CASTOR (Alpha Geminorum). In classical mythology, Castor is the mortal twin of Pollux, the twin warriors whose stars dominate the bright zodiacal constellation of Gemini.
The distance of Castor, or Alpha Geminorum, from Sol was estimated by the HIPPARCOS mission to be about 51.6 light-years (ly) using a space-based parallax (Plx=0.06327, e_Plx= +/- 0.00123).
A real example of a system with hierarchy 3 is Castor, also known as Alpha Geminorum or α Gem. It consists of what appears to be a visual binary star which, upon closer inspection, can be seen to consist of two spectroscopic binary stars.
Double stars in Gemini Alpha Geminorum is a well-known binary with the companion currently (2000.0) at a PA of 65º and separation 3.9". The visual magnitudes are 1.9 and 3.0.
The twin to the right is Castor, whose brightest star is alpha Geminorum (more commonly called Castor), is of the second magnitude, and represents Castor's head.
Castor Alpha Geminorum
Cebalrai Beta Ophiuchi
Celaeno 16 Tauri
Chara Alpha Canum Venaticorum
Cheleb Beta Ophiuchi
Chertan Theta Leonis
Chort Theta Leonis
Chow Beta Serpentis
Cor Caroli Alpha Canum Venaticorum
Cursa Beta Eridani ...
[3940] alpha Geminorum, Castor, is a visual binary composed of two separate binary stars.
Systems with up to six stars are known; for example, Castor (Alpha Geminorum), which consists of a binary pair in a distant orbit of two closer binary pairs.
The head of the Gemini twin nearest the ecliptic. An orange giant just above the ecliptic. The other head of a Gemini twin is Castor. Although it has the Bayer designation "beta", it is actually brighter than Alpha Geminorum (Castor).
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Fomalhaut ...
 See also: Castor, Star, Sky, Distance, Orbit
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