Editing Open cluster (section)

==Stellar composition==
[[File:Tarantula nebula detail.jpg|thumb|A cluster of stars a few million years old at the lower right illuminates the [[Tarantula Nebula]] in the [[Large Magellanic Cloud]].]]

Because open clusters tend to be dispersed before most of their stars reach the end of their lives, the light from them tends to be dominated by the young, hot blue stars. These stars are the most massive, and have the shortest lives, a few tens of millions of years. The older open clusters tend to contain more yellow stars.<ref>{{Cite web |author1=Stefanie Waldek |date=2021-10-06 |title=What are star clusters? |url=https://www.space.com/star-clusters |access-date=2024-07-19 |website=Space.com |language=en}}</ref>

The frequency of [[binary star]] systems has been observed to be higher within open clusters than outside open clusters. This is seen as evidence that single stars get ejected from open clusters due to dynamical interactions.<ref>{{cite journal|doi=10.3847/1538-4357/ac1585|title=Long-term Spectroscopic Survey of the Pleiades Cluster: The Binary Population|year=2021|last1=Torres|first1=Guillermo|last2=Latham|first2=David W.|last3=Quinn|first3=Samuel N.|journal=The Astrophysical Journal|volume=921|issue=2|page=117|arxiv=2107.10259|bibcode=2021ApJ...921..117T|s2cid=236171384 |doi-access=free }}</ref>

Some open clusters contain hot blue stars which seem to be much younger than the rest of the cluster. These [[blue straggler]]s are also observed in globular clusters, and in the very dense cores of globulars they are believed to arise when stars collide, forming a much hotter, more massive star. However, the stellar density in open clusters is much lower than that in globular clusters, and stellar collisions cannot explain the numbers of blue stragglers observed. Instead, it is thought that most of them probably originate when dynamical interactions with other stars cause a binary system to coalesce into one star.<ref name=baas35/>

Once they have exhausted their supply of [[hydrogen]] through [[nuclear fusion]], medium- to low-mass stars shed their outer layers to form a [[planetary nebula]] and evolve into [[white dwarf]]s. While most clusters become dispersed before a large proportion of their members have reached the white dwarf stage, the number of white dwarfs in open clusters is still generally much lower than would be expected, given the age of the cluster and the expected initial mass distribution of the stars. One possible explanation for the lack of white dwarfs is that when a [[red giant]] expels its outer layers to become a planetary nebula, a slight asymmetry in the loss of material could give the star a 'kick' of a few [[kilometre per second|kilometres per second]], enough to eject it from the cluster.<ref name=apj595/>

Because of their high density, close encounters between stars in an open cluster are common.{{Citation needed|date=April 2018}} For a typical cluster with 1,000 stars with a 0.5&nbsp;parsec half-mass radius, on average a star will have an encounter with another member every 10 million years. The rate is even higher in denser clusters. These encounters can have a significant impact on the extended circumstellar disks of material that surround many young stars. Tidal perturbations of large disks may result in the formation of massive planets and [[brown dwarf]]s, producing companions at distances of 100&nbsp;[[Astronomical Unit|AU]] or more from the host star.<ref name=apj717_1_577/>