How big is Proxima Centauri?

Proxima Centauri is a dM5e star (dwarf M5 emission-line star) with a luminosity of 0.00006 times the sun that was discovered in 1913 when it was at that time 0.1 light years closer than Alpha Centauri. The mass of this star is 0.1 times the Sun, and its radius is probably about 1/2 that of the Sun or so.

Today, its orbit around Alpha Centauri (distance = 4.395 light years) now puts it at a distance of 4.223 light years according to the Hipparcos Satellite.

Proxima is located about 13,000 AUs from Alpha Cantauri A and B. The star is located roughly a fifth of a light-year from the AB binary pair and, if gravitationally bound to it, may have an orbital period of around half a million years. According to Anosova et al (1994), however, its motion with respect to the AB pair is hyperbolic.

Alpha Centauri A and B orbit each other at a distance of about 2.2 billion miles (3.6 billion kilometers), a bit more than the distance from the Sun to planet Uranus. It takes 80 years for them to complete an orbit. Proxima Centauri is nearer to Earth than the other two stars, by the rather large distance -- roughly 10,000 times the distance from Earth to the Sun. All three known stars in the system were born about 4.85 billion years ago, astronomers believe. Our Sun began shining about 4.6 billion years ago. The A and B stars are both about the same temperature as the Sun. Proxima Centauri is about seven times smaller than the Sun. It contains just enough mass to cause hydrogen to burn, and it is much cooler and, intrinsically, only about 1/150th as bright as the Sun. This small star is barely a star at all, in fact. Its mass is just above that of brown dwarfs, a class of object that seems to straddle the definition between stars and planets. Though 150 times more massive than Jupiter, Proxima Centauri is only about 1.5 times bigger than the planet.