An open cluster is a system of related stars that usually originates from a common molecular cloud, comprised of dust and cosmic gases. Typically, an open cluster of stars originates in galaxies where active star formation is taking place, such as in spiral or irregular galaxies. Since they are present within a galaxy’s disk, open clusters may also be referred to as galactic clusters. Open cluster stars are usually relatively small in size, containing an average of several hundred to a few thousand individual stars.
Approximately 1100 open clusters have been identified in the Milky Way Galaxy, although many scientists believe this to be only a small amount of the total number of open clusters that actually exist. Open clusters can sometimes be seen with the naked eye. Pleiades, Hyades, and the Alpha Persei clusters are all examples of visible open clusters. Other clusters, such as the Double Cluster, can only be seen using telescopes or binoculars.
The stars within an open cluster are relatively young in age, usually less than a few hundred million years old. Scientists can estimate the age of open clusters by examining the ratio of blue, yellow, and red stars within the clusters. Clusters that have more blue stars are usually younger than clusters with more red and yellow stars.
As a general rule, each open cluster star shares several key characteristics. Given this, open clusters can offer scientists and astrophysicists the ability to more easily study the impact of variables on star properties. The stars in an open cluster are usually comparable in both age and initial chemical make-up. In addition, these stars are typically spaced at roughly the same distance from the earth.
Open cluster stars ordinarily have different masses. These masses range in size, with the largest stars in a young cluster comprising approximately 80 to 100 solar masses. Smaller stars can be less than 0.08 solar masses.
Open clusters are usually bound to one another loosely by gravity. They can, however, become disturbed as they orbit the galactic center. This disruption is often caused when they experience close encounter with gas clouds or other clusters. They may also lose individual stars through internal close encounters.
Some scientists estimate that open clusters are lost after about a billion years, with some stars drifting to the far side of the galaxy and others to the near side. The time period for this disbursement can vary depending on the cluster’s initial density. A tightly-packed cluster frequently lasts for a longer period of time than a loosely-packed cluster.