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.