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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