The Binary Tour

Castor

t makes historical sense to start with , or Alpha (α) Geminorum. Except for the period from 1963 to 1973 when the components were less than 2” apart, Castor has been (and will remain) an accessible binary for smaller telescopes. Although a spectacular and bright blue-white pair, there’s much more here than visual observers can see. Both the A and B components are , making this a quadruple system. There’s also a third gravitationally bound component (in its turn