G SCO (G Scorpii). This fairly bright third magnitude (3.21),
rather ordinary, orange class K (K2) giant does not have a lot going
for it physically, but it makes up for the lack with its odd name.
Greek letters were assigned to
the stars of the ancient constellations by Johannes Bayer in his
great atlas, the Uranometria of 1603. (Others did the stars
of the modern figures.) When he ran out of Greek, he started in on Roman letters, a few
of which survive. But "G Sco" is not one of his creations. The
star, just to the east of the "Stinger" of
Scorpius (made of Shaula and Lesath
near the southern boundary of the constellation), is among the
brightest that carry no Greek letter name. Curiously, it is
ghosted in without name on the Uranometria plates for both Scorpius and neighboring Sagittarius. Why Bayer ignored it while still entering
it on his maps is a mystery. When the ancient constellations were
supplemented, in some cases assaulted, by the moderns (which were
invented in the 18th and 19th centuries), the star became Gamma
Telescopii, of Telescopium (the
Telescope), assigned by the Abbe Nicolas de Lacaille (1713-1762).
But constellation boundaries were at the time flexible and depended
on the astronomer involved. Others kept the star un-named in
Scorpius. Finally the prominent 19th century astronomer B. A.
Gould (1824-1896), the founder of the still-vital Astronomical
Journal, called it G Sco simply because a star that bright
should have some kind of common name. Why "G" is another mystery,
but it was probably picked in remembrance of it once having been
Lacaille's "Gamma."
At a very well determined distance of 126 light years (known to
half a unit) and a temperature of 4540 Kelvin, the star shines with
a luminosity of 104 times that of the Sun,
a fair bit coming out in the infrared, which yields a radius of
16.6 times solar. Direct measure of angular diameter gives a
radius of 17.6 Suns, just six percent greater. Mass is always a
problem for giant stars like this one, which are quietly fusing
their core helium into carbon and oxygen, as luminosity and
temperature are not very sensitive to it, and stars over a large
mass range look rather alike. The best estimate is around double
that of the Sun. But oscillations to the rescue. Subtle
vibrations of the star observed with the WIRE satellite lead to a
very well determined mass of 1.44 Suns, which leads to an age of
about three billion years. G Sco is listed as having two companions, one of 14th
magnitude at a separation of 200 seconds of arc, the other 15th at
a distance of 40. Both are almost certainly line-of-sight
coincidences, not surprising given the density of stars in the
background Milky Way. "G" does, though, have
one very nice other thing going for it. It's the gateway to the
rich and quite beautiful naked-eye open cluster Messier 7, which
lies just 2.5 degrees to the north. Actually, the cluster is so prominent, that it could almost be called
the gateway to finding G! (The history of the name is in part
taken from Lost Stars by Morton Wagman, McDonald and
Woodward, Blacksburg VA, 2003. Thanks to Jerry Diekmann for
suggesting the star.)
Written by Jim Kaler 7/02/10. Return to STARS.