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.