C399
Published on Sep 5, 2006 at 11:35 am.
No Comments.
Filed under astronomy, skywatching.
This time of year is one of the best for viewing my favorite star cluster. Interistingly, it was noted by Messier, but never cataloged. Likewise, Herschel noted, and so did Dreyer, but it didn’t make it into the NGC catalog, either. In fact, most astronomers never really thought of it as a star cluster. However, Swedish astronomer Per Collinder included it in his 1931 catalog of open star clusters. At that time, clusters were determined by physical proximity of stars, and this one normally hadn’t made most lists due to a dearth of stars. But it never was clear that this was an actual star cluster. 
Collinder 399 is also known as Brocchi’s Cluster (after a prominent American amateur astronomer at the beginning of the 20th Century, D. F. Brocchi). But, most people know it as the Coathanger Cluster, because it looks a lot like a coathanger. Here is my sketch of the cluster.
Sadly, it turns out that this isn’t really a star cluster after all. Around the middle of the 20th Century, it was shown that several of the brightest stars neatly fit an H-R diagram, indicating that they are roughly the same distance away (about 420 lightyears). But, by the 1970’s and 1980’s, further studies showed that the dimmer stars are not really associated with the rest of group. Furthermore, not even all of the brighter stars share a common motion in space, something that would be true of a star cluster. So, rather than being a real open star cluster, this is merely an asterism — a random apparent pattern of stars. But that doesn’t make it any less interesting to look at.
It is also a very easy to find object. It is located in the constellation Vulpecula. However, Vulpecula (the Fox) is a very dim constellation, and I always have trouble with it. I simply know that it is between Sagitta (the Arrow) and Cygnus (the Swan). 
I have included a finder chart for it.  In very dark skies, you can even pick it out with the naked eye. The brighter stars are about magintude 5, and the stars along the base of the coathanger are at the naked eye limit, but they make what looks like a little fuzz in the area. It shows up great in binoculars. In fact, that is probably the best way to look at C399. It is huge (for a star cluster), being nearly three times the diameter of the Full Moon across. You can easily see it in a telescope’s finder scope, but normally it is not so impressive through the telescope. Most telescopes have too much magnification to thet the entire object.
So, if you get clear skies, go look for Collinder 399, the Coathanger Cluster. Even if it isn’t really a star cluster, then it is fun to look at.
-Astroprof
