The Open Star Clusters (OC) are ragged and irregular groups of dozens or hundreds of stars that have a closer gravitational relation than that of the surrounding star field. These open clusters define the galactic equatorial plane and form a flattened disk-like system 1,000 parsecs thick and with a diameter of 10,000 parsecs. There is a distinct anti-center emphasis among open clusters with amaximum concentration of these objects toward a point 350 parsecs from our Sun, in the vicinity of galactic longitude 280°. The spatial distribution of the open clusters defines the spiral arm structure of our galaxy much as the globular clusters define the nucleus and spherical shape of the galactic system. The open clusters are much smaller than the globulars, with a maximum linear diameter for the largest of not over 15 parsecs, the smallest around 1.5 parsecs, and a range of 2-6 parsecs for those clusters of average size.
They are also much closer to us than the globulars, or at least the ones visible from our vantage point in the galaxy. The open clusters occur in the disk or plane of the galaxy along with the great concentration of gas and dust clouds. For this reason, we cannot see the more distant members of this group. Keep in mind that not only are the globular clusters intrinsically brighter, but their relatively higher galactic latitudes put them out of the dusty galactic plane and into view. All open clusters show a concentration to the plane of the galaxy with the exception of those clusters, which are situated so near to us that they appear projected in high galactic latitudes. The Coma Berenices cluster is one of these. While some 1000 open clusters are known, it is estimated that there are about 18,000 of these objects distributed throughout the galactic plane.
We have noted that the dense globular clusters are able to defy the disruptive tidal forces within our galaxy and survive forever in terms of the life of the galaxy, not so for the open clusters. The great disk of our galaxy revolves like some great wheel through space and time. While to us, this spinning disk appears stationary (we are like a flashbulb picture), it has a powerful motion in terms of the life span of stellar objects. The open clusters have much shorter life spans than do the globular clusters. The fact that so many open clusters are found tells us that these objects are continuously being born or formed. Otherwise they would have vanished from the galaxy long ago.
In fact, the open clusters are the Johnny Appleseeds of our galaxy. In endless formation, they arise together in clusters from the great dust and gas clouds and move together through space, away from their birthplace. They cohere or hang together as long as possible in defiance of the galactic tidal waves, yet suffer loss of star after star until all are dispersed in an endless trail across the galactic plane. The speed of "evaporation" of the cluster stars depends greatly on their density in the cluster. A dense, compact cluster will be much more stable than a cluster of low density. The Hyades cluster is perhaps safe for about a thousand million years, whereas the Pleiades and Praesepe may endure for ten times that period.
Copyright (c) 1997-99 Michael Erlewine