Castor

Second-brightest star in the constellation Gemini and the 23rd brightest star in the night sky. Along with the brighter Pollux, it forms a prominent pair at the eastern end of Gemini, representing the head of the twins.

Second-magnitude Castor is 45 light years from Earth, and is one of the finest binary stars in the sky for small telescopes. The two main components orbit each other over a period of 350 years. A third, much fainter, star orbits the main pair over a period probably exceeding 10,000 years. Each of the three visible components is a spectroscopic binary, making Castor a sextuple star system.