Second brightest star in the constellation
Gemini and the 23rd brightest star in the
sky. Along with Pollux, it forms a prominent
pair at the eastern end of Gemini. 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 400-500 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.