CHI AQL (Chi Aquilae). "All the naked-eye stars are well
understood." Nobody actually said that, or would say it. It's a
raw fallacy, as witnessed by our star Chi Aquilae. Among the more
familiar figures in the sky is the three-star lineup centered on Altair, the first magnitude luminary of Aquila (the Eagle) and the southern anchor
of the Summer Triangle. Running
southeast to northwest, the tight trio is made of bright-fourth-
magnitude Alshain (Gamma Aql), Altair
(Alpha), and almost-second-magnitude Tarazed (Gamma), the line not quite five
degrees long. Extending it to the northwest by about the same
angular distance as between either of the pairs gets you to fifth
magnitude (5.3) Chi Aquilae, making it a four-star line.
Unfortunately, ease of location is the only straightforward thing
there is about the star. The rest is confusion, resulting in part
from Chi Aql being a close double whose members are quite
difficult to untangle. First, the combined magnitude is suspect.
The Bright Star Catalogue
(BSC) gives 5.27, Hipparcos 5.31 (not bad agreement actually),
while the Washington Double Star Catalog lists individual values of
5.37 and 6.57 for Chi Aql A and B, which sum to 5.06. Keeping just
to the tenth (5.3), we scale A and B down to 5.6 and 6.8. So far
so good. Then there is the distance, which is nominally 855 light
years, but with a mighty uncertainty of 115 light years (in part
because of the duplicity). Worse are the classes. The BSC gives
them as class F and A (dF3, an old way of expressing it as dwarf,
coupled to an A3 dwarf). Later notions made them out as A3 dwarf
plus G0 giant, while still
another source calls them B5.5 dwarf plus G2 supergiant! To add to the
mess, Chi is in the middle of the dark Great Rift of the Milky Way, and is probably afflicted with
dimming from interstellar dust. But without good classes to
estimate true color (dust reddening stars too, the reddening tied
to total absorption), it's impossible to say how much. If the
primary is an A3 dwarf, it is by far too bright, so for argument's
sake, adopt B5.5 (dwarf) plus G0 (giant) and zero absorption, guess
the temperature from the classes (14,500 and 5800 Kelvin: there are
no measurements), and out come luminosities of 800 and 115 Suns, which lead to 5 and 3 solar masses. With
a maximum observed separation of under a second of arc, the two
stars orbit at a minimum distance of 185 Astronomical units and
take at least 900 years to make their mutual circuit. But given
all the uncertainties (including that of interstellar dust
absorption, which if present would increase luminosities and
masses), if you believe any of this, as the American saying goes,
"we have a bridge we can sell you."
Not being satisfied, Chi Aql throws another one at us. The
listings make it into a veritable cluster with five more
components for a total of seven. Motions are small, so it's hard
to know which (if any!) actually belong to the inner pair. At
separations that range from 81 seconds of arc to 139 seconds are
three twelfth/thirteenth magnitude outliers (Chi Aql C, D, and E).
Accompanying "D" is "G," while "E" is split into a pair of 12th
magnitude members ("E" and "F") 8 seconds apart. If real and at
the distance of "AB," they are all roughly solar or just subsolar
in mass, separated by tens of thousands of AU from "AB" and have
orbital periods of more than a million years. None appears
trustworthy, scaling us back to our original duo, which just begs
for more study.
Written by Jim Kaler 8/26/11. Return to STARS.