Editing Open cluster (section)

==Numbers and distribution==
[[File:NGC 346 in Small magellanic cloud.jpg|thumb|[[NGC 346]], an open cluster in the [[Small Magellanic Cloud]]]]

There are over 1,100 known open clusters in our galaxy, but the true total may be up to ten times higher than that.<ref name=aaa389/> In [[spiral galaxy|spiral galaxies]], open clusters are largely found in the spiral arms where gas densities are highest and so most star formation occurs, and clusters usually disperse before they have had time to travel beyond their spiral arm. Open clusters are strongly concentrated close to the galactic plane, with a [[scale height]] in our galaxy of about 180&nbsp;light years, compared with a galactic radius of approximately 50,000&nbsp;light years.<ref name=aj108/>

In [[irregular galaxy|irregular galaxies]], open clusters may be found throughout the galaxy, although their concentration is highest where the gas density is highest.<ref name=pasp109/> Open clusters are not seen in [[elliptical galaxy|elliptical galaxies]]: Star formation ceased many millions of years ago in ellipticals, and so the open clusters which were originally present have long since dispersed.<ref name=binney_merrifield98/>

In the Milky Way Galaxy, the distribution of clusters depends on age, with older clusters being preferentially found at greater distances from the [[Galactic Center]], generally at substantial distances above or below the [[galactic plane]].<ref name=Friel/> [[Tidal force]]s are stronger nearer the center of the galaxy, increasing the rate of disruption of clusters, and also the giant molecular clouds which cause the disruption of clusters are concentrated towards the inner regions of the galaxy, so clusters in the inner regions of the galaxy tend to get dispersed at a younger age than their counterparts in the outer regions.<ref name=aaa88/>