Castor — Double Star in Gemini

Castor is one of the most remarkable multiple star systems in the sky, appearing as a single star of magnitude 1.98 in Gemini. A telescope reveals two bright components (A and B) orbiting each other, and each of those is itself a spectroscopic binary. A third, fainter eclipsing binary (C) orbits the inner pair, making Castor a sextuple star system — six stars gravitationally bound together. Located about 51 light-years from Earth.

Castor is the more northerly of the twin heads in Gemini, slightly fainter and whiter than the orange Pollux below it. A small telescope at 100x easily splits the A and B components (currently about 5 arcseconds apart), revealing a lovely pair of white stars. The faint C component is visible nearby in larger telescopes. Best observed January through May.

Named after one of the Dioscuri twins of Greek mythology — Castor was the mortal twin, a skilled horseman, while Pollux was the immortal boxer. Despite being the fainter twin, Castor received the Alpha designation, likely because Bayer considered it the 'first' twin positionally. William Herschel studied the orbital motion of Castor A and B, helping establish that binary stars are gravitationally bound.