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GAMMA SCT (Gamma Scuti). ModernScutum(the Shield) is more or less defined by its two luminaries(such as they are), fourth magnitudeAlpha andBetaScuti (respectively 3.85 and 4.22), which lie at the northernand southern edges of theScutum starcloud, an obvious naked-eye bright patch in theMilky Way south ofAquila. The rest of theGreek-lettered Scutum crowd rather indistinguishably lie scatteredamong fifth magnitude, including (at 4.70) Gamma Scuti, by far thesouthernmost of the set to carry one (and the only one without aFlamsteed number, all of whichwere assigned to Aquila since Flamsteed did not recognize theShield as a formalconstellation, and yes,it does get messy). The star's apparent dimness (and it will notstay that way, as seen below) is due to a substantial distance of319 light years (give or take just 8), in spite of which -- and ofthe star's location within the Milky Way -- there is no evidencefor dimming by intervening interstellar dust. A white class Astar, originally known as an A3 hydrogen-fusing dwarf, the star'sparameters (as given below) are much better fit by a laterclassification as an A1 subgiant-dwarf, so we will go with that. Since there are no temperature measures, we adopt 9400 Kelvin fromthe class, which with distance gives a luminosity of 113 times thatof theSun and a radius of 4.0 times solar. One of Gamma Sct's outstanding properties is very rapid rotation,at least 223 kilometers per second at the equator, which leads toa rotation period of under one day (as opposed to nearly a monthfor the Sun). Quick spin makes for ellipsoidal distortion andtemperature variation over the surface, making abundance analysisdifficult. The theory of stellar structure and evolution thenyields a mass of 2.7 to 2.8 times solar and shows the 450-million-year-old star to be a true subgiant that has just given up (or isabout to give up) core hydrogen fusion, thus validating the revisedspectral class. Gamma Scuti is perhaps best known as defining oneof four directions for the EROS2 "microlensing" project, in whichmillions of stars are monitored to detect intervening bodies thatbrighten the stars through "gravitational lensing" caused by thestars' local distortions of Einstein's spacetime (the others BetaScuti, Theta Muscae, andGammaNormae). Of more personal interest is that Gamma Sct is comingalmost directly at us with a speed of some 41 kilometers persecond. Calculations then show that 2.1 million years from now,Gamma Sct will pass only 14 light years away from us and shine atminus second magnitude (-2.1, almost twice as bright asSirius is today), making it most likely tobe the brightest star in a future sky. Written byJim Kaler 9/02/11. Return toSTARS.
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