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.