ZUBENHAKRABI (Sigma Librae = Gamma Scorpii). There are a small
number of "linking stars" that traditionally belong to two
constellations. Alpheratz, Alpha
Andromedae, is also Delta Pegasi, while Elnath, Beta Tauri, is also Gamma Aurigae
(the two alternatives no longer used since the stars are within the
modern boundaries of Andromeda and
Taurus). Zubenhakrabi is more extreme.
The star, whose name means "the scorpion's claw," was originally
part of Scorpius along with the more-
traditional "claws" Zubeneschamali and
Zubenelgenubi (which refer to the
"northern" and "southern" claw). As such, Bayer called it Gamma
Scorpii. But it was so far west of Scorpius proper and so much a
part of Libra that in the nineteenth
century B. A. Gould (who in 1851 founded the "Astronomical Journal,"
one of the world's premier research publications) gave it to Libra
as Sigma Librae, which is how it is referred to today. To add to
the confusion, early in that century Elijah Burritt coupled the
name "Zubenhakrabi" to Eta Librae! The star is as
interesting as the story of its name. Notably fainter than
Zubenelgenubi or Zubeneschamali, third magnitude (3.29)
Zubenhakrabi is a cool class M (M3) rather-luminous red giant.
From a distance of 290 light years, it radiates 1900 solar
luminosities from a reddish 3600 Kelvin surface that is swollen to
a radius 110 times that of the Sun (0.52
astronomical units, which would take the star about halfway between
the orbits of Mercury and Venus). It is a subtle "semi-regular
variable" (in the astro-trade, an "SRb") that changes its
brightness by only 0.16 magnitudes over a 20-day period. This
dying 2 (?) solar mass star, with its dead carbon-oxygen core, is
expanding and brightening as a giant for the second time (the first
brightening was with a dead helium core) fueled by internal
nuclear-burning shells of helium and hydrogen. It is on its way to
becoming a much larger, brighter, Mira-type
"long-period variable" that will eventually slough off its outer
envelope, its now-quiet carbon-oxygen core to become yet another of
the white dwarfs (a lower mass version of Sirius-B) that flock around us. The star
has been well-examined for various quirks, but comes away clean,
with no indication of a surrounding dusty shell or any anomalies in
surface chemical composition, although both may well pop up in the
distant future.