6 GEM (6 Geminorum). Less than three degrees east-southeast of the Summer Solstice and less than a degree northwest of Eta Geminorum (in Gemini, the Twins), lies an unassuming but quite spectacular star. At dim sixth magnitude (6.4), 6 Geminorum (its Flamsteed number) is possibly visible to excellent eyes, though it and its reddish color are far better seen in binoculars. Were it not dimmed roughly 1.8 magnitudes by the Milky Way's interstellar dust, 6 Gem would be almost fourth magnitude and be readily seen with no optical aid. The star's faintness hides a red supergiant so far away that its distance cannot be measured through parallax (the shift in position caused by the orbital movement of Earth). As a rare class M1 (maybe M2) red supergiant, one near the high end of such stars, it should have an absolute magnitude (what the apparent magnitude would be at 32.6 light years) of around -6.5. Comparison with the actual apparent magnitude of 4.6 as undimmed by intervening dust then implies a distance of 5400 light years. Though very uncertain, the distance is supported by 6 Gem's membership in the Gemini OB1 association of hot stars, which is estimated to be 5000 light years away. Allowance for a lot of infrared radiation from a 3700 Kelvin "surface" (such as it is for a distended supergiant) suggests a whopping luminosity of 150,000 times that of the Sun and a radius of 950 solar, or about 4.4 Astronomical Units, which in our Solar System would bring the star 85 percent of the way out to the orbit of Jupiter. Assumption of the star as a lesser supergiant brings the luminosity down to a "mere" 105,000 Suns. In any case, 6 Geminorum, with a mass of around 20 Suns, falls in league with much closer Betelgeuse and Antares (though falling well below such stars as VV and Mu Cephei). Such red supergiants are usually pretty unstable, and 6 Gem is no different. It's listed as an irregular variable that goes between magnitudes 6.1 and 7.2 over an ill-defined interval of 6.7 years (giving it the variable star name BU Gem). Keep an eye out, and if it is near its peak, it could be seen without aid. There is another reason to keep watch, at least over the next million or so years. Born as a blue class O hydrogen-fusing dwarf (hence its appearance in an association of hot stars), 6 Gem is probably now fusing helium into carbon and oxygen in its deep core. Well above the 8-10 solar mass limit beyond which stars eventually generate unstable iron cores (the Sun will not), 6 Gem is destined to explode as a supernova. If so, it would for a time probably top Venus in brightness and make a fine mess of the outline of the celestial Twins.

Written by Jim Kaler 12/7/12. Return to STARS.