Three Sadly-Neglected Star Clusters in Ophiuchus

Last week, we examined the odd little asterism called Taurus Poniatowski, an obsolete constellation that’s now part of Ophiuchus.  Though this asterism is not in a star-rich region of the Milky Way, it does host a few lovely star clusters.  The best among them is called IC4665, a splendid but little-known open cluster that looks stunning in binoculars or a small telescope.  Here’s how to see it…

In dark sky, IC4665 is just visible to the unaided eye roughly 1 degree NE of Celebrai.  If you’ve got a little light pollution, you’ll need binoculars to spot it.  The cluster 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 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 IC4665 means few if any stars have evolved into red giants or supergiants.  So the color of its stars is fairly uniform.

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

Three star clusters near Taurus Poniatowski in the constellation Ophiuchus (click to enlarge)

If southwest is “up” in your field of view when you look at IC4665, 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.

IC 4665 is a little unusual because it’s about 15 degrees 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

If it’s so darned pretty and easy to see, why isn’t IC4665 better known?  Well, it doesn’t have a very catchy name, I’m sure you’ll agree.  And it’s a little too faint to be easily seen with the unaided eye.  It’s also a little too spread out to see easily in a telescope.  But never mind.  Try to find this cluster for yourself just above the little bull in Ophiuchus.  It presents a lovely spray of stars in a modest pair of binoculars for observers north or south, city or country.

A bonus: in this same area, you’ll find two more dispersed open clusters IC4756 and NGC 6633.  Steve O’Meara calls these two clusters Tweedledee and Tweedle-dum, since they are chubby and fat with stars and seemingly match quite closely.  These clusters were also known by de Cheseaux and Caroline Herschel, though neither had the wide-field instruments to enjoy their splendour.