Mu Geminorum

α Geminorum

Alpha Geminorum, (Castor, The Horseman) is a wonderful multiple system (Struve 1110), considered one of the finest binaries in the heavens:
   The two stars A and B are similar in magnitude, and are easily resolved in small telescopes. Indeed, Castor was the first star to be shown to be a true binary, a star with another gravitationally attached to it, as Sir William Herschel announced in 1803.
     AB: 1.9, 3.0; 55, 4.6". (orbit)
     AC: 1.9, 9.8; 165, 70.5".
     AD: 1.9, 10.1; 222, 181.9"

The twins Castor and Pollux were known from antiquity and even featured on coins in Greece and throughout the Roman Empire. Romans knew them as the Sons of Zeus.

Gemini is one of the more easily found constellations, found to the northeast of Orion or due north of the bright and solitary Procyon (alpha Canis Minoris).

From the brightest star in the Winter sky, Sirius, describe a curving line northward, through Procyon and then slightly westward to the twin stars.

Castor and Pollux are in the same field of view: binoculars. Castor is pale blue and Pollux is orange (or yellow).


Just west of Castor a half-binocular field can be seen tau Geminorum: binoculars. This is Castor's 'chest' and from this point two stars mark his hands: theta Geminorum to the northwest one and a half binocular fields, and iota Geminorum.
     Iota is the one common star between the twins, forming the right hand of Castor and the left hand of Pollux.

Move back to Castor and Pollux, now down one field of view. This is kappa, a fine telescopic binary with a colour contrast.
      Kappa Gem (Otto Struve 179), orange and blue: 3.7, 8.2; 242, 7.5".

The other binary here, Struve 1108, is best seen if you move southwest one field to delta Gem, which is also a telescopic binary.

Click on delta on the map to continue.

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1999-2014 by Richard Dibon-Smith.