KAPPA SER (Kappa Serpentis). Serpens, the Serpent, wraps around Ophiuchus, the Serpent Bearer. Divided in two, the Serpent's head (Serpens Caput) lies to the west of the Bearer, while the tail (Serpens Cauda) falls the east, the Greek letters also divided twixt the two parts. In the middle of the snake's rather squared-off head lies a fourth magnitude (4.09) reddish "eye," the class M (M0.5) red giant Kappa Serpentis. Temperature is a problem. Angular diameter measures combined with a distance of 348 light years give a physical diameter of 71 times that of the Sun and a calculated temperature of 3575 Kelvin, notably lower than expected for the class. From apparent brightness, distance, and this temperature (needed to assess the large amount of invisible infrared radiation), we find a luminosity of 1390 times that of the Sun and a much larger radius of 98 times solar. Using a more typical temperature of 3800 Kelvin for an M0.5 giant gives a lower luminosity of 820 solar and a more satisfactory radius of 66 times solar. With the lower temperature, the mass comes in at 2.5 times that of the Sun, while it is reduced to about double solar with the higher value. In either case, Kappa Serpentis stands out as a red giant at a peak luminosity at which it is about to fire up its core helium to fuse to carbon and oxygen (if it has not done so already), after which it will shrink and dim by some fifty percent while the helium merrily burns along. Such stars can be unstable, but there is no firm evidence for variability. One study suggests a very uncertain five percent variation over a 30 day period. Aside from its important lesson in stellar evolution, Kappa Ser (which has no known companions) is mostly used as a rather staid calibrator for those measuring angular diameters and magnetic activity.
Written by Jim Kaler 7/11/08. Return to STARS.