GAMMA SCT (Gamma Scuti). Modern Scutum (the Shield) is more or less defined by its two luminaries (such as they are), fourth magnitude Alpha and Beta Scuti (respectively 3.85 and 4.22), which lie at the northern and southern edges of the Scutum star cloud, an obvious naked-eye bright patch in the Milky Way south of Aquila. The rest of the Greek- lettered Scutum crowd rather indistinguishably lie scattered among fifth magnitude, including (at 4.70) Gamma Scuti, by far the southernmost of the set to carry one (and the only one without a Flamsteed number, all of which were assigned to Aquila since Flamsteed did not recognize the Shield as a formal constellation, and yes, it does get messy). The star's apparent dimness (and it will not stay that way, as seen below) is due to a substantial distance of 319 light years (give or take just 8), in spite of which -- and of the star's location within the Milky Way -- there is no evidence for dimming by intervening interstellar dust. A white class A star, originally known as an A3 hydrogen-fusing dwarf, the star's parameters (as given below) are much better fit by a later classification as an A1 subgiant-dwarf, so we will go with that. Since there are no temperature measures, we adopt 9400 Kelvin from the class, which with distance gives a luminosity of 113 times that of the Sun 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 to a rotation period of under one day (as opposed to nearly a month for the Sun). Quick spin makes for ellipsoidal distortion and temperature variation over the surface, making abundance analysis difficult. The theory of stellar structure and evolution then yields 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 is about to give up) core hydrogen fusion, thus validating the revised spectral class. Gamma Scuti is perhaps best known as defining one of four directions for the EROS2 "microlensing" project, in which millions of stars are monitored to detect intervening bodies that brighten the stars through "gravitational lensing" caused by the stars' local distortions of Einstein's spacetime (the others Beta Scuti, Theta Muscae, and Gamma Normae). Of more personal interest is that Gamma Sct is coming almost directly at us with a speed of some 41 kilometers per second. Calculations then show that 2.1 million years from now, Gamma Sct will pass only 14 light years away from us and shine at minus second magnitude (-2.1, almost twice as bright as Sirius is today), making it most likely to be the brightest star in a future sky.
Written by Jim Kaler 9/02/11. Return to STARS.