Proxima Centauri (from Latin, meaning 'nearest [star] of Centaurus'[1]), or Alpha Centauri C, is a red dwarf, a small low-mass star, about 4.25 light-years (1.30 pc)[9] from the Sun in the constellation of Centaurus.[17][18] It was discovered in 1915 by the Scottish astronomer Robert Innes, the Director of the Union Observatory in South Africa, and is the nearest-known star to the Sun.[15] With an apparent magnitude of 11.05, it is too faint to be seen with the naked eye. Proxima Centauri forms a third component of the Alpha Centauri trinary star system, currently with a separation of about 12,950 AU (1.94 trillion km) and an orbital period of 550,000 years.[8] At present Proxima is 2.18° to the southwest of Alpha Centauri.[19][nb 2]

Because of Proxima Centauri's proximity to Earth, its angular diameter can be measured directly. The star is about one-seventh the actual diameter of the Sun.[15] It has a mass about an eighth of the Sun's mass (M☉), and its average density is about 33 times that of the Sun.[nb 3] Although it has a very low average luminosity, Proxima is a flare star that undergoes random dramatic increases in brightness because of magnetic activity.[20] The star's magnetic field is created by convection throughout the stellar body, and the resulting flare activity generates a total X-ray emission similar to that produced by the Sun.[21] The mixing of the fuel at Proxima Centauri's core through convection and its relatively low energy-production rate mean that it will be a main-sequence star for another four trillion years,[22] or nearly 300 times the current age of the universe.[23]

In 2016, the European Southern Observatory announced the discovery of Proxima b,[24][25][26] a planet orbiting the star at a distance of roughly 0.05 AU (7.5 million km) with an orbital period of approximately 11.2 Earth days. Its estimated mass is at least 1.3 times that of the Earth. The equilibrium temperature of Proxima b is estimated to be within the range of where water could exist as liquid on its surface, thus placing it within the habitable zone of Proxima Centauri,[24][27][28] although because Proxima Centauri is a red dwarf and a flare star, whether it could support life is disputed.[29][30] Previous searches for orbiting companions had ruled out the presence of brown dwarfs and supermassive planets.[31][32]