Castor

Castor is the traditional name of Alpha Geminorum, a bright multiple star system in the constellation Gemini.

Castor is the traditional name of Alpha Geminorum, a bright multiple star system in the constellation Gemini. It appears to the naked eye as a single star, but is actually a six-star system arranged in three binary pairs. With an apparent magnitude of about 1.58, Castor is one of the brightest stars in the night sky and is the second-brightest object in Gemini after Pollux.

Identity

Castor is the traditional name for Alpha Geminorum, one of the brightest objects in the constellation Gemini. In modern astronomy it is classified as a multiple star system rather than a single star.

System structure

The Castor system contains six stars organized into three close binary pairs: Castor A, Castor B, and Castor C. Each of the three visible components is itself a spectroscopic binary, meaning the individual stars are too close to separate visually.

Brightness and visibility

Castor has an apparent magnitude around 1.58, which makes it easily visible without a telescope. Because of this brightness, it is a familiar naked-eye star in the Gemini constellation.

Distance and location

Castor lies roughly 51 light-years from Earth in the zodiac constellation Gemini. It is usually identified with the bright star near Pollux, the other major star associated with Gemini.