71 AQL (71 Aquilae). Aquila's Flamsteed numbers extend from fifth magnitude 4 Aquilae (the story told there), which lies just above the celestial equator in the far western part of the constellation southwest of Altair, to fourth magnitude (4.32) 71 Aql, which sits just below the equator in Aquila's far eastern portion almost on the border with Aquarius. Separated by more than 25 degrees, the two provide us with a fine contrast, 4 Aquilae a B9 dwarf with a radiating disk, 71 Aql an apparent triple star dominated by an evolving class G (G8) giant. The triple nature of 71 Aql is, however, illusory. At a distance of 337 light years (give or take 26), eleventh magnitude 71 Aql B, set 35 seconds of arc away, is moving too fast relative to 71 Aql A to be a real, gravitationally bound, companion, and is clearly just a line of sight coincidence. The remaining duplicity, however, is real, 71 A orbited by a close companion (detected spectroscopically through its induced movement in 71 proper) every 205.2 days. From its distance and temperature of 5000 Kelvin (needed to adjust for a small amount of infrared radiation), 71 Aquilae glows at a luminosity just over 200 times that of the Sun (after some adjustment for the companion), which leads to a radius of 19 times solar (not all that much for a "giant" star) and from theory a mass of about 3.5 Suns. The best guess is the 71 Aql is not yet a true giant, but is just finishing its transition from being a hydrogen-fusing dwarf to the point where it is about ready to undergo serious core contraction as it brightens as a genuine giant, a star that will eventually stabilize itself by fusing its helium core into a mix of carbon and oxygen. The nature of the companion can be found through a combination of the wobbles it introduces both spectroscopically (back and forth) and in position (from side to side). It appears to be a coolish class A dwarf (say A8) with a mass of perhaps 1.7 times that of the Sun. Kepler's Laws then suggest an orbital radius of about 1.2 Astronomical Units. Now about 250 million years old, 71 Aql A will eventually die as a white dwarf of about three quarters of a solar mass, while the companion (with a total dwarf lifetime of 1.8 billion years) will follow at 0.6 solar, provided of course that the two do not interact during their respective evolutions, which they probably will.

Written by Jim Kaler 9/27/13. Return to STARS.