The Astrogators' Guide to

Alpha Centauri

Back to Contents

Also known as Rigel Kentaurus, the right front hoof of the Centaur, the obvious destination of Humanity's first interstellar expedition, Alpha Centauri floats closer to Sol than does any other star in the galaxy. Less commonly known as Toliman.

Location in Space

Radial Distance:

    Parallax = 0.75060.0046 arc-seconds, which leads to;

       1.  4.3452 lightyears 0.0266 lightyear (1.332 0.0081545 parsecs).

       2.  274,800 AU 1685 AU.

Equatorial Coordinates:

        Right Ascension; 14 hr, 39 min, 36 sec + 408t sec.

        Declination; -60 deg, 50 min - 1500t arc-sec.

            [t measured in centuries, Jan 2000 is t = 0]

Imagine standing on the north side of a plane in which Earth's equator lies with the north celestial pole directly overhead and look toward the First Point of Aries (now in Pisces just southeast of the Circlet).  Slide your gaze eastward (to the left) by a tenth of a degree less than 220 degrees along the celestial equator, toward the east end of Virgo, and tilt your gaze south (down) by about 60-5/6 degrees.

Ecliptic Coordinates:

        Ecliptic Latitude; -42.587 deg - 108.6t arc-sec.

        Ecliptic Longitude; 239.488 deg - 354.7t arc-sec.

            [t measured in centuries, Jan 2000 is t = 0]

Look toward the First Point of Aries (the point on the sky that the sun occupies on the first day of spring), shift your gaze a little less than two hundred and forty degrees eastward along the Ecliptic (the line that the sun traces through the Zodiac in the course of a year), and then shift your gaze southward forty-two and a half degrees along a line perpendicular to the Ecliptic.

Galactic Coordinates:

        Galactic Latitude; +0.7333 deg - 29.1479t arc-sec.

        Galactic Longitude; +315.8166 deg - 369.815t arc-sec.

            [t measured in centuries, Jan 2000 is t = 0]

Look  toward the radio source Sagittarius-A by casting your gaze across the Orion-Sagittarius Gap toward the Sagittarius-Carina Arm of the galaxy and at a point about six degrees south of the Ecliptic on the west (right) side of Sagittarius and two degrees south of X Sagittarii (the point of the arrow in the Archer's bow).  Move your gaze in an easterly direction along the plane of the Milky Way by almost 315-5/6 degrees.  Then tilt your gaze by almost 3/4 of a degree in a northerly direction, more or less upward, at right angles to the plane of the Milky Way.

Annual Proper Motion

in Right Ascension = -3.67819 arc-sec/yr (4.9 AU/yr = 23.22948 km/sec).

in Declination = +0.48184 arc-sec/yr (0.64192 AU/yr = 3.043 km/sec).

    Total Proper Motion = 3.709616 arc-sec/yr (4.942 AU/yr = 23.42795 km/sec) in a direction 277.46 degrees counterclockwise from due celestial north, 252.98 degrees counterclockwise from due Ecliptic north, and 265.49 degrees counterclockwise from due galactic north.

in Radial Distance = -21.6 km/sec (4.5565 AU/yr).

        Total motion = 6.72199 AU/year = 31.8654 km/sec.


From the present; in 30,262 years Alpha Centauri will become an eclipsing binary (for about 1-1/2 centuries) as its orbital plane passes over the Sol-Alpha Centauri line and in 27,703 years Alpha Centauri will reach its perihelion 3.195 lightyears (202,000 AU) from Sol in the NW corner of Centaurus after crossing 42.67 degrees of sky.

Orientation in Space

Orbit size: 23.7 AU semi-major axis (e=0.519); 11.4 - 36 AU.

Inclination; the angle between the plane of the stars' orbits and the plane of the sky.

i=79.24 deg (0.10 deg)t.

Position angle of the secondary's ascending node; the angle between the Ecliptic north vector and the line of nodes (the line where the plane of the orbit crosses the plane of the sky), measured counterclockwise toward the nodal point between 0 degrees and 180 degrees (the nodal point at which the secondary moves slightly toward the viewer).

Ω =204.87 deg - (0.55 deg)t.

Longitude of Periastron; the angle between the line of nodes and the orbit's major axis (line of apsides), measured in the prograde direction (the direction of the secondary's motion) in the plane of the true orbit, from the secondary's ascending node to the secondary's periastron

ω =231.56 deg - (0.02 deg)t.

    On a piece of stiff paper draw an ellipse of eccentricity e=0.516 and draw an arrow indicating the direction of the star's motion on the orbit that the ellipse represents. Look toward Alpha Centauri and so hold the paper that the line of apsides coincides with your line of sight and the north vector of the orbit (defined by the right-hand rule: when your right thumb, extended in a thumbs-up gesture, points north, the fingers of that hand curl in the same way that the body moves on its orbit) points Ecliptic north. Rotate the paper counterclockwise about the line of apsides by 114.87 degrees. Turn the paper 51.56 degrees in the prograde direction about the orbit's north vector. And then tilt the paper 10.76 degrees about the line of nodes, turning the part closer to you Ecliptic northwest (toward your upper right). With the paper in that position you can interpret the ellipse in two ways:

1. if you see the apastron closer to you than the periastron, then the ellipse represents the orbit of Alpha Centauri B about the system's center of mass.

2. if you see the apastron farther from you than the periastron, then the ellipse represents the orbit of Alpha Centauri A about the system's center of mass.

Orbital Period: 79.92 yr.

Time of Periastron passage:

        1. 1955.56 (AD 1955 Jul 24)

        2. 2035.48 (AD 2035 Jun 25)

        3. 2115.40 (AD 2115 May 27)

        4. 2195.32 (AD 2195 Apr 27)

        5. 2275.24 (AD 2275 Mar 29)

The Stars Themselves

Alpha Cent. A:

    Diameter; 1,708,000 km (1.227 Sol)

    Harvard Class; G2 V (5800 K)(5770 K)

    Age; 5-6 Gyr

    Mass; 1.09 Sol

    Brightness; 1.54 Sol

    Habitable zone: 1.2 - 1.3 AU (1.25 AU, 1.34 yr)

    Surface composition: hydrogen 71.5%, helium 25.8%, other 2.74% (Sol = hydrogen 73.7%, helium 24.5%, other 1.81%)

Alpha Cent. B:

    Diameter; 1,204,000 km (0.865 Sol)

    Harvard Class; K1 V (5300 K)

    Age; 5-6 Gyr

    Mass; 0.90 Sol

    Brightness; 0.44 Sol

    Habitable zone: 0.73 - 0.74 AU (0.74 AU, 0.66 yr)

    Surface composition: hydrogen 69.4%, helium 27.7%, other 2.89%

Proxima: (Discovered 1915)

    Harvard Class; M5 (2700 K)

    Age; 1 Gyr

    Mass; 0.1 Sol

    Brightness; 0.00006 Sol (0.000138 Sol)

    Location; 13,000 AU from A-B

    Surface composition: hydrogen 69.5%, helium 27.8%, other 2.90%

Planetary system properties:

    Stable planetary orbits lie within 1/5 of closest approach of components, 2.2 AU in this case.


Back to Contents