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Alpha Geminorum, 66 Geminorum

Proper NameCastor
Bayer DesignationAlpha2,1 Geminorum, Alpha Geminorum A, B
Flamsteed Number66 Geminorum
HR (BSC)2891 (A), 2890 (B)
HD60179 (A), 60178 (B)
Right Ascension7h 34m 36s
Declination+31° 53' 18"
Distance51 light years
16 parsecs
MagnitudeApparent: +1.6 (A), +2.9 (B)
Absolute: +0.6 (A), +1.9 (B)
Spectral ClassA1V white main sequence star (A), A2Vm white main sequence star (B)
Optimum VisibilityJanuary / February
NotesCastor is a multiple star system consisting of three close binary pairs. Two of these binaries, Castor A and Castor B, follow a relatively close orbit around one other, while the fainter variable binary Castor C pursues a more distant orbit around the primary pair.

A complex multiple star that shines brightly in the north of Gemini. Though designated 'Alpha', it is not the brightest star in its constellation: its magnitude is actually somewhat less than that of its mythical twin, Pollux.

The relatively bright star seen here to the west (right) of Castor is Rho Geminorum. This bright yellow dwarf is only a few light years more distant from the Sun than Castor, but is a cooler and less luminous star than its prominent neighbour in the sky. Imagery provided by Aladin sky atlas


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