SIGMA GEM (Sigma Geminorum). Tucked right next to Pollux in Gemini on the line between it and Castor, fourth magnitude (4.28) Sigma Geminorum is largely ignored. In its own way, however, it outshines its brighter neighbors. At first seeming like just another class K (K1) giant, Sigma Gem is a fine example of a cool and very well studied "RS Canum Venaticorum" star rather like Epsilon Ursae Minoris, Lambda Andromedae, and Rana (Delta Eridani). From its distance of 125 light years (plus or minus just one), the star radiates 39 solar luminosities from its 4600 Kelvin surface, from which we derive a radius 10.2 times that of the Sun. Direct measure of angular diameter coupled with distance gives 9.3 solar radii, showing that the parameters are close to the mark. Sigma Gem is most likely a fairly low mass (perhaps 1.25 solar, though it is very difficult to derive an accurate number) giant star that is now fusing its internal helium into carbon and oxygen. The "RS Canum Venaticorum" tag derives from a close, fainter companion (possibly a class G or K dwarf) that orbits the bigger star in 19.605 days at a distance no more than about 0.2 Astronomical Units. An equatorial rotation speed of between 22 and 27 kilometers per second shows that the star rotates at the orbital period, the pair synchronized quite like the rotation of the Moon (which keeps one face pointed toward us). This tidal locking has spun up the giant to a higher than normal speed, and has created considerable magnetic activity (as a result of a natural dynamo caused in part by the rotation; the Sun does the same thing). Up to 30 percent of the star can be covered by cool, magnetic (3500 Kelvin) "starspots" (akin to sunspots) that lie at mid-latitudes north and south of the stellar equator and cause it to vary in brightness about a tenth of a magnitude as it rotates. The magnetic activity produces active outer regions and a hot corona, that make Sigma Gem one of the brightest stellar X-ray sources in the sky and even make it shine nicely in the radio spectrum. Sigma is even seen to launch mighty flares that have brightened it in the high energy ultraviolet part of the spectrum by as much as a factor of 9. As a result of the close binary interaction, the star is an "antisolar rotator," that is, it rotates faster as we proceed from the equator to the poles rather than slower, the reverse of what we see in the Sun. About three minutes of arc away lies an eleventh magnitude "companion." The two have been drawing apart, however, revealing the apparent duplicity to be just a line-of-sight coincidence, our Sigma clipping along at a goodly pace of 62 kilometers per second relative to the Sun, about four times normal.
Written byJim Kaler 3/19/04; revised 9/09/15. Return to STARS.