Among the various types of deep sky objects, open star clusters are usually the poor cousins. They simply don't excite us as much as other objects. Many of them are so loose and star-poor that they're even difficult to identify as discrete objects. But a few are so dense with stars that we enjoy observing them again and again.
The constellation Cassiopeia sits astride the winter Milky Way in an area filled with open clusters. This hints at a unique aspect of these objects - they are also referred to as "galactic clusters" because they're concentrated within the plane of our galaxy. The richest open clusters have hundreds of stars. Contrast that with hundreds of thousands of members in many globular clusters, which reside in a halo at the outskirts of our galaxy. Stars tend to form in clusters, with open clusters being among the youngest objects in space. The fact that the vast majority of stars we see are not in clusters is due to the fact that over time the gravitational field of a galaxy subjects clusters to disintegration. And open clusters being so loose are least likely to survive for very long in a galaxy.
Some clusters, such as the Pleiades and Beehive, are bright enough to be easily visible to the naked eye, but are so loose that they appear more interesting in binoculars or a rich field telescope. Three of the most highly concentrated clusters that lend themselves to telescopic observation are M11 in Scutum, the Double Cluster (NGC 869/884) in Perseus, and NGC 7789 in Cassiopeia. The last two are in an area of the sky that is particularly well suited for observation in the fall and winter months.
The Double Cluster is familiar to all amateur astronomers - it's easy to locate even in light polluted skies. Although it's located within Perseus, I find it easiest by following an imaginary line from Gamma (?) Cassiopeia roughly through Delta (?), then double that distance and the cluster will be visible in a finder scope. The star-hopping chart here from the Cartes du Ciel program shows the route. In a dark sky, the cluster is a naked eye object. It's a wonder that Charles Messier never included the Double Cluster in his famous catalog. A wide field eyepiece gives it a more concentrated aspect and frames it nicely against the background. Look for three M-class orange-colored stars in NGC 884 and a few others surrounding the cluster. It's unusual to find older stars like these in a galactic cluster.NGC 7789 was discovered by Caroline Herschel in the 18th Century and while not as familiar to many amateur astronomers, I think it's one of the finer objects in the sky. The stars that compose this rich cluster are fainter than in many clusters; the brightest members are only about 11th magnitude. As shown in the above chart, it can be located by following a line from Kappa (?) Cassiopeia through Beta (?). I first saw it about twenty years ago when observing with Greg Lutes from a dark site in western Illinois. Greg noticed a naked eye fuzzy patch near the "W" of Cassiopeia and when we turned our telescopes on it, we were amazed to see this rich cluster with stars arranged in what appeared to be concentric circles with no central condensation. NGC 7789, with about 1000 members, is much richer and older than most galactic clusters. That unusual aspect leads some to consider it as intermediate between a galactic cluster and a less condensed globular. But studies have shown that it is a true galactic cluster. The photo of NGC 7789 is from the Palomar Sky Survey. It actually stands out better from the background than what this image portrays, especially in a dark sky.
The problem for observers is that open clusters by their very nature are set against the star-filled background of the Milky Way, so they often are hard to identify. I found that one answer is to use a small rich-field telescope or a very wide field eyepiece in a larger scope. The finder chart on the previous page shows only the brightest clusters, though there are many more in the area. This is a good hunting ground to add objects to your observing log. Here are my condensed observing notes on a few other objects that are shown in the chart:
M103 - Appears as a small, loose cluster including a bright double star and fainter background stars. Easily identified among field stars.
NGC 129 - Stands out in a small rich field scope as six fairly bright stars against a background of stardust. Includes the star DL, a Cepheid variable.
NGC 457 - A loose elongated cluster with a prominent orange-colored star toward one side.
NGC 654 - A small, loose, inconspicuous cluster composed of relatively faint stars.
Stock 2 - A large cluster with an abundance of bright stars; stands out well from field stars.