EPS SCT (Epsilon Scuti). Between Aquila and Sagittarius lies the modern constellation Scutum (the Shield), which is renown for an especially bright patch in the Milky Way called the Scutum star cloud whose brightest part is bordered rather raggedly at the northern end by Beta Scuti and on the southern side by fifth magnitude (4.90) Epsilon Scuti. Oddly, Epsilon and Beta Sct share a property: both are relatively rare class G "bright giants," a type of star intermediate between ordinary giants and supergiants, Beta subclass G5, Epsilon a G8, which, at a temperature of 4920 Kelvin, is about that expected. An additional claim to fame (such as it is) is that along with Alpha, Beta, Delta, and Eta Sct (restricting the list to Greek letter names), Epsilon lies within "The Box," a remarkable 6 X 8 (EW) degree rectangle defined by Lambda Aquilae near its northeast corner that contains six open clusters named in the New General Catalogue (NGC), four variable stars (including Delta and R Scuti), two planetary nebulae, and one NGC globular cluster. As augured by its spectral class (as a G8 bright giant), Epsilon itself is no weakling. After accounting for a distance of 538 light years (give or take 57), temperature (to assess the amount of infrared light), and not quite half a magnitude of dimming by interstellar dust, we find a luminosity of 460 times that of the Sun, which gives a radius of 30 Suns, 35 percent the size of Mercury's orbit. A projected equatorial rotation speed of not quite six kilometers per second gives a rotation period that could be as long as 255 days. Theory then tells that Eps Sct carries a substantial mass of 4 times that of the Sun. Some 180 million years ago, the star began life as a blue-white class B6 hydrogen-fusing dwarf. That phase ended 15 million years ago when the core hydrogen ran out, leaving Epsilon now as an ordinary, though massive, helium- fusing bright giant, the helium fusing to a mix of carbon and oxygen. After the helium runs out, the star will brighten and then die as a fairly massive (about 0.8 solar) white dwarf. The star is surrounded by a rather large number of faint "companions" that range from Eps Sct B through "F." "D" and "E," at 15 and 43 seconds separation, have but one measure and we can for now reject them. "B" is moving far too rapidly (from 20 to 14 seconds separation in 24 years) to be anything other than in the line-of-sight. The closest to belonging are Eps C and F. If 14th magnitude Eps Sct C, 39 seconds of arc distant, were "real," it would be a K6 dwarf with a minimum 6500-AU-wide, 150,000-year orbit. But the separation increase can be nicely accounted for by Epsilon A's motion across the line of sight. The same can be said for F, leaving no other option but to think the star is in fact all alone.
Written by Jim Kaler 9/16/11. Return to STARS.