Star Cluster - Open Clusters

Open Clusters

Open clusters (OC) are very different from globular clusters. Unlike the spherically distributed globulars, they are confined to the galactic plane, and are almost always found within spiral arms. They are generally young objects, up to a few tens of millions of years old, with a few rare exceptions as old as a few billion years, such as Messier 67 (the closest and most observed old open cluster) for example. They form from H II regions such as the Orion Nebula.

Open clusters usually contain up to a few hundred members, within a region up to about 30 light-years across. Being much less densely populated than globular clusters, they are much less tightly gravitationally bound, and over time, are disrupted by the gravity of giant molecular clouds and other clusters. Close encounters between cluster members can also result in the ejection of evaporation'.

The most prominent open clusters are the Pleiades and Hyades in Taurus. The Double Cluster of h+Chi Persei can also be prominent under dark skies. Open clusters are often dominated by hot young blue stars, because although such stars are short-lived in stellar terms, only lasting a few tens of millions of years, open clusters tend to have dispersed before these stars die.