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G SCO (G Scorpii). This fairly bright third magnitude (3.21),rather ordinary, orange class K (K2)giant does not have a lot goingfor it physically, but it makes up for the lack with its odd name.Greek letters were assigned tothe stars of the ancient constellations by Johannes Bayer in hisgreat atlas, theUranometria of 1603. (Others did the starsof the modern figures.) When he ran out ofGreek, he started in on Roman letters, a fewof which survive. But "G Sco" is not one of his creations. Thestar, just to the east of the "Stinger" ofScorpius (made ofShaula andLesathnear the southern boundary of the constellation), is among thebrightest that carry no Greek letter name. Curiously, it isghosted in without name on the Uranometria plates for bothScorpius and neighboringSagittarius. Why Bayer ignored it while still enteringit on his maps is a mystery. When the ancient constellations weresupplemented, in some cases assaulted, by the moderns (which wereinvented in the 18th and 19th centuries), the star became GammaTelescopii, ofTelescopium (theTelescope), assigned by the Abbe Nicolas de Lacaille (1713-1762). But constellation boundaries were at the time flexible and dependedon the astronomer involved. Others kept the star un-named inScorpius. Finally the prominent 19th century astronomer B. A.Gould (1824-1896), the founder of the still-vitalAstronomicalJournal, called it G Sco simply because a star that brightshould have some kind of common name. Why "G" is another mystery,but it was probably picked in remembrance of it once having beenLacaille's "Gamma."
At a very well determined distance of 126 light years (known tohalf a unit) and a temperature of 4540 Kelvin, the star shines witha luminosity of 104 times that of theSun,a fair bit coming out in the infrared, which yields a radius of16.6 times solar. Direct measure of angular diameter gives aradius of 17.6 Suns, just six percent greater. Mass is always aproblem for giant stars like this one, which are quietly fusingtheir core helium into carbon and oxygen, as luminosity andtemperature are not very sensitive to it, and stars over a largemass range look rather alike. The best estimate is around doublethat of the Sun. But oscillations to the rescue. Subtlevibrations of the star observed with the WIRE satellite lead to avery well determined mass of 1.44 Suns, which leads to an age ofabout three billion years. G Sco is listed as having twocompanions, one of 14thmagnitude at a separation of 200 seconds of arc, the other 15th ata distance of 40. Both are almost certainly line-of-sightcoincidences, not surprising given the density of stars in thebackgroundMilky Way. "G" does, though, haveone very nice other thing going for it. It's the gateway to therich and quite beautiful naked-eyeopen cluster Messier 7, whichlies just 2.5 degrees to the north. Actually, thecluster is so prominent, that it could almost be calledthe gateway to finding G! (The history of the name is in parttaken fromLost Stars by Morton Wagman, McDonald andWoodward, Blacksburg VA, 2003. Thanks to Jerry Diekmann forsuggesting the star.) Written byJim Kaler 7/02/10. Return toSTARS.
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