KAPPA GEM (Kappa Geminorum). Due south of Gemini's Pollux, separated from it by about the angular distance between Pollux and Castor, lies easily-found almost-but-not- quite-third-magnitude (3.57, so it's barely fourth) Kappa Geminorum. Not really part of the classical outline, it at first comes across as just one more helium-fusing class K or G (this one G8) giant. But we should not dismiss such stars so lightly, as along with class A and B hydrogen-fusing dwarfs, they form the hearts and souls of the patterns we have come to know and love. Though in outward appearance these stars all seem rather alike, no two are quite the same, this one especially harboring a delightful secret (as noted below). A large number of temperature measures (in part as a result of the star being used as a statistical standard) yield a well-determined average of 4990 Kelvin. From a VERY well determined distance of 141 light years (accurate to just 2 l-y), the star shines at us with a luminosity (that includes the invisible infrared radiation estimated from temperature) of 74 times that of the Sun, from which we find a radius of 11.5 solar and (from theory) a rather hefty mass of around (with low accuracy) 2.7 Suns with an age of some 500 million years. Kappa Gem is near and large enough for direct measure of angular diameter by interferometry. An observation in yellow light gives a radius of 11.6 times that of the Sun, right on the mark, while another at a redder color makes the star seem much larger at 14 solar radii. While distended giants and supergiants commonly appear to have different sizes when seen in different colors, this discrepancy is disconcertingly large. A projected equatorial rotation speed of 5 kilometers per second (with a very large range of measures from 2 to 9 km/s, and using the smaller radius) gives a rotation period that could be as long as 115 days.

What makes Kappa Gem special, however, are not these statistics, but that it is linked to an intriguing solar-type companion. Set off at a current separation of 7.2 seconds of arc lies eighth magnitude (8.2) Kappa Gem B. Smythe and Chambers in the 19th century called the pair "a very delicate double star" with colors "orange" (certainly appropriate for bright Kappa A) and "pale blue" for Kappa B. Hardly. Over nearly 200 years, the separation has changed by only 2.2 seconds of arc, so they are probably a real orbiting binary. (If they were a line-of-sight coincidence, the rate of separation should have been much greater.) Given that they do belong to each other, Kappa-B's brightness however shows it to be a class G4 dwarf that is rather much like our Sun, which is yellow-white. The "pale blue" is the result of a visual contrast effect. Placement of stars right next to each other enhances, or even changes, the stars' apparent colorings. The effect is so strong, and the apparent colors so vivid, that old-time astronomers once thought that the components to such "wide" (non-interacting) double stars were physically different from single stars. Given a true separation of at least 310 Astronomical Units between the stars and that "B" has about a solar mass, Kepler's Laws give an orbital period greater than 2900 years. X-rays coming from the system are probably due to magnetic activity associated with the solar-type companion. If it (Kappa B) has any planets (which is unknown), Kappa A would present a fantastic sight, shining in their skies with the light of as many as 275 full Moons, which when up would quite blot out the stars. Thanks to Paolo Colona, who suggested this star, to Bill Hartkopf for expert opinion, and to Thomas Dutkiewicz, for uncovering errors in the original script.
Written by Jim Kaler 2/11/11; updated 11/06/15. Return to STARS.