MU AQL (Mu Aquilae). Wander around through the Milky Way in Vulpecula with a telescope or binoculars and you may come across a charming cluster" called "the Coathanger." It's an extreme example of a "non-cluster," a chance alignment of differently-colored stars. At the other extreme of alignment is the region around Mu Aquilae, a rather non-descript common fifth magnitude (4.45) class K (K3) giant that lies some 4.5 degrees west-southwest of Altair, the luminary of Aquila, the celestial eagle. K giants are common as dirt. Most of them are stars that have used their internal hydrogen fuel, the cores now slowly heating and contracting in size as the stars grow larger and cooler on the outside, or have fired up their internal helium to fuse to carbon and oxygen. This one, Mu Aquilae, is no different. Take the K giants from the sky and many of the constellations would nearly disappear or at least be severely altered. Imagine for example Taurus without Aldebaran, Bootes without Arcturus. And what happened to that second twin, Pollux, anyway? (Though we must admit that removing Mu Aql from Aquila's outline would do little harm.) They are all going to slough off their outer hydrogen envelopes and reveal their inner, now-dead cores as nascent white dwarfs, perhaps producing expanding shells, planetary nebulae, along the way. At a distance of 108 light years (with an uncertainty of just 1), Mu Aql has a typical temperature of 4520 Kelvin, which means that a fair amount of infrared radiation must be added to the visible starlight to get a luminosity of 25 times that of the Sun, which in turn yields a radius of 8.1 times that of the Sun. That's not much for a "giant," but then neither is the mass very high, at most just one and half Suns. Even the metal content is near-solar as well. The only thing that's a little off about the star is the velocity of 48 kilometers per second relative to the Sun, some three times normal. What brings us back to the Coathanger is that Mu Aquilae is surrounded by "companions" of tenth to thirteenth magnitude (Mu Aql B through F), and one's first reaction is that we've found a "mini-cluster" with Mu Aql A at the top. Measurements of motion, however, disabuse one of such a notion. What IS odd is that in looking at this loose swarm as a cluster, we inadvertently seem to have found a REAL double, as Mu Aql B and C (each about a minute of arc from Mu-A) have almost identical motions relative to A and are probably a real pair, but of unknown distance and type. Best perhaps to go back and admire the real Coathanger before the motions of its member stars dissipate, in the case of Mu Aql leaving the real double of Mu Aql behind.

Written byJim Kaler 8/25/17. Return to STARS.