A group of stars formed together in the spiral arms of a galaxy; sometimes called a galactic cluster. Open clusters are usually irregular in shape and contain anything from a few dozen to several hundred relatively young stars in a volume up to 50 l.y. across. The Hyades and the Pleiades are well-known examples. Open clusters are divided into various types according to the Trumpler classification. They are more loosely packed than globular clusters, but may still have a central star density of up to 10 000 times that in the Sun’s neighbourhood. Over 2000 are known, all in the galactic disk. Their ages range from a few million to several billion years, and the youngest are still surrounded by traces of the nebula from which they formed. The less dense of them are gradually disrupted by gravitational interaction with the rest of the Galaxy.

http://www.wilton.unifei.edu.br/ocdb/ Database of open clusters.