the Sun and Saturn when closest, and between the Sun and Pluto at most distant. Both stars are about five to six billion years old, just like our Sun.

The red dwarf Proxima Centauri, the outcast, weighs only 1/10th of the Sun and accordingly burns its nuclear fuel much slower, meaning it is less bright and hot. In fact, the only reason we can observe this red dwarf at all is that it is located so close to our home system. Proxima Centauri orbits Beta and Alpha Centauri at a distance of 11,000 AU, or about 0.21 light years, which is 1/20th of the way between the Sun and Alpha Centauri. At first, we were not even certain if Proxima should be considered part of the same star system, but observations have shown that it orbits the binary center and shares approximately the same motion through space as the rest of the star system.