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.