Chandra and XMM-Newton observations of the red dwarf star Proxima Centauri have shown that its surface is in a state of turmoil. Flares, or explosive outbursts, occur almost continually. In the cores of low mass stars, nuclear fusion reactions that convert hydrogen to helium proceed very slowly, and create a turbulent, convective motion throughout their interiors. This motion stores up magnetic energy which is often released explosively in the star’s upper atmosphere where it produces flares in X-rays and other forms of light.
The same process produces X-rays on the Sun, but the magnetic energy is released in a less explosive manner through heating loops of gas, with occasional flares. The difference is due to the size of the convection zone, which in a more massive star such as the Sun, is smaller and closer to its surface.
From its spectrum, Proxima Centauri is classified as a “late M-dwarf star”. Such stars are among the smallest and faintest, but also the most numerous in our Milky Way galaxy. With a mass of only 15% of that of the Sun, Proxima Centauri is in the extreme low-mass end of the M-stars. In fact, if it had only half of its present mass, it would be too light for hydrogen fusion to ignite in its interior. It would then have been a “Brown-Dwarf” rather than a real star.
In the case of Proxima Centauri, both the mass and the diameter are about 1/7 of those of the Sun. Contrarily, while it is 150 times more massive than Jupiter, it is only about 1.5 times larger than that planet.
The measured angular diameter of Proxima Centauri is 1.02 ± 0.08 milliarcsec, or about the size of an astronaut on the surface of the Moon as seen from the Earth (or a head of a pin on the surface of the Earth, as seen from the International Space Station).
It is the nearest known star to the solar system and a member of a triple stellar system that includes the bright double star Alpha Centauri. Proxima is the nearest of the three. It was discovered in 1915 by a Scottish astronomer, Robert T. A. Innes (1861-1933), during a stay at the Cape Observatory (South Africa), due to its very fast motion on the sky, about 3.9 arcsec/year. It is also designated GJ 551. The visual magnitude is 11, or 100 times fainter than what can be perceived with the unaided eye on a dark sky; the parallax measured by the ESA Hipparcos astrometric satellite is 772.33 ± 2.42 milliarcsec, corresponding to a distance of 4.22 light-years.
