Castor appears singular to the naked eye, but it is actually a sextuple star system organized into three binary pairs.

Although it is the 'α' (alpha) member of the constellation, it is half a magnitude fainter than 'β' (beta) Geminorum, Pollux.

It is an eclipsing binary with additional variations due to areas of different brightness on the surface of one or both stars, as well as irregular flares.

BY Draconis variables are cool dwarf stars which vary as they rotate due to starspots or other variations in their photospheres.

[19] All the red dwarfs in the Castor system have emissions lines in their spectra, and all are flare stars.

[20] In Chinese, 北河 (Běi Hé), meaning North River, refers to an asterism consisting of Castor, Rho Geminorum, and Pollux.

[21] Consequently, Castor itself is known as 北河二 (Běi Hé èr, English: the Second Star of North River.