What is an Open Cluster?

   A galactic cluster, or open cluster, is a group of associated stars that travel together through Earth's galaxy, the Milky Way. About 1,100 such clusters, each with a dozen to several hundred stars, have been catalogued. Almost all of them are located close to the plane of the galaxy. Open clusters range from about 5 to about 70 light-years in diameter. Often a thin, misty light, caused by reflection off cosmic dust and gas, envelops the entire cluster.
   Open clusters are classified by the number of stars in them and by the degree to which the stars are concentrated toward the center of the cluster. All clus­ters in the same class are roughly the same size.
A study of galactic clusters was carried out in 1930 by R. J. Trumpler of the Lick Observatory. His investi-gations provided the first clear evidence of the existence of cosmic dust and gas throughout the galaxy.

Open Star cluster Pleiades