Standing along the stream of the summer Milky Way is the large constellation Ophiuchus, the Serpent Bearer. According to mythology, Ophiuchus was the physician who accompanied Jason and the Argonauts in their quest for the Golden Fleece. Appropriately, he is depicted in the sky as holding a serpent, a symbol of wisdom and healing.
Aside from his serpent, Ophiuchus bears many fine examples of nearly every type of deep sky object. Some are big and splashy, while others appear as modest enhancements of the background star field. IC 4665 lies somewhere in between.
This large open cluster just northeast of Beta Ophiuchi is a nice binocular sight, consisting of several dozen 7th- and 8th-magnitude stars. Though its stellar density is not very high, IC 4665 stands out well thanks to the sparseness of its environment. On dark, clear nights, and just with the naked eye, the cluster may be seen as a hazy spot measuring nearly two Full Moons across.
IC 4665 is still an attractive star cluster in a telescope, at low magnification – not from any virtue of concentration, which it most certainly lacks, but from the uniform brilliance of its brightest stars, which with larger apertures are really bright. The group appears as a ring with a short “handle” on its northwestern side and a single bright star almost at its center. The “handle” consists of four progressively fainter stars arching northwest and then west away from the ring.
The stars of IC 4665 are scattered over a 1° area; because the cluster is about 1,400 light-years distant, its full diameter must therefore be around 30 light-years. IC 4665 is anywhere from 30 to 40 million years old – relatively young in astronomical terms! This places it somewhere between the age of the Jewel Box cluster in the southern constellation Crux and the Pleiades.
Finder map – field width 15°, stars to magnitude +8.5.