Editing Open cluster (section)

==Studying stellar evolution==
[[File:Open cluster HR diagram ages.gif|thumb|[[Hertzsprung–Russell diagram]]s for two open clusters. [[NGC 188]] is older, and shows a lower turn-off from the [[main sequence]] than that seen in [[Messier 67|M67]].]]

When a [[Hertzsprung–Russell diagram]] is plotted for an open cluster, most stars lie on the [[main sequence]].<ref name=orsa/> The most massive stars have begun to evolve away from the main sequence and are becoming [[red giant]]s; the position of the turn-off from the main sequence can be used to estimate the age of the cluster.<ref name=BOB4767>{{cite book |last1=Carroll |first1=B. W. |last2=Ostlie |first2=D. A. |title=An Introduction to Modern Astrophysics |edition=2nd |publisher=[[Cambridge University Press]] |date=2017 |isbn=978-1-108-42216-1 |pages=476–477}}</ref>

Because the stars in an open cluster are all at roughly the same distance from [[Earth]], and were born at roughly the same time from the same raw material, the differences in apparent brightness among cluster members are due only to their mass.<ref name=orsa/> This makes open clusters very useful in the study of stellar evolution, because when comparing one star with another, many of the variable parameters are fixed.<ref name=BOB4767/>

The study of the abundances of [[lithium]] and [[beryllium]] in open-cluster stars can give important clues about the evolution of stars and their interior structures. While [[hydrogen]] nuclei cannot fuse to form [[helium]] until the temperature reaches about 10 million&nbsp;[[Kelvin|K]], lithium and beryllium are destroyed at temperatures of 2.5 million&nbsp;K and 3.5 million&nbsp;K respectively. This means that their abundances depend strongly on how much mixing occurs in stellar interiors. Through study of their abundances in open-cluster stars, variables such as age and chemical composition can be fixed.<ref name=pasp116/>

Studies have shown that the abundances of these light elements are much lower than models of stellar evolution predict. While the reason for this underabundance is not yet fully understood, one possibility is that [[convection]] in stellar interiors can 'overshoot' into regions where [[radiant energy|radiation]] is normally the dominant mode of energy transport.<ref name=pasp116/>