Editing Open cluster (section)

{{Short description|Large group of stars less bound than globular clusters}}
{{redirect-distinguish|Galactic cluster|galaxy cluster}}{{Infobox astronomical formation|caption=The Pleiades is one of the most famous open clusters.|image=File:Pleiades.jpg|name=Open cluster|thing=Loose cluster of stars|commonscat=Open clusters|qid=Q11387|density=~ 1.5 stars / cubic {{abbr|ly|light year}}|size=< 30 ly in diameter}}
An '''open cluster''' is a type of [[star cluster]] made of tens to a few thousand [[star]]s that were formed from the same [[giant molecular cloud]] and have roughly the same age. More than 1,100 open clusters have been discovered within the [[Milky Way]] galaxy, and many more are thought to exist.<ref name="seds" /> Each one is loosely bound by mutual [[gravity|gravitational attraction]] and becomes disrupted by close encounters with other clusters and clouds of gas as they orbit the [[Galactic Center]]. This can result in a loss of cluster members through internal close encounters and a dispersion into the main body of the galaxy.<ref name="Karttunen_et_al_03" /> Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years. In contrast, the more massive [[globular cluster]]s of stars exert a stronger gravitational attraction on their members, and can survive for longer. Open clusters have been found only in [[spiral galaxy|spiral]] and [[irregular galaxy|irregular galaxies]], in which active [[star formation]] is occurring.<ref name="payne-gaposchkin79" />

Young open clusters may be contained within the molecular cloud from which they formed, illuminating it to create an [[H II region]].<ref name=apj62/> Over time, [[radiation pressure]] from the cluster will disperse the molecular cloud. Typically, about 10% of the mass of a gas cloud will coalesce into stars before radiation pressure drives the rest of the gas away.

Open clusters are key objects in the study of [[stellar evolution]]. Because the cluster members are of similar age and [[chemical element|chemical composition]], their properties (such as distance, age, [[metallicity]], [[Extinction (astronomy)|extinction]], and velocity) are more easily determined than they are for isolated stars.<ref name="seds" /> A number of open clusters, such as the [[Pleiades]], the [[Hyades (star cluster)|Hyades]] and the [[Alpha Persei Cluster]], are visible with the naked eye. Some others, such as the [[Double Cluster]], are barely perceptible without instruments, while many more can be seen using [[binoculars]] or [[optical telescope|telescopes]]. The [[Wild Duck Cluster]], M11, is an example.<ref name=neata09/>