NU GEM (Nu Geminorum). At fourth magnitude (4.15) bright enough sometimes to be considered part of the "connect the dots" pattern of its constellation, Gemini, one would think Nu Geminorum (of no proper name) would be rather well understood. It isn't. It's a double, probably triple, maybe multiple star that has yet to be fully sorted out. Even the names of its components are hard to get straight. However many there are. At a healthy distance of 500 light years, Nu Gem is also a "Be" star (class B6), one that exhibits emissions in its spectrum that come from a surrounding disk (like Gamma Cassiopeiae, Zeta Tauri, Delta Scorpii, and a host of others). But we don't even know which of the members of the system is the culprit. The bright member, seemingly a class B6 giant, is a very close double (with the B6 giant dominant) with a likely (but uncertain) period of 53.72 days. These two are orbited by a third member that takes (best guess) about 13 years to make a circuit. These are collectively all called Nu Gem A. Farther out, nearly two minutes of arc away, is eighth magnitude class B8 Nu Gem B, which is too faint for such a classification. Either the star is just a line-of-sight coincidence or the class is wrong. Or both. The inner pair then has Nu Gem Ab orbiting the tight double Aa1 and Aa2. Different people, however, use different nomenclature, leading to considerable confusion. Nu Gem Aa and Ab are very difficult to separate, being at most only one or two tenths of a second of arc apart. Best estimate is that Aa1 (the bright B6 star) shines with the light of about 840 Suns (Aa2 is probably inconsequential), while Ab puts out maybe 330. These, with a temperature for Aa1 of 14,000 Kelvin, gives us respective masses for Aa1 and Ab of 4.5 and 4.0 solar (or as high, depending on the exact state of evolution, of 4.8 and 4.2). Bright Aa1 is then seen not to be a real giant, but more likely a subgiant that has just recently given up its core hydrogen fusion (or will shortly). The 54-day orbital period gives an average orbital separation between Aa1 and Aa2 of about half an Astronomical Unit. The orbit of Ab around Aa is much wider, around 10 AU, but with a very high eccentricity that takes the stars from as far as 20 AU to as close as 1.5. Nu Gem Aa1 spins with an equatorial speed of at least 220 kilometers per second, typical for the class, but low for a Be star. The emissions, however, do not follow the orbit of the Aa1-Aa2 pair. Odds are that the emissions are actually coming from Ab. Nobody really knows. If Nu Gem B is really a member, it is at least 17,000 AU away from the inner trio and takes at least 1500 years to orbit. Four other listed members, faint stars between one and two minutes of arc out oddly called Nu Gem P, Q, R, and S (suggesting that a lot of others were once considered), probably again just lie along line of sight. Little about Nu Gem is certain. (Much of this description was taken from a summary by Th. Rivinius, S. Stefl, and D. Baade in Astronomy and Astrophysics. Thanks to Bill Hartkopf for discussion.)
Written by Jim Kaler 1/26/07. Return to STARS.