The term 'Twin Stars' refers to the two stars of the CASTOR star system, which is located in the constellation Gemini. CASTOR is a binary star system composed of two stars, CASTOR A and CASTOR B, which are both visible to the naked eye. CASTOR A is a white main sequence star, while CASTOR B is a yellow-white main sequence star. Both stars are about twice as massive as the Sun and have similar temperatures. The two stars orbit each other at a distance of about 20 astronomical units.