Open Cluster
Open Cluster, or galactic cluster, group of associated stars that travel together through space. Astronomers have cataloged about 1200 open clusters in the earth’s galaxy, each containing from ten to many hundreds of stars (see Milky Way ). Astronomers have also discovered hundreds of open clusters in other galaxies. The stars in a galaxy usually orbit in a plane around a common center, and almost all open clusters lie close to the plane of the galaxy in which they’re located. Clusters range from about 5 light-years to about 70 light-years in diameter. Often a thin, misty light, caused by reflection of starlight off the cosmic dust and gas in the cluster, envelops an 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. The stars in an open cluster are usually relatively young, and the cluster is considered the same age