POLLUX (Beta Geminorum). Star with planet. In northern spring evenings, the "twin" stars Castor and Pollux of the constellation Gemini descend the northwestern sky looking like a pair of eyes staring down at the Earth. They are twins only in mythology, these warriors, Pollux fathered by Zeus and divine, Castor mortal, both placed in the sky to allow them to be together for all time. The northernmost of the zodiacal constellations, Gemini is also among the brightest, helped by first magnitude Pollux and second magnitude Castor. An exception to the rule, brighter Pollux, the sky's 17th brightest star, was given the Beta designation by Bayer, while somewhat fainter Castor is known as Alpha Geminorum. In fact, Pollux and Castor are nothing like twins, bright Castor a white quadruple star with fairly class A hot components (sextuple if you count a distant pair of companions) and Pollux an orange-colored cool (4770 Kelvin) class K (K0) giant with a planet, the nice pairing with Castor making Pollux's color more vivid. From its distance of 34 light years, we calculate a total luminosity (incuding infrared radiation) for Pollux 46 times that of the Sun, and coupled with its temperature, a diameter some 10 times solar, making it smaller than most of its cool giant brethren and only a quarter the dimension of Aldebaran. Direct measures of angular size, however, yield a somewhat smaller diameter 9 times that of the Sun. From luminosity and temperature, the mass comes in at around 1.8 times solar. Like so many others of the nighttime sky, Pollux is a typical red giant that is quietly fusing helium into carbon and oxygen in its deep core. It emits X-rays and seems to have a hot, outer, magnetically supported corona perhaps similar to that surrounding our Sun.

The Planet: Pollux's greatest claim to fame is an orbiting planet, making one of the very few giants with such companions, and the brightest planet-holding star. With a mass at least 2.9 times that of Jupiter, the planet orbits in a nearly circular path at a average distance of 1.69 Astronomical Units (11 percent farther than Mars is from the Sun) with a period of 590 days (1.6 years). Like many other planet-holding stars, Pollux is metal-rich, with an iron content (relative to hydrogen) 55 percent higher than solar. From the planet, which is bathed with a radiation intensity 16 times the amount we get from the Sun, Pollux would glower in the sky with an angular diameter of nearly 3 degrees, 5.7 times bigger than we see the Sun. Not far away, in Taurus, is another giant with a planet, Ain (Epsilon Tauri). (Gemini has two other stars with orbiting planets: HR 2877 and HD 50554.)
Written by Jim Kaler. Return to STARS.