ZUBENELGENUBI (Alpha Librae). Dim Libra, which 2000
years ago held the autumnal equinox in its balance pans, is
identified chiefly by two stars to the northwest of Scorpius that have delightful names, Zubeneschamali and Zubenelgenubi. They
harken back to the ancient times when they were considered the
outstretched claws of the Scorpion, making the two something of a
double constellation. "Zubenelgenubi" derives from an Arabic
phrase meaning the "Scorpion's southern claw," while the name of
its mate (to which it is not physically related) refers to the
northern claw. Bright third magnitude (2.75) and somewhat dimmer than
Zubeneschamali, Zubenelgenubi still received the Alpha designation.
Rather like Mizar and Alcor, it is a naked
eye double, flanked four minutes of arc (0.06 degree) to the
northwest by a fifth magnitude (5.15) companion. The two are probably a
physical pair, as they move through space together, both of them 77
light years away from us. The fainter (called Alpha-1 because it
is the westerly of the two) is a class F (F4) star that
at about 6700 Kelvin is 1000 Kelvin
hotter than the Sun, while the brighter
(Alpha-2) is a much warmer white class A (A3) subgiant with a temperature of
8500 Kelvin. The two are separated by at least 5500 Astronomical
Units, nearly 140 times Pluto's distance from the Sun, and maybe
more since we do not know the exact difference in distance. At
that separation, the orbital period would be over 200,000 years.
From Alpha-2, Alpha-1 would appear as a brilliant star of the minus
tenth magnitude, 100 times brighter than Venus does in our sky.
From Alpha-1, Alpha-2 would be 10 times brighter yet and rival our
full Moon. Alpha-2, the brighter, is deceptive however. It is
itself a double made of two class A stars, one 45 percent brighter
than the other. From Earth they are a mere hundredth of a second
arc, only a few tenths of an astronomical unit, apart, comparable
to Mercury's distance from the Sun. Even from Alpha-1 they would
be inseparable with a human eye. There is some evidence that this
triple star system belongs to a hugely extended group of stars
(the "Castor Moving Group") that
move together through space and that include Castor, Vega, and Fomalhaut. Zubenelgenubi proper (Alpha-
2) has an enhancement of metals in its atmosphere most likely
caused by separation of elements, some rising up, others drifting
down, the culprit probably the brighter of the very close pair that
makes it up.