Editing Open cluster (section)

==Formation==
[[File:Trapezium cluster optical and infrared comparison.jpg|thumb|[[Infrared]] light reveals the dense open cluster forming at the heart of the [[Orion nebula]].]]

The formation of an open cluster begins with the collapse of part of a [[giant molecular cloud]], a cold dense cloud of gas and dust containing up to many thousands of times the [[solar mass|mass of the Sun]]. These clouds have densities that vary from 10<sup>2</sup> to 10<sup>6</sup> molecules of [[neutral hydrogen]] per cm<sup>3</sup>, with star formation occurring in regions with densities above 10<sup>4</sup> molecules per cm<sup>3</sup>. Typically, only 1–10% of the cloud by volume is above the latter density.<ref name=ptrsa368_1913_713/> Prior to collapse, these clouds maintain their mechanical equilibrium through magnetic fields, turbulence and rotation.<ref name=araa25_23/>

Many factors may disrupt the equilibrium of a giant molecular cloud, triggering a collapse and initiating the burst of star formation that can result in an open cluster. These include shock waves from a nearby [[supernova]], collisions with other clouds and gravitational interactions. Even without external triggers, regions of the cloud can reach conditions where they become unstable against collapse.<ref name=araa25_23/> The collapsing cloud region will undergo hierarchical fragmentation into ever smaller clumps, including a particularly dense form known as [[infrared dark cloud]]s, eventually leading to the formation of up to several thousand stars. This star formation begins enshrouded in the collapsing cloud, blocking the protostars from sight but allowing infrared observation.<ref name=ptrsa368_1913_713/> In the Milky Way galaxy, the formation rate of open clusters is estimated to be one every few thousand years.<ref name=mnras249/>

[[File:Eagle nebula pillars.jpg|right|thumb|The so-called "[[Pillars of Creation]]", a region of the [[Eagle Nebula]] where the molecular cloud is being evaporated by young, massive stars]]
The hottest and most massive of the newly formed stars (known as [[OB star]]s) will emit intense [[ultraviolet radiation]], which steadily ionizes the surrounding gas of the giant molecular cloud, forming an [[H II region]]. [[Stellar wind]]s and [[radiation pressure]] from the massive stars begins to drive away the hot ionized gas at a velocity matching the speed of sound in the gas. After a few million years the cluster will experience its first [[core-collapse supernova]]e, which will also expel gas from the vicinity. In most cases these processes will strip the cluster of gas within ten million years, and no further star formation will take place. Still, about half of the resulting protostellar objects will be left surrounded by [[circumstellar disk]]s, many of which form accretion disks.<ref name=ptrsa368_1913_713/>

As only 30 to 40 percent of the gas in the cloud core forms stars, the process of residual gas expulsion is highly damaging to the star formation process. All clusters thus suffer significant infant weight loss, while a large fraction undergo infant mortality. At this point, the formation of an open cluster will depend on whether the newly formed stars are gravitationally bound to each other; otherwise an unbound [[stellar association]] will result. Even when a cluster such as the Pleiades does form, it may hold on to only a third of the original stars, with the remainder becoming unbound once the gas is expelled.<ref name=mnras321_4_699/> The young stars so released from their natal cluster become part of the Galactic field population.

Because most if not all stars form in clusters, [[star cluster]]s are to be viewed as the fundamental building blocks of galaxies. The violent gas-expulsion events that shape and destroy many star clusters at birth leave their imprint in the morphological and kinematical structures of galaxies.<ref name=kroupa05/> Most open clusters form with at least 100 stars and a mass of 50 or more solar masses. The largest clusters can have over 10<sup>4</sup> solar masses, with the massive cluster [[Westerlund 1]] being estimated at 5 × 10<sup>4</sup> solar masses and [[R136]] at almost 5 x 10<sup>5</sup>, typical of globular clusters.<ref name=ptrsa368_1913_713/> While open clusters and globular clusters form two fairly distinct groups, there may not be a great deal of intrinsic difference between a very sparse globular cluster such as [[Palomar 12]] and a very rich open cluster. Some astronomers believe the two types of star clusters form via the same basic mechanism, with the difference being that the conditions that allowed the formation of the very rich globular clusters containing hundreds of thousands of stars no longer prevail in the Milky Way.<ref name=apj480/>

It is common for two or more separate open clusters to form out of the same molecular cloud. In the [[Large Magellanic Cloud]], both [[Hodge 301]] and [[R136]] have formed from the gases of the [[Tarantula Nebula]], while in our own galaxy, tracing back the motion through space of the [[Hyades (star cluster)|Hyades]] and [[Praesepe]], two prominent nearby open clusters, suggests that they formed in the same cloud about 600 million years ago.<ref name=mnras120/> Sometimes, two clusters born at the same time will form a binary cluster. The best known example in the Milky Way is the [[Double Cluster]] of NGC 869 and NGC 884 (also known as h and χ Persei), but at least 10 more double clusters are known to exist.<ref name=aaa302/> New research indicates the [[Cepheid]]-hosting [[Messier 25|M25]] may constitute a ternary star cluster together with NGC 6716 and Collinder 394.<ref name=Majaess2024/>  Many more binary clusters are known in the [[Small Magellanic Cloud|Small]] and Large Magellanic Clouds—they are easier to detect in external systems than in our own galaxy because [[projection effect (disambiguation)|projection effect]]s can cause unrelated clusters within the Milky Way to appear close to each other.