Star clusters are large groups of individual stars that are centered around the same gravity force. Each individual star may be located near the center of the gravitational pull or along the outskirts. Together the group of stars resemble a larger circle. There are several main types of star clusters, including global and open clusters.
The main characteristic that identifies star clusters is the fact that each star in the cluster travels around the same center. That common center is a force of gravity that binds all of the stars together. Star clusters are also known as systems and may be a part of a larger galaxy, such as the Milky Way.
Since the same gravitational force binds clusters of stars together, the center of star clusters tends to be quite bright. That brightness seems to dissipate as groups of clustered stars settle further away from the center. Despite the fact that the same force binds the stars, a star cluster does not remain stationary. Over time, these clusters can move away from each other or separate into two segments.
An open star cluster is more vulnerable to these separations. These clusters tend to contain stars that are younger and can be broken up by molecular clouds. Open star clusters are also characterized by a smaller amount of stars. Clusters continue to move and expand over a period of time and the individual stars can keep the same orbit direction if they become separated.
If a star cluster does become separated, the individual stars can be grouped together as an association. This is a group of stars that are moving in the same direction, but are not gathered together around a common center. An example of this would be the earth's sun and the other stars in the solar system that are considered to be "neighbors."
Open clusters tend to contain stars that are made up of heavy particles. This is due to the fact that these particles remain when stars become supernovas and die. Since stars in an open cluster are younger, they tend to use this material when they form.
In contrast to open clusters, global star clusters are made up of older stars. This type of cluster usually contains a great number of stars that are tightly bound together. In one galaxy there may be several hundred of these clusters. Global clusters are considered to have been formed when the galaxies first came into existence.