RHO BOO (Rho Bootis). Though separated by 12 minutes of arc (0.2) degrees, Ursa Major's Mizar and Alcor are most likely a real orbiting pair, and thus give something of an imprimatur of reality to other reasonably close stellar pairings. Two of them are found in the constellations Bootes, Rho Boo 0.8 degrees from Sigma, W Boo 0.6 degrees from Izar (Epsilon Boo). In these cases, however, we are mightily fooled, as the individuals of the pairs are at vastly different distances and thus bear no relation to each other. At a distance of 150 light years, Rho Boo, a class K (K3) giant, is three times Sigma's distance. With a temperature of 4400 Kelvin, it radiates with a luminosity 112 times that of the Sun, which in turn yields a radius 17.9 times solar. The measured angular diameter of 0.0038 seconds of arc gives a similar radius of 18.6 solar. A very slow equatorial rotation speed of at least 1.3 kilometers per second yields a rotation period of at most 1.89 years (the uncertainty caused by our not knowing the axial tilt). Luminosity and temperature plus the rules of stellar structure and evolution then lead to a mass between 2 and 2.5 times that of the Sun. Though Rho Boo seems at first to be just another stable helium-fusing star of the general pack (the "clump"), it is more likely to be in a current state of transition, either brightening with a dead helium core (giving an age of roughly a billion years) and a shell of fusing hydrogen, dimming after just having fired its helium core to burn to carbon and oxygen, or brightening for the second time with a dead carbon- oxygen core (in which case it is a bit older). The lack of variability seems to rule out the last option. Less important is a lower-than-solar, but not unusual, metal content (about 75 percent). Rho Boo appears to have a 12th magnitude companion 42 seconds of arc away. However, the two are separating far faster than orbital motion would allow. The apparent duplicity is thus -- like Rho and Sigma Boo -- just another line-of-sight coincidence.
Written by Jim Kaler. Return to STARS.