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.