RHO GEM (Rho Geminorum). The bright stars of Gemini, Castor (Alpha Gem, the brightest of second magnitude) and first magnitude Pollux (Beta Gem, the more southern of the two), draw the eye. Placed symmetrically against them both is an equally-otherwise-unrelated fourth magnitude pair that are thus ridiculously easy to locate. Look for Sigma Gem (magnitude 4.28) just a degree to the northwest of Pollux, and Rho (continuing the symmetry the previous letter of the Greek alphabet, the star at magnitude 4.18) about equally placed just to the west of Castor. Now, however, our journey through symmetry ends, as Sigma is an orange K giant 122 light years away, while Rho is a more solar, white class F (F0) hydrogen-fusing dwarf at half that distance (58.9 light years, with an uncertainty of just a quarter l-y). With a temperature of 7000 Kelvin, it radiates a total of 5.4 Suns into space, most of it in the visual spectrum. Temperature and luminosity then conspire to give us a radius of 1.6 Suns. As do many of its warm-F class, Rho Gem is a fairly rapid rotator, a projected equatorial rotation speed of 63 kilometers per second leading to a rotation period of under 1.3 days. It's probably spinning even faster, as there is enough stirring in the star's atmosphere to prevent it from having any weird composition anomalies (wherein some chemical elements fall downward under the influence of gravity, while others are radiatively lofted, as in for example Chi Lupi). Its relatively low metal content (compared to hydrogen) of about half the solar value is not all that unusual. Application of theory then reveals a mass of just under 1.5 times that of the Sun and not far from having begun its six or so billion-year hydrogen-fusing lifetime. Rho Gem radiates some in the X-ray, which suggests a minimal outer chromosphere, but no variation or cycle has been found.

While there is no evidence for any planets or circumstellar debris disk, Rho Gem is listed as having a quartet of stellar companions (not all of which really belong). Just 3.4 seconds of arc away lies 12.5-magnitude Rho Gem B. Keeping a nice pace with Rho A, it is most likely a true mate. If so, from its brightness, it is a class M4 dwarf, which fits with its observed class of M5, which in turn confirms the physical relation between them. At least sixty Astronomical Units away, from Kepler's laws it would take Rho Gem B more than 370 years to make a full orbit of Rho Gem proper. Twelfth magnitude Rho C, which appears coupled to 13th mag Rho D, the two 3.7 minutes of arc away from A, is pretty clearly a line-of-sight coincidence. C's K5 class also puts it much too far away.

Then there is eighth-magnitude Rho Gem E, which is better known by its own name as the eighth-magnitude (7.8) class K2 variable star V 376 Geminorum. The variation of just a few percent is apparently caused by starspots swinging in and out of view as the star rotates. While separated by a huge angle of 12.6 minutes of arc, the partnership between A and E seems quite secure in that the two stars are at the same distance from us, that they are tracking each other through space, and that Rho E has just the right brightness for its class. More than 13,500 AU apart, the two must take more than a million years to orbit, making them a "fragile binary" indeed, one reminiscent of Proxima Centauri. Were one at Rho E, Rho A would shine with the brightness of up to some 15 times that of Venus, while Rho B would appear not much brighter than zeroth magnitude, the two at best roughly a quarter of a degree apart.
Written by Jim Kaler 3/04/11. Return to STARS.