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.