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.