Viewed through a small telescope, Castor appears to be a double star. The two components A and B are of magnitude 2.0 and 2.8 (see Burnham). There is however a dim third C component to the system. A and B revolve around their common center of mass with a period of four or five hundred years. The separation between the stars is about 100AU, that is about 25% more than the diameter of the orbit of planet Pluto.
C is separated from A and B by 73 arc seconds, corresponding to a projected distance of 1200 AU. According to Burnham, the period of the orbit of C about A and B probably exceeds 10,000 years.
The The Bright Star Catalog lists Castor A as A1V, that is, a white main sequence star. B is A5m, that is a somewhat smaller and cooler main sequence star. C is of spectral type M1V, that is, a red main sequence dwarf.
Close Binary Stars
Each of the components of Castor proves to be a close binary star. Castor A consists of two almost identical main sequence stars of spectral type A in a rather eccentric elliptical orbit with a period of 9.2 days. The separation between the stars is about four million miles. Each star is twice the diameter of the sun and 12 times its luminosity according to Burnham. The total mass of the pair is 3.2 times the mass of the sun.
Castor B consists of two stars revolving in a circular orbit with a period of 2.9 days. Both stars are of type A5 according to Burnham. Each star is about 1.5 times the diameter of the sun and 6 times its luminosity. The total mass of the pair is 2.3 solar masses.
Castor C consists of two stars revolving in an orbit almost edge-on to our line of sight. The two stars are separated by about 1.67 million miles and orbit with a period of 19.5 hours. Each star has about 2.5% of the luminosity of the sun, about 0.6 times the mass of the sun, and 0.7 to 0.8 times the diameter.