CASTOR (Alpha Geminorum). In classical mythology, Castor is the mortal twin of Pollux, the twin warriors whose stars dominate the bright zodiacal constellation of Gemini. Though Castor is the fainter of the two, it still received the Alpha designation from Bayer, who made Pollux the Beta star. Castor's intimate mythological and celestial association with Pollux commonly sometimes lofts it into the "first magnitude" category, though in fact it is the brightest of the second magnitude stars (1.58), coming in just behind Adhara in Canis Major. Castor and Pollux make a most attractive sight at the northern end of Gemini, Pollux an orange giant, Castor a contrasting white. To the naked eye, Castor shines down to us as a seemingly ordinary hydrogen-fusing class A star that appears much like Vega. Castor has no physical relation with Pollux, and at a distance of 51 light years (second Hipparcos reduction) is half again as far away as its mythological companion. The telescope reveals Castor's real claim to fame to be as a remarkable multiple star. Even a modest amateur instrument shows bright Castor to consist of a pair of similar stars only a few seconds of arc apart. The brighter (Castor A) is mid-second magnitude (1.93), the fainter (Castor B) mid-third (2.97), both of them class A (A1 and A5). (As a "metallic-line star, Castor B's class is uncertain, and is sometimes given as warm as A2). About 1.2 minutes of arc away to the south lies a third companion, ninth magnitude (9.1) Castor C. Each of the three is a spectroscopic double, making the system sextuple. How ironic that one of the "twins" should in fact be made of three sets of twins, Castor certainly the sky's ranking sextuple, double-double-double, star. Castor may also be part of a hugely extended, physically related, group of stars called the "Castor Moving Group" that includes Vega, Fomalhaut, Zubenelgenubi, and Alderamin.

A AND B. Castor A and B are in elliptical orbit about each other with a 445 year period, and are now separating, making them ever more easy to observe.
Castor Castor B goes around Castor A (the brighter and more massive of the two, placed at the cross) with an orbital period of 445 years at an average separation of 100 Astronomical Units. In reality, both go around each other. North is down, and the scale around the edges is in seconds of arc. The dot-dash line is the orbit's major axis. The orbital plane is tilted to the line of sight by 25 degrees, so that Castor A does not appear at the focus of the orbit (where it actually is). The period is so long that we have yet to see the stars make a full turn around each other. Each of the two is again double. Castor C lies 1.2 minutes of arc away. (From W. I. Hartkopf and B. D. Mason, Sixth Catalog of Orbits of Visual Binary Stars, U. S, Naval Observatory.)
Averaging 104 Astronomical Units apart, they get as close as 71 AU and as far apart as 138 AU. Kepler's laws give a total mass to the four stars of 5.7 times that of the Sun. From the distance, respective temperatures for the main components A and B of approximately 9500 and 8300 Kelvin, and the respective luminosities of 37 and 13 Suns, we find masses of 2.4 and 1.9 solar. Though their companions remain mysterious, they respectively seem fall into cool K and warm M, making their masses (on average) about half solar, giving a total system mass of 5.3 solar, close to that derived from the above orbit. The small companions (Ab and Bb) orbit their brighter mates with respective periods of 9.21 and 2.93 days at very close separations of about 0.12 and 0.03 AU. Each of the binaries (A and B) are sources of X-rays that in all likelihood arise from magnetic activity on the cool companions.

CASTOR C (Alpha Geminorum C = YY Gem). Castor C is an eclipsing spectroscopic double that also goes by the variable-star name YY Geminorum, the light and velocity curves allowing precise determination of stellar parameters. YY Gem consists of nearly identical class M (M1) dwarfs with temperatures of 3820 Kelvin that orbit each other on circular paths every 19.54 hours, the masses averaging 0.61 times that of the Sun. They are separated from the Castor AB pair by at least 1000 AU and would therefore take at least 14,000 years to orbit the bright quadruple. The two, with radii of 0.62 solar, are only 3.9 solar diameters (6.3 stellar radii) apart. One or both are flare stars, stars that will suddenly change their brightnesses as a result of the erratic release of surface magnetic energy (as does Proxima Centauri). The system points up a fundamental problem. The parameters of Castor A and B show them to be 370 million years old, while Castor C is only 30 to 85 million years old. Put another way, 370 million year old red dwarfs should be some 20 percent smaller than observed. The problem may be related to the magnetic activity that causes the flaring. (Principal data for Castor C from G. Torres and I. Ribas in The Astrophysical Journal.)
Written by Jim Kaler 12/11/98; revised 7/19/02, 8/15/08, 7/10/09. Return to STARS.