TAU GEM (Tau Geminorum). While we might overlook a common orange class K (K2) giant, fourth magnitude (4.41) Tau Geminorum, in Gemini five degrees west-southwest of Castor, is a significant part of the Twins' outline, so it deserves to be explored. And as usual, we find buried treasure. Though in the thin part of the Milky Way not far from the galactic anticenter (in Taurus), the star's distance of 321 light years (give or take 8) is enough to produce an uncertain 0.29 magnitudes of dimming by interstellar dust. A temperature of 4420 Kelvin (needed to account for infrared radiation) then shows the star to shine with the light of 316 Suns, which leads to a radius 30 times solar. Theory yields a mass of 3.0 times that of the Sun. A recently-published study (see below) gives a mass of 2.3 Suns, which, given the similarity of properties among such stars over a range of masses, is not unreasonable agreement. Tau Gem is a slight variable that goes between magnitudes 4.3 and 4.6, which supports theory's suggestion that the star has ceased core helium fusion (or is close to doing so) and is beginning to brighten for the second time as a giant with a dead carbon/oxygen core. It will someday eject its outer layers to create an ephemeral planetary nebula, after which it will die as a white dwarf with a mass between 65 and 70 percent of the Sun. Tau Gem is also a quadruple star. At a separation from the luminous giant of just under two seconds of arc we find eleventh magnitude (11.0) Tau Gem B, which from its brightness is probably a K0 dwarf. Farther out, 59 seconds of arc away, is thirteenth magnitude (13.0) Tau C, which is most likely a K6 dwarf, if indeed it is a member of the system, which is contended. But over the past century, "C" has tracked Tau Gem A to within a couple tenths of a second of arc while Tau Gem itself has sailed along against the background by nearly five arcseconds, so Tau C seems to be a member, albeit a fragile one that may be separating from its mates. With a physical separation of at least 187 Astronomical Units, Tau B takes at least 1450 years to orbit Tau A, while (at least 5800 AU away) Tau C might take over a quarter million. Of far greater interest, Tau Geminorum harbors a closely-orbiting brown dwarf, a body with such a low mass (under eight percent solar) that it cannot initiate fusion of hydrogen to helium in its core. With an orbital period of 305.5 days, Tau Ab is at least 1.17 AU from its parent star and weighs in with a mass of at least 20.6 Jupiters, or 0.020 Suns. We might call it a massive planet, but the mass is higher than the 13 Jupiter-mass limit above which stars can fuse their deuterium (a nucleus with a proton and a neutron) into helium, which makes Ab most likely to be a brown dwarf that collapsed from its birth cloud along with Tau Aa, instead of being made by accumulation of dust and gas from the leftover circulating disk. But nobody knows the cutoff, so it might yet be a giant planet. If anyone's there, from Tau B, Tau A could shine with the light of four full Moons, while from Tau A, Tau B might be as bright as Venus. (Brown dwarf properties and stellar mass from D.S. Mitchell et al., "Astronomy and Astrophysics," 555, A87, 2013.)
Written byJim Kaler 1/01/16. Return to STARS.