LAMBDA GEM (Lambda Geminorum), along with neighboring BQ GEM, a two-for-one special! Five or so degrees to the south of the main outline of classical Gemini lies yet another white class A (A3) dwarf, fourth magnitude (but 3.58, perilously close to third) Lambda Gem. While outwardly ordinary, close examination reveals surprising complexity and confusion. First up is Lambda proper, or Lambda A, as the system is clearly triple. There is little in the way of temperature measure, but from one citation and the class, it falls around 8600 Kelvin. From the star's distance of 101 light years (known to better than one l- y), we straightforwardly calculate a luminosity of 28 Suns, a radius of 2.4 times solar, a mass of 2.2 solar, and an age decently along its core hydrogen-fusing lifetime of 900 million years. A rapid projected equatorial rotation speed of 154 kilometers per second leads to a rotation period under 19 hours, which appears to keep the star's atmosphere stirred up enough that elemental abundances are sufficiently normal (there are no actual measures) that there are no "peculiar" labels put on the star as a result of elemental diffusion (gravitational settling of some chemical elements, radiative lofting of others). Not quite 10 seconds of arc away lies eleventh magnitude (10.7) Lambda Geminorum B. While there is no significant orbital movement, "B" has been tracking "A" almost perfectly for nearly two centuries, so that they obviously belong to each other. Writing in the nineteenth century, Smythe and Chambers refer to the pair as: "A delicate double star ... A 4 1/2, brilliant white, B 11, yellowish." From the calculated absolute brightness, Lambda B is a cool class K (K8) dwarf with a mass of around six-tenths solar, which rather fits with the visual color. A minimum average separation of some 300 Astronomical Units then leads, via Kepler's Laws, to an orbital period of greater than 3000 years.

So far, so good. But there is a serious complication. Lambda Gem is close enough to the ecliptic that it is occasionally occulted by the Moon, revealing it to have a very close-in second companion at most just a few hundredths of a second of arc away. Playing hide and seek, Lambda Ab (rendering Lambda A as "Aa") could sometimes be seen by occultation, sometimes not, interferometry failing as well. One study claims that the difference in apparent visual brightness is no more than a full magnitude, with Aa at magnitude 4.0, Ab at 5.0. If that is the case, "Ab" is a class A9 dwarf with luminosity of 9 times solar and a mass of 1.6 Suns. The luminosity of "Aa" then drops to 19 Suns and the mass to double solar, the change cutting Lambda B's orbital period to greater than 2500 years. However, from another measure of the magnitude difference, this time in infrared light, the visual difference should be significantly greater than 3.5 magnitudes, allowing us pretty much to ignore Ab's contribution to Aa. Only further close examination will tell. In any case, from Lambda B, Lambda A would shine at level of at least 100 of our full Moons, the inner pair appearing perhaps up to a dozen minutes of arc apart.

Whatever the truth of the matter, Lambda Gem acts as a fine gateway to an interesting fifth magnitude (5.0) semi-regular variable star, a cool class M4 advanced giant known as BQ Geminorum (also by its Flamsteed number as 51 Gem) that lies just 1.2 southwest of Lambda. Lambda's angular proximity and steady light then allows one to easily monitor BQ's 0.4 magnitude variation over its 50-day period. Given BQ's distance of 540 light years, more than five times farther than Lambda, the two obviously have no physical connection. A luminosity of nearly 2000 Suns suggests that BQ began life as a 1.5-to-2 solar mass star, that it is now dying with a dead carbon core, and that it will "shortly" shed its outer envelope while the core turns into a white dwarf. Thanks to Paolo Colona, who suggested Lambda Gem, and to Bill Hartkopf for his usual double-star expertise.
Written by Jim Kaler 2/18/11. Return to STARS.