Editing Star cluster (section)

==Open cluster==
[[File:Pleiades large.jpg|thumb|The [[Pleiades]], an [[open cluster]] dominated by hot [[OB star|blue stars]] surrounded by [[reflection nebulosity]]]]
{{Main|Open cluster}}
Open clusters are very different from globular clusters. Unlike the spherically distributed globulars, they are confined to the [[galactic plane]], and are almost always found within [[spiral arm]]s. They are generally young objects, up to a few tens of millions of years old, with a few rare exceptions as old as a few billion years, such as [[Messier 67]] (the closest and most observed old open cluster) for example.<ref name="ArchinalHynes2003">{{cite book | author1 = Brent A. Archinal | author2 = Steven J. Hynes | date = 2003 | title = Star Clusters | publisher = Willmann-Bell | pages = | isbn = 978-0-943396-80-4 | url = https://books.google.com/books?id=2WJ3AAAACAAJ}}</ref> They form in [[H II region]]s such as the [[Orion Nebula]].

Open clusters typically have a few hundred members and are located in an area up to 30 light-years across. Being much less densely populated than globular clusters, they are much less tightly gravitationally bound, and over time, are disrupted by the gravity of [[giant molecular cloud]]s and other clusters. Close encounters between cluster members can also result in the ejection of stars, a process known as "evaporation".

The most prominent open clusters are the [[Pleiades]] and [[Hyades (star cluster)|Hyades]] in [[Taurus (constellation)|Taurus]]. The [[Double Cluster]] of [[NGC 869|h]]+[[NGC 884|Chi Persei]] can also be prominent under dark skies. Open clusters are often dominated by hot young blue stars, because although such stars are short-lived in stellar terms, only lasting a few tens of millions of years, open clusters tend to have dispersed before these stars die.

A subset of open clusters constitute a binary or aggregate cluster.<ref name=PeickaPaunzen2021>{{citation
 | title=Aggregates of clusters in the Gaia data
 | last1=Piecka | first1=M. | last2=Paunzen | first2=E.
 | display-authors=1
 | journal=Astronomy & Astrophysics
 | volume=649 | issue=A54 | pages=12 | date=May 2021
 | doi=10.1051/0004-6361/202040139 | doi-access=free | postscript=. | arxiv=2106.08920 | bibcode=2021A&A...649A..54P }}</ref>  New research indicates [[Messier 25]] may constitute a ternary star cluster together with NGC 6716 and Collinder 394.<ref name=Majaess2024>{{citation
 | title=A Rare Cepheid-hosting Open Cluster Triad in Sagittarius
 | last1=Majaess | first1=D. | last2=Turner | first2=D.
 | last3=Usenko | first3=I. | display-authors=1
 | journal=Research Notes of the AAS
 | volume=8 | issue=8 | pages=205 | date=August 2024
 | doi=10.3847/2515-5172/ad7139 | doi-access=free | bibcode=2024RNAAS...8..205M | postscript=. }}</ref>

Establishing precise distances to open clusters enables the calibration of the period-luminosity relationship shown by [[Cepheids]] [[variable stars]], which are then used as [[standard candle]]s. Cepheids are luminous and can be used to establish both the distances to remote galaxies and the expansion rate of the Universe ([[Hubble constant]]). Indeed, the open cluster NGC 7790 hosts three [[classical Cepheids]] which are critical for such efforts.<ref name=sa1958>{{cite journal |bibcode=1958ApJ...128..150S |doi=10.1086/146532 |title=Cepheids in Galactic Clusters. I. CF Cass in NGC 7790 |year=1958 |last1=Sandage |first1=Allan |journal=The Astrophysical Journal |volume=128 |page=150 |doi-access=free }}</ref><ref name=ma2013>{{cite journal |bibcode=2013A&A...560A..22M |doi=10.1051/0004-6361/201322670 |title=Anchors for the cosmic distance scale: The Cepheids U Sagittarii, CF Cassiopeiae, and CEab Cassiopeiae |year=2013 |last1=Majaess |first1=D. |last2=Carraro |first2=G. |last3=Moni Bidin |first3=C. |last4=Bonatto |first4=C. |last5=Berdnikov |first5=L. |last6=Balam |first6=D. |last7=Moyano |first7=M. |last8=Gallo |first8=L. |last9=Turner |first9=D. |last10=Lane |first10=D. |last11=Gieren |first11=W. |last12=Borissova |first12=J. |last13=Kovtyukh |first13=V. |last14=Beletsky |first14=Y. |journal=Astronomy & Astrophysics |volume=560 |pages=A22 |arxiv=1311.0865 |s2cid=55934597 }}</ref>

===Embedded cluster===
[[File:Chandra_X-ray_View_of_Orion.jpg|thumb|The [[embedded cluster|embedded]] Trapezium cluster seen in [[X-ray astronomy|X-rays]] which penetrate the surrounding clouds]]
{{main|Embedded cluster}}

Embedded clusters are groups of very young stars that are partially or fully encased in [[Cosmic dust|interstellar dust or gas]] which is often impervious to optical observations. Embedded clusters form in [[molecular clouds]], when the clouds begin to collapse and [[star formation|form stars]]. There is often ongoing star formation in these clusters, so embedded clusters may be home to various types of [[young stellar object]]s including [[protostar]]s and [[pre-main-sequence star]]s. An example of an embedded cluster is the [[Trapezium Cluster]] in the [[Orion Nebula]]. In [[Rho Ophiuchi cloud complex|ρ Ophiuchi cloud]] (L1688) core region there is an embedded cluster.<ref>{{cite journal|bibcode=1995ApJ...450..233G|title=An Infrared Spectroscopic Survey of the rho Ophiuchi Young Stellar Cluster: Masses and Ages from the H-R Diagram|journal=Astrophysical Journal |volume=450|pages=233|last1=Greene|first1=Thomas P|last2=Meyer|first2=Michael R|year=1995|doi=10.1086/176134}}</ref>

The embedded cluster phase may last for several million years, after which gas in the cloud is depleted by star formation or dispersed through [[radiation pressure]], [[stellar wind]]s and [[astrophysical jet|outflows]], or [[supernova explosion]]s. In general less than 30% of cloud mass is converted to stars before the cloud is dispersed, but this fraction may be higher in particularly dense parts of the cloud. With the loss of mass in the cloud, the energy of the system is altered, often leading to the disruption of a star cluster. Most young embedded clusters disperse shortly after the end of star formation.<ref name="Lada Lada 2003 pp. 57–115">{{cite journal | last1=Lada | first1=Charles J. | last2=Lada | first2=Elizabeth A. | title=Embedded Clusters in Molecular Clouds | journal=Annual Review of Astronomy and Astrophysics | volume=41 | issue=1 | year=2003 | issn=0066-4146 | doi=10.1146/annurev.astro.41.011802.094844 | pages=57–115| bibcode=2003ARA&A..41...57L | arxiv=astro-ph/0301540 | s2cid=16752089 }}</ref>

The open clusters found in the Galaxy are former embedded clusters that were able to survive early cluster evolution. However, nearly all freely floating stars, including the [[Sun]],<ref name="Gounelle Meynet p=A4">{{cite journal | last1=Gounelle | first1=M. | last2=Meynet | first2=G. | title=Solar system genealogy revealed by extinct short-lived radionuclides in meteorites | journal=Astronomy & Astrophysics | publisher=EDP Sciences | volume=545 | date=2012-08-27 | issn=0004-6361 | doi=10.1051/0004-6361/201219031 | page=A4| bibcode=2012A&A...545A...4G | arxiv=1208.5879 | s2cid=54970631 }}</ref> were originally born into embedded clusters that disintegrated.<ref name="Lada Lada 2003 pp. 57–115" />