Castor isn't a single star. If you look through an amateur telescope you can see that there are two stars. Here's a nice picture by English astrophotographer Damian Peach:
Castor. Each of these two stars has a companion that even big telescopes can't see. They use special equipment to detect them. But there are two more, because Castor is a sextuple - yes, SIX! - star system. The last two are a pair of red dwarfs which aren't visible in Damian's photo.