The Astrogators' Guide to
Alpha Centauri
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 = 0.7506±0.0046 arc-seconds, which leads to;
4.3452 lightyears ±0.0266 lightyear
274,800 AU ±1685 AU
Equatorial Coordinates:
Right Ascension; 14 hr, 39 min, 36 sec + (6 min, 48 sec)t
Declination; -60 deg, 50 min - (25 min)t
[t measured in centuries, Jan 2000 is t = 0]
Imagine standing on the north side of a plane in which Earth's equator lies and look toward the east end of Virgo and tilt south (down) sixty degrees.
Ecliptic Coordinates:
Ecliptic Latitude; -42.587 deg -0.1862t deg.
Ecliptic Longitude; +59.478 deg +0.8812t deg.
[t measured in centuries, Jan 2000 is t = 0]
Galactic Coordinates:
Galactic Latitude; +0 deg, 44 min.
Galactic Longitude; +315 deg, 49 min.
Look across the Orion-Sagittarius Gap toward the Sagittarius-Carina Arm and at a point slightly galactic north of the galactic equator and 45 degrees east of the line from Sol to the galactic center.
Annual Proper Motion
in Right Ascension = -3.606 sec (4.75 AU per yr)
in Declination = +0.705 sec (0.93 AU per yr)
in Parallax = +0.751 sec (0.9895 AU per yr)
Total motion = 4.94 AU per year = 31.68 km/sec.
in Ecliptic latitude = -0.1862 degree per yr
in Ecliptic longitude = +0.8812 degree per yr
From the present; in 11,000 years Alpha Centauri will become an eclipsing binary (for about 2-1/2 centuries) as its orbital plane passes over the Sol-Alpha Centauri line and in 28,000 years Alpha Centauri will reach its perihelion 3.048 lightyears (192,771 AU) from Sol in the NW corner of Centaurus.
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 204.87 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
Composition: (1.3 - 2.3 x Sol's heavy element load)
Age: 6.5 - 8 billion years.
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.
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