in Astronomy, the Twins, a constellation of the zodiac, the third in order, representing Castor and Pollux. These names are also given to two stars in its constellation. Castor is a star of the third, and Pollux of the second magnitude. The former, seen through a telescope, is a beautiful double star, that is, two stars of the fifth and sixth magnitudes, one of which revolves about the other, forming a binary system.