| Proper Name | Castor | | Bayer Designation | Alpha2,1 Geminorum, Alpha Geminorum A, B | | Flamsteed Number | 66 Geminorum | | HR (BSC) | 2891 (A), 2890 (B) | | HD | 60179 (A), 60178 (B) | | Constellation | Gemini | | Right Ascension | 7h 34m 36s | | Declination | +31° 53' 18" | | Distance | 51 light years
16 parsecs | | Magnitude | Apparent: +1.6 (A), +2.9 (B)
Absolute: +0.6 (A), +1.9 (B) | | Spectral Class | A1V White Main Sequence Star (A), A2Vm White Main Sequence Star (B) | | Optimum Visibility | January / February | | Notes | Castor 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.
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