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 1915 by the Scottish astronomer Robert Innes, the Director of the Union Observatory in South Africa, when it was at that time 0.1 light years closer to our sun than Alpha Centauri. Because of Proxima Centauri's proximity to Earth, its angular diameter can be measured directly. It is about one-seventh the diameter of the Sun

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.

On August 24, 2016, the European Southern Observatory announced the discovery of Proxima b, a planet orbiting the star at a distance of roughly 0.05 AU (7,500,000 km) and 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 where water could exist as liquid on its surface, thus placing it within the habitable zone of Proxima Centauri. Previous searches for orbiting companions had ruled out the presence of brown dwarfs and supermassive planets orbiting Proxima.

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