Castor (star)

Castor is the second brightest object in the zodiac constellation of Gemini. It has the Bayer designation α Geminorum, which is Latinised to Alpha Geminorum and shortened to Alpha Gem or α Gem. It has an apparent visual magnitude of 1.93, which makes it one of the brightest stars in the night sky. Castor appears as a single star to the naked eye, but it is actually a sextuple star system (six stars) organized into three binary pairs of stars designated Castor A, Castor B and Castor C, each containing two stars. Even though it is the 'α' (alpha) member of the constellation, it is fainter than 'β' (beta) Geminorum, Pollux.

Stellar system

Castor is a multiple star system made up of six different stars; there are three visual stars, all of which are spectroscopic binaries. Appearing as a single star to the naked eye, Castor was first recorded as a double star in 1718 by James Pound, but it may have been changed into at least two sources of light by Cassini as early as 1678. The separation between the binary systems Castor A and Castor B has increased from about 2″ (2 arcseconds of angular measurement) in 1970 to about 6″ in 2017.
 These stars have magnitudes of 1.9 and 3.0, respectively.

Castor Aa and Ba both have orbits of a few days with a much fainter star.

Castor C, or YY Geminorum, was discovered to change in brightness with a regular period. It is an eclipsing binary with additional changes due to areas of different brightness on the surface of one or both stars, as well as irregular flares. The Castor C stars orbit in less than a day. Castor C is believed to be in a orbit around Castor AB, but with an extremely long period of several thousand years. It is 73″ distant from the bright stars in the system

The combined apparent magnitude of all six stars is +1.58.

 References 

Category:Stars