Castor

Castor is one of the most remarkable Mehrfachsternsystems in the sky, appearing as a single star of magnitude 1.98 im Sternbild Zwillinge. A Teleskop reveals two bright components (A and B) orbiting each other, and each of those is itself a spektroskopischer Doppelstern. A third, fainter bedeckungsveränderlicher Doppelstern (C) orbits the inner pair, making Castor a sextuple star system — six stars gravitationally bound together. Located about 51 Lichtjahre von der Erde entfernt.

Castor is the more northerly of the twin heads im Sternbild Zwillinge, slightly fainter and whiter than the orange Pollux below it. A kleines Teleskop at 100x easily splits the A and B components (currently about 5 Bogensekunden apart), revealing a lovely pair of white stars. The faint C component is visible nearby in larger Teleskops. Best observed Januar through Mai.

Named after one of the Dioscuri twins of griechische Mythologie — 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 Doppelsterns are gravitationally bound.