UPS GEM (Upsilon Geminorum). In northeastern Gemini, just a bit over two degrees to the southwest of Pollux (the brighter and southeastern of the Castor and Pollux pair), lies rather anonymous mid-fourth-magnitude Upsilon Geminorum, whose initial characteristic is its orange color. A class M (M0) giant (even "bright giant"), the star bears roughly the same relation to Pollux as Rho Gem does to Castor (though at a somewhat different angle). Ups's cool class is consistent with a rather diverse array of published temperatures that average around 3900 Kelvin, which tells that most of the star's radiation comes out in the infrared part of the spectrum. From this consideration and a distance of 271 light years (give or take 15), we find the star to radiate at a rate of 421 times that of the Sun, which leads to a radius of 45 times solar. Published indirect estimates of angular diameter plus distance yield the same value. A measured projected equatorial rotation velocity of just over five kilometers per second gives a rotation period that could be as long as 435 days. Luminosity, temperature, and theory then suggest a mass of around double that of our Sun, though the value is uncertain, as in this region of the parameter realm stars of differing mass have similar characteristics. Upsilon Gem's evolutionary status is not very clear either. Most likely the star is getting brighter with a dead central carbon/oxygen core (brightening for the second time), though it you push the errors, it could be near the maximum of its first brightening with a dead helium core, the helium about to be ignited. Upsilon Gem thus retains a bit of a mystery about it. A thirteenth magnitude companion hovers 55 seconds of arc away. But over the past century it has changed position enough to say that it clearly just lies in the line of sight, Rho Gem itself moving along relative to the Sun at 49 kilometers per second, more than twice normal. That does not mean the star is alone. Ups Gem is a part of the Wolf 630 "moving group" that consists of stars that share a more or less common motion. Seemingly all over the sky, members include Omicron-1 Ori, BQ Gem, R Scl, 7 Ceti, R Leo, R CrB, 82 Aqr, 56 Peg, and 160 or so others. Moving groups are important. It seems that our Galaxy was built largely from mergers with other systems. Study of such groups can then provide us with something of a history of how our Milky Way came to be assembled. That is not to say that Wolf 630 represents a merged galaxy. The stars could simply have had some commonality in their birthplace. Nevertheless, they all seem somehow related.
Written by Jim Kaler 3/02/12. Return to STARS.