page_title: Alpha Centauri
section_title: Orbital properties
hierarchical_section_title: Alpha Centauri / Stellar system / Orbital properties
caption_attribution_description: English: Trajectory of Alpha Centauri B relative to A (fixed to the coordinate origin) as seen from the Earth (inclined ellipse) and face-on (horizontal ellipse). The orbit parameters are taken from Pourbaix et al. (2002). The graph has been created with gnuplot based on data generated by solving Kepler's equation. Note 1: The time stamps refer to the motion as seen from Earth, i.e. delayed with respect to the true motion by the travel of light by about 4.4 years. North is down, as usual in many astronomical charts. Note 2: For simplicity, the points refer to steps of 1/80 of an orbit rather than to exact 1-year steps (which would be 1/79.91 of an orbit). Note 3: The ascending node is the point where B (with respect to A) penetrates the celestial plane and becomes more distant to Earth than A.
context_page_description: Alpha Centauri is the closest star system and closest planetary system to Earth's Solar System at 4.37 light-years from the Sun. It is a triple star system, consisting of three stars: α Centauri A, α Centauri B, and α Centauri C.
  Alpha Centauri A and B are Sun-like stars, and together they form the binary star Alpha Centauri AB. To the naked eye, the two main components appear to be a single star with an apparent magnitude of −0.27, the brightest star in the southern constellation of Centaurus and the third-brightest in the night sky, outshone only by Sirius and Canopus.
  Alpha Centauri A has 1.1 times the mass and 1.519 times the luminosity of the Sun, while Alpha Centauri B is smaller and cooler, at 0.907 times the Sun's mass and 0.445 times its luminosity. The pair orbit around a common centre with an orbital period of 79.91 years. Their elliptical orbit is eccentric, so that the distance between A and B varies from 35.6 AU, or about the distance between Pluto and the Sun, to 11.2 AU, or about the distance between Saturn and the Sun.
  Alpha Centauri C, or Proxima Centauri, is a small and faint red dwarf.
context_section_description: The A and B components of Alpha Centauri have an orbital period of 79.91 years. Their orbit is moderately eccentric, e = 0.5179; their closest approach or periastron is 11.2 AU (1.68 billion km), or about the distance between the Sun and Saturn; and their furthest separation or apastron is 35.6 AU (5.33 billion km), about the distance between the Sun and Pluto. The most recent periastron was in August 1955 and the next will occur in May 2035; the most recent apastron was in May 1995 and will next occur in 2075.
  Viewed from Earth, the apparent orbit of A and B means that their separation and position angle (PA) are in continuous change throughout their projected orbit. Observed stellar positions in 2019 are separated by 4.92 arcsec through the PA of 337.1°, increasing to 5.49 arcsec through 345.3° in 2020. The closest recent approach was in February 2016, at 4.0 arcsec through the PA of 300°. The observed maximum separation of these stars is about 22 arcsec, while the minimum distance is 1.7 arcsec. The widest separation occurred during February 1976, and the next will be in January 2056.
  Alpha Centauri C is about 13,000 AU away from Alpha Centauri AB. This is equivalent to 0.21 ly or 1.9 trillion km—about 5% the distance between Alpha Centauri AB and the Sun. Until 2017, measurements of its small speed and its trajectory were of too little accuracy and duration in years to determine whether it is bound to Alpha Centauri AB or unrelated.
  Radial velocity measurements made in 2017 were precise enough to show that Proxima Centauri and Alpha Centauri AB are gravitationally bound. The orbital period of Proxima Centauri is approximately 547000+6600
  −4000 years, with an eccentricity of 0.50 ± 0.08, much more eccentric than Mercury's. Proxima Centauri comes within 4300+1100
  −900 AU of AB at periastron, and its apastron occurs at 13000+300
  −100 AU.
caption: Apparent and true orbits of Alpha Centauri. The A component is held stationary, and the relative orbital motion of the B component is shown. The apparent orbit (thin ellipse) is the shape of the orbit as seen by an 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,[14] the radial separation of A and B along the line of sight had reached a maximum in 2007, with B being further from Earth than A. The orbit is divided here into 80 points: each step refers to a timestep of approx. 0.99888 years or 364.84 days.
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