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GAMMA CMI (Gamma Canis Minoris). The classic outline ofCanis Minor,Orion's smaller hunting dog, is among the simplest inthe sky, consisting of a line between bright, first magnitudeProcyon and third magnitudeGomeisa (Beta CMi). The other stars, Gammathrough Zeta (plus a fewFlamsteed stars) within thissmall figure, do not much seem to count. Gamma maybe even less so,the fourth magnitude (4.32) class K (K3)giant rather hiding just two-thirds of a degree northeast of Beta, though at 318 light years(give or take 15) considerably farther. What Gamma lacks inluster, it makes up for in confusion, as it is a very closeunresolved "spectroscopic"double, the fainter one (Ab, "A" since there are also much fainterGamma B, C, and D) adding a very uncertain component to thebrighter (Aa), making Gamma itself difficult to assess. Rightly orwrongly, we make the assumption that the two have the same(estimated) temperature of 4400 Kelvin. After accounting for someinfrared radiation from the coolish surface, we find a totalluminosity of 265 times that of theSun. Ameasure of the light ratio, with Gamma Ab a quarter the visualbrightness of Gamma Aa (the actual K giant) gives 212 Suns for thebrighter of the pair and 53 for the fainter. The problem is thatif the temperatures are different, the visible brightness ratiowill not be the same as the ratio considering the TOTAL luminosity. The giant's radius then comes in at 25 solar and the mass at anapproximate 3 times that of the Sun, with an age of 400 millionyears. One source suggests that the other star is also evolvedfrom hydrogen-fusing dwarfhood, with a mass ratio Ab/Aa of 0.90,but we really do not know. There is no measure of rotation speed,though there is a suggestion that the iron abundance is a bit low. What we DO know is that the mutual orbital period of the pair is389.32 days, the synchrony to a year making full observationalcoverage of the orbit rather difficult. Assuming 3 solar massesfor the brighter and 2.7 for the fainter, the average separation is1.9 Astronomical Units, a precisely known eccentricity of 0.259taking them as close as 1.4 AU and as far from each other as 2.3AU. Other than all this, the star presents itself as a fineexample of a fake multiple. At separations of 27, 115, and 138seconds of arc lie 13th magnitude Gamma CMi B, 12th magnitude GammaC, and 10th magnitude Gamma D. Observations over long periods oftime show each moving far too much relative to Gamma A (ourconfusing close binary), clearly showing all to be just "optical"line-of-sight coincidences. Written byJim Kaler 3/19/10. Return toSTARS.
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