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.