Alpha Centauri - Binary System

Binary System

With the orbital period of 79.91 years, the A and B components of this binary star can approach each other to 11.2 astronomical units, equivalent to 1.67 billion km or about the mean distance between the Sun and Saturn, or may recede as far as 35.6 AU (5.3 billion km—approximately the distance from the Sun to Pluto). This is a consequence of the binary's moderate orbital eccentricity e = 0.5179. From the orbital elements, the total mass of both stars is about 2.0 M_☉—or twice that of the Sun. The average individual stellar masses are 1.09 M_☉ and 0.90 M_☉, respectively, though slightly higher masses have been quoted in recent years, such as 1.14 M_☉ and 0.92 M_☉, or totalling 2.06 M_☉. Alpha Centauri A and B have absolute magnitudes of +4.38 and +5.71, respectively. Stellar evolution theory implies both stars are slightly older than the Sun at 5 to 6 billion years, as derived by both mass and their spectral characteristics.

Viewed from Earth, the apparent orbit of this binary star means that the separation and position angle (P.A.) are in continuous change throughout the projected orbit. Observed stellar positions in 2010 are separated by 6.74 arcsec through the P.A. of 245.7°, reducing to 6.04 arcsec through 251.8° in 2011. Next closest approach will be in February 2016, at 4.0 arcsec through 300°. Observed maximum separation of these stars is about 22 arcsec, while the minimum distance is 1.7 arcsec. Widest separation occurred during February 1976 and the next will be in January 2056.

In the true orbit, closest approach or periastron was in August 1955, and next in May 2035. Furthest orbital separation at apastron last occurred in May 1995 and the next will be in 2075. The apparent distance between the two stars is presently rapidly decreasing, at least until 2019.