IC 4665, the “HI” Star Cluster

The open star cluster IC 4665 in the constellation Ophiuchus.

The open star cluster IC 4665 in the constellation Ophiuchus (credit: Wikipedia)

One of the best targets for a quick stargazing session this time of year is the pretty but underappreciated open star cluster IC 4665. It’s a snap to find, beautiful to behold, and like most sky sights on Cosmic Pursuits, it’s visible from the northern and southern hemispheres.

IC 4665 is spread out over a full degree, more than twice the diameter of the full Moon, so it looks fainter than its integrated magnitude of 4.7. In dark sky, IC4665 is just barely visible to the unaided eye roughly 1° NE of the star Celebrai in the constellation Ophiuchus. Celebrai, or β (beta) Ophiuchi, is one of the stars in the distinctive asterism called Taurus Poniatowski, the “little bull”. If you’re battling light pollution, you’ll need binoculars to spot the cluster.

In binoculars, you’ll see perhaps a dozen stars; a small telescope at 25-35x shows a few dozen blue-white stars. At an age of 35 million years, this little cluster is young compared to robust grand-dad clusters like the Beehive (M44) which looks similar but is more than 600 million years old.  The youth of IC 4665 means few if any stars have evolved into red giants or supergiants.  So the color of its brightest stars is fairly uniform blue, blue-white, and white.

Since the cluster doesn’t need much optics to resolve, you won’t be surprised to know it was discovered long ago, in 1745 by Philippe Loys de Cheseaux, a quarter century before Messier compiled his famed list.  Caroline Herschel saw it too.

Location of the star cluster IC 4665 in the constellation Ophiuchus and NE of the asterism called Taurus Poniatowski.

Location of the star cluster IC 4665 in the constellation Ophiuchus and NE of the asterism called Taurus Poniatowski.

If southwest is “up” in your field of view when you look at IC 4665, look carefully at the inner stars. They form the pattern of the word “HI”, like a big friendly cosmic greeting.  While not obvious at first, it’s a little unnerving when the pattern finally jumps out at you!   This image (from AllTheSky.com) shows the “HI” reversed: move your mouse over the image to reveal the annotated circle around the cluster.  Within the yellow circle, the “I” is on the lower left, and the “H” is on the upper right. If you’re observing with binoculars or a telescope with a star diagonal, the “HI” pattern is backyards, but you can still easily make it out.

IC 4665 is a little unusual because it’s about 15° above the plane of the Milky Way.  Most young open star clusters are along the plane of our galaxy.  The cluster is about 1,100 light years away and some 20 light-years across.

 

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