Alpha Centauri

triple star system in the constellation Centaurus, where one of the three stars, Proxima Centauri, being the closest star to the Earth

Alpha Centauri (also known as Rigil Kentaurus) is the brightest star in the southern Centaurus constellation.[2] It is the fourth brightest star in the night sky, with a magnitude of -0.01. It is visible in the Southern Hemisphere, and is too far south for most of the Northern Hemisphere to see.

Position of Alpha Centauri
The Apparent and True Orbits of Alpha Centauri.
The Apparent Orbit (thin ellipse) is the shape of the orbit as seen by the observer on Earth. The True Orbit is the shape of the orbit viewed perpendicular to the plane of the orbital motion. According to the radial velocity vs. time,[1] the orbit is divided here into 80 points, and each step refers to a timestep of almost a year.

Alpha Centauri is a binary star system of two stars A & B. The distance between them is quite close. To the naked eye, the stars are too close for the eye to be able to see them as separate. Their orbit is about the distance of the giant planets from our Sun.

There is a third star, Proxima Centauri (or Alpha Centauri C). This is usually considered separately, but in fact it is also gravitationally connected to the other two. It is actually slightly closer to us, with a very much larger orbit around A and B.


Relative sizes of stars

Viewed as a triple star system, Alpha Centauri is the closest to our own, being 4.2-4.4 light years (ly) away. It consists of two main stars, Alpha Centauri A and Alpha Centauri B (which form a binary star together) at a distance of 4.36 ly, and a dimmer red dwarf named Proxima Centauri at a distance of 4.22 ly. Both of the two main stars are rather similar to the Sun. The larger star, Alpha Centauri A, is the most similar to the Sun, but a little larger and brighter.

Diameter and radius


Alpha Centauri is 1,702,240 km in diameter and 851,120 km in radius.

  1. Pourbaix, D.; Nidever, D.; McCarthy, C.; Butler, R. P.; Tinney, C. G.; Marcy, G. W.; Jones, H. R. A.; Penny, A. J.; Carter, B. D.; Bouchy, F.; Pepe, F.; Hearnshaw, J. B.; Skuljan, J.; Ramm, D.; Kent, D. (2002). "Constraining the difference in convective blueshift between the components of alpha Centauri with precise radial velocities". Astronomy and Astrophysics. 386 (1): 208–85. arXiv:astro-ph/0202400. Bibcode:2002A&A...386..280P. doi:10.1051/0004-6361:20020287. S2CID 14308791.
  2. Other names are Rigil Kentaurus or Rigil Kent, Toliman and Bungula. Alpha Centauri A is also known as HD 128620, HR 5459, CP-60°5483, GCTP 3309.00A, and LHS 50. Alpha Centauri B is also known as HD 128621, HR 5460, GCTP 3309.00B, and LHS 51.