Alpha Centauri

Alpha Centauri (α Centauri, α Cen; also known as Rigel Kent /ˈraɪdʒəl ˈkɛnt/—see Names) is the brightest star in the southern constellation of Centaurus, it is currently inside the G-cloud. Although it appears to the unaided eye as a single object, Alpha Centauri is actually a binary star system (designated Alpha Centauri AB or α Cen AB) whose combined visual magnitude of −0.27 makes it the third-brightest star in the night sky after the −1.46 magnitude Sirius and the −0.72 magnitude Canopus.

Its individual component stars are named Alpha Centauri A (α Cen A), with 110% of the mass and 151.9% the luminosity of the Sun, and Alpha Centauri B (α Cen B), at 90.7% of the Sun's mass and 44.5% of its luminosity. During the pair's 79.91-year orbit about a common center, the distance between them varies from about that between Pluto and the Sun to that between Saturn and the Sun. They are 1.34 parsecs or 4.37 light years from the Sun.

A third star, known as Proxima Centauri, Proxima or Alpha Centauri C (α Cen C), is probably gravitationally associated with Alpha Centauri AB. Proxima is at the slightly smaller distance of 1.29 parsecs or 4.21 light years from the Sun, making it the closest star to the Sun, even though it is not visible to the naked eye. The separation of Proxima from Alpha Centauri AB is about 0.06 parsecs, 0.2 light years or 13,000 astronomical units (AU); equivalent to 400 times the size of Neptune's orbit.

The system also contains at least one planet, the Earth-sized Alpha Centauri Bb, which is the closest known exoplanet to Earth. It has a mass at least 113% of Earth's and orbits Alpha Centauri B with a period of 3.236 days. Orbiting at a distance of 6 million kilometers from the star, 4% of the distance of the Earth to the Sun and ten times closer than the orbit of Mercury, the planet has an estimated surface temperature of 1500 K (roughly 1200 °C), too hot to be habitable.