Castor and Pollux, the twins from Greek mythology, are also bright stars which form the constellation Gemini in the Northern hemisphere. Castor was given the designation of alpha star of the constellation in the 17th century by Johannes Bayer, even though Pollux is slightly brighter, the 17th brightest star in the night sky - this may indicate that one of the two stars has changed in luminosity in the last 400 years.

Pollux is an old and cool red-orange giant star 34 light years distant from our solar system, with a magnitude of 1.2. It is approximately 11 times our sun's diameter (although angular measurements yield a figure of 8.3 times), which is quite small for a red-orange giant - only a quarter the size of Aldebaran, for example. Its surface temperature is 4500 kelvin.

Pollux is most likely younger than our sun, and its relatively high mass (1.7 to 4 times that of the sun) indicates that it has not advanced very far in its life cycle as a giant - it is definitely already fusing helium into carbon and oxygen at its core, but at some point in the future Pollux will begin to lose mass as its outer, more diffuse layers are pushed out by internal forces, eventually leaving the star's immediate gravitational pull to form planetary nebulae, and leaving the core of the star to cool as a white dwarf.

Pollux was named after the mortal son of Zeus and Leda, conceived when the horny old god turned into a swan for some long-necked nookie. Most cultures have grouped Castor and Pollux together - for example, many Native American peoples group them, with other nearby stars, into Mato Tipila, the Bear's Lodge, while in China Castor and Pollux are Yin Yang, the dual principles of Tao. Babylonians called them Mastabbagalgal, the twins; ancient Hebrews named them after the brothers Simeon and Levi; and in India they were Aswins, the twin horsemen of the dawn.