DELTA AQL (Delta Aquilae). One of the prominent third magnitude (3.36) stars that help make the extended southern portion of Aquila (the Eagle) southwest of Altair, Delta plays three other roles. It is also the western-most star of the three-star string that the Arabs called "Al Mizan," the "Scale Beam" (the others Eta Aql in the middle, Theta to the east). As such, we could emulate Sagittarius where the three stars that make the Bow are Kaus Borealis, Kaus Media, and Kaus Australis (for the northern, middle, and southern stars). Delta might then carry its own proper name of "Almizan Occidental," a term never used, so we'll drop it here. Delta plays an equally strong role as representing part of the body of a now-defunct constellation called Antinous (his other stars Theta, Iota, Kappa, Lambda, Nu, Sigma, and Eta), who was honored as companion to the Roman emperor Hadrian by placing him as flying with the Eagle. The pattern, no longer formally recognized, is unusual in being neither "ancient" nor "modern," but in-between, having been noted by Ptolemy, appearing on Bayer's Uranometria," but then finally dropped. The star is fairly close to us, only 50 light years away. Classed as a mid-temperature (7610 Kelvin) F (F3) subgiant (one just beginning to become a giant), Delta shines with the light of 8.2 Suns, almost all of it in the visual spectrum (there being no corrections for ultraviolet of infrared), that and temperature giving it a radius of 1.5 times solar. A projected equatorial rotation velocity of 88 kilometers per second gives a rotation period of less than 0.9 days. Then we get a bit of controversy. The theory of stellar structure and evolution indicates a mass 1.65 times that of the Sun, but show also the Delta Aql is clearly a dwarf that is somewhat past the middle of its hydrogen-fusing lifetime of five billion years (half that of the Sun). Controversies and confusion then accelerate. The star was once thought to be double from variations in its spectrum that indicated a companion with a period of a mere four hours, but further work suggests simple variation from low-level pulsation. It also seems to be a "Delta Scuti" variable (multi-periodic, low level variables of class F), but with just one measured period of 1.05 days and a variation of just 0.003 magnitudes (the four day period not surfacing). But it may yet have a companion, as wobbles in the motion across the sky suggest one with a period of 3.422 years and an orbital radius of 0.9 Astronomical Units. These numbers are not at all compatible with the stellar mass, so the companion remains doubtful. But then maybe there is a real companion, an 11th magnitude star 110 seconds of arc away. But in 81 years, the two stars separated by 14 seconds of arc, far too much for orbital motion, rendering the "companion" yet another line-of-sight coincidence. Factoring in all the data, Delta is probably a single rather weak example of a Delta Scuti variable.
Written by Jim Kaler 10/05/07. Return to STARS.