Editing Open cluster (section)

==Eventual fate==
[[File:Nursery of New Stars - GPN-2000-000972.jpg|left|thumb|[[NGC 604]] in the [[Triangulum Galaxy]] is a very massive open cluster surrounded by an [[H II region]].]]

Many open clusters are inherently unstable, with a small enough mass that the [[escape velocity]] of the system is lower than the average [[velocity]] of the constituent stars. These clusters will rapidly disperse within a few million years. In many cases, the stripping away of the gas from which the cluster formed by the radiation pressure of the hot young stars reduces the cluster mass enough to allow rapid dispersal.<ref name=apj235_1/>

Clusters that have enough mass to be gravitationally bound once the surrounding nebula has evaporated can remain distinct for many tens of millions of years, but, over time, internal and external processes tend also to disperse them. Internally, close encounters between stars can increase the velocity of a member beyond the escape velocity of the cluster. This results in the gradual 'evaporation' of cluster members.<ref name=pasp110/>

Externally, about every half-billion years or so an open cluster tends to be disturbed by external factors such as passing close to or through a molecular cloud. The gravitational [[tidal force]]s generated by such an encounter tend to disrupt the cluster. Eventually, the cluster becomes a stream of stars, not close enough to be a cluster but all related and moving in similar directions at similar speeds. The timescale over which a cluster disrupts depends on its initial stellar density, with more tightly packed clusters persisting longer. Estimated cluster [[half life|half lives]], after which half the original cluster members will have been lost, range from 150–800 million years, depending on the original density.<ref name=pasp110/>

After a cluster has become gravitationally unbound, many of its constituent stars will still be moving through space on similar trajectories, in what is known as a [[stellar association]], moving cluster, or [[Stellar kinematics|moving group]]. Several of the brightest stars in the '[[Big Dipper|Plough]]' of [[Ursa Major]] are former members of an open cluster which now form such an association, in this case the [[Ursa Major Moving Group]].<ref name=aj105_1/> Eventually their slightly different relative velocities will see them scattered throughout the galaxy. A larger cluster is then known as a stream, if we discover the similar velocities and ages of otherwise well-separated stars.<ref name=aspcs92/><ref name=baas38/>