The term 'Star in Gemini' refers to the two brightest stars in the constellation of Gemini, CASTOR and POLLUX. CASTOR is a binary star system composed of two stars orbiting each other, while POLLUX is a single star. Both stars are visible to the naked eye and have been known since antiquity. CASTOR has a magnitude of 1.58 and POLLUX has a magnitude of 1.14, making them the sixth and seventh brightest stars in the night sky respectively.