The Flying Unicorn Cluster
Posted by Brian Ventrudo
The constellation Aquila, the Eagle, has few sights for a small telescope. Which is surprising, since the constellation lies along the northern Milky Way between the rich star clouds of Cygnus and the galactic opulence of Sagittarius. But a handful of clusters and nebulae fleck patches of stars in Aquila that stand out from the dark clouds of the Great Rift of the Milky Way. Perhaps the finest open cluster in the constellation is NGC 6709. It’s a sprawling irregular group of stars which, to some stargazers, looks like a sparkling unicorn.
NGC 6709 lies off the northern wing of Aquila near the intersection with the constellations Hercules and Ophiuchus. Look for the group about 5º SW of ζ (zeta) Aquilae. It forms a narrow right triangle with zeta and ε (epsilon) Aquilae (see map above).
This 7th-magnitude cluster is easily visible in 7×50 binoculars, and higher-magnification binoculars begin to resolve the group. In a 3-inch or larger telescope the cluster bursts open like a small backyard firework, with chains and loops of stars spread over a quarter degree of sky. The cluster reveals detail at all magnifications. At 25-40x, you can take in the overall shape of the group and enjoy the dense tapestry of background stars. Medium and high magnifications reveal, to the patient observer, numerous double stars among the cluster’s loops and chains.
At low power, look for two irregular groups to the east and west of the cluster’s sparse center. The western group looks like an irregular “V” while the eastern appears as an inverted “J” with fainter loops to the west. Let your eye and imagination relax, and you may see the western “V” as the front leg, head, and horn of a unicorn, with the rounded body and rear leg to the east. Look carefully at the image at the top of the page. The unicorn is facing right…
NGC 6709 is about 3,900 light years away and spans about 17 light years. Spectroscopic studies of the group’s 110 members suggest it formed about 300 million years ago.