Castor (star)

Lore & Background

Castor was first recorded as a double star in 1718 by James Pound, though it may have been resolved 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″ in 1970 to about 6″ in 2026. These pairs have magnitudes of 1.9 and 3.0, respectively. Castor Aa and Ba both have orbits of a few days with a much fainter companion. Castor C, also known as YY Geminorum, is an eclipsing binary with variations due to starspots and irregular flares; its components orbit in less than a day. Castor C is believed to orbit Castor AB with an extremely long period of several thousand years.

Reader's Guide

Castor is significant as a nearby multiple star system that has been studied for centuries, providing insights into stellar multiplicity and binary dynamics. Its six stars form three binary pairs, including two A-type main-sequence stars and four red dwarfs, all of which are flare stars with emission lines. The system's variable star Castor C (YY Geminorum) is a BY Draconis variable, and since 2018 a possible brown dwarf has been suspected orbiting it. Culturally, Castor and Pollux are the twin stars of Gemini, named after the mythological twin sons of Zeus and Leda. The star has historical designations in Arabic and Chinese astronomy, and in 2016 the IAU approved the name Castor for α Geminorum Aa. Its legacy lies in its role as a benchmark for understanding stellar evolution and binary star systems.

Did You Know?

• Castor is a sextuple star system consisting of three binary pairs.
• Castor C (YY Geminorum) is an eclipsing binary with variations from starspots and flares.
• The separation between Castor A and Castor B increased from about 2″ in 1970 to about 6″ in 2026.
• Since 2018, a brown dwarf with mass at least 49±7 times Jupiter's mass may orbit Castor C.