GAMMA LIB (Gamma Librae). On the bright side of fourth magnitude
(3.91), Gamma Librae anchors an eastern corner of the traditional
figure of Libra, the Scales, which in
ancient times held the autumnal
equinox in its weighing pan. Gamma Lib, however, has the
misfortune to be not only a common sort of star, a class G (G8.5)
helium-fusing giant (some say
a K0 giant, making it even more common), but has to compete with two
of the most beloved stars, or at least star names, in the sky: Zubenelgenubi (Alpha Librae, to the west of
Gamma) and Zubeneschamali (Beta Librae,
to the northwest of Gamma), which in mythology represent the
outstretched claws of Scorpius, the
Scorpion, found to the southeast of Libra. Yet common as the star
is, an air of mystery lingers about it. Various "Zubens" (from
"claw" in Arabic) have been applied to it, Gamma at one time or another
garnering Zuben Elekrab, Zuben Hakraki, Zuben el Hakrabi. Since
we've already assigned a version of the latter to Sigma Librae, it's best to let any proper
names go and just use Bayer's Gamma. A look at the star itself
deepens the mystery. Observation from decades ago makes the star out
to be a close double, its
components nearly equal (differing by 0.2 magnitudes) and separated
by 0.1 second of arc. Yet no modern observation, including
spectroscopic, has confirmed such duplicity. So for now we proceed
as if Gamma is single. An accurate distance of 163 light years (good
to just 2) and a well-determined temperature of 4730 Kelvin (needed
to assess the amount of infrared radiation) leads to a
luminosity of 82 times that of the Sun and
a radius of 13.5 times solar. There is no measure of rotation.
Application of the theory of stellar structure and evolution suggests
a mass around 2.5 times that of the Sun (not all that far from 2.15
solar masses as previously measured) and clearly shows the star to
be a "clump giant" that is quietly fusing helium into carbon and
oxygen and that joins a large number of others with similar
properties. Having spent some 580 million years as a hotter class
A hydrogen-fusing dwarf, Gamma is now around 700 million years old.
The radius presents yet another problem. Direct measure of angular
diameter gives radii that range from close to that given above to
twice as large. (If Gamma Lib were composed of two nearly identical
stars, which seems unlikely, the luminosity of each would be 41 Suns,
the radii about 10 solar, the masses of each would drop to about double
solar, and they would be some 50 AU apart.) Though Gamma Librae
itself does not seem to be a close double, it does indeed appear to
have a companion. At a current separation of 42.5 seconds of arc,
eleventh magnitude Gamma Librae B has been tracking its giant mate
pretty well since the first measure in 1878. From its brightness
and distance, "B" is probably a K7 dwarf with a mass a bit above
half solar. If the two are truly related, Gamma Lib B must be at
least 2100 Astronomical Units from "A" and has to take more than
55,000 years to orbit. Remarkably, such fragile links are fairly
common. Given the above separation, from Gamma B, Gamma A would be
just a bright stellar point six times brighter than our full Moon, while from Gamma A, Gamma B
would shine with the light of eight times that of Venus at her best.
Written byJim Kaler 1/27/14. Return to STARS.