Editing Globular cluster (section)

==Formation==
[[File:NGC 2808 HST.jpg|thumb|right|alt=Thousands of white-ish dots scattered on a black background, strongly concentrated towards the center|[[NGC 2808]] contains three distinct generations of stars.<ref>{{cite journal |last1=Piotto |first1=G. |last2=Bedin |first2=L. R. |last3=Anderson |first3=J. |last4=King |first4=I. R. |last5=Cassisi |first5=S. |last6=Milone |first6=A.P. |last7=Villanova |first7=S. |last8=Pietrinferni |first8=A. |last9=Renzini |first9=A. | title=A Triple Main Sequence in the Globular Cluster NGC 2808
 | journal=The Astrophysical Journal | volume=661| issue=1
 | pages=L53–L56 |date=May 2007
 | doi=10.1086/518503 | bibcode=2007ApJ...661L..53P |arxiv = astro-ph/0703767 |s2cid=119376556 }}</ref><br />''NASA image'']]

The formation of globular clusters is poorly understood.<ref name="gratton">{{cite journal |last1=Gratton |first1=Raffaele |last2=Bragaglia |first2=Angela |last3=Carretta |first3=Eugenio |last4=D'Orazi |first4=Valentina |last5=Lucatello |first5=Sara |last6=Sollima |first6=Antonio |title=What is a globular cluster? An observational perspective |journal=The Astronomy and Astrophysics Review |date=2019 |volume=27 |issue=1 |page=8 |doi=10.1007/s00159-019-0119-3 |bibcode=2019A&ARv..27....8G | arxiv=1911.02835|s2cid=207847491 }}</ref> Globular clusters have traditionally been described as a simple star population formed from a single [[giant molecular cloud]], and thus with roughly uniform age and [[metallicity]] (proportion of heavy elements in their composition). Modern observations show that nearly all globular clusters contain multiple populations;<ref name="bastian">{{cite journal |last1=Bastian |first1=Nate |last2=Lardo |first2=Carmela |title=Multiple Stellar Populations in Globular Clusters |journal=Annual Review of Astronomy and Astrophysics |date=September 14, 2018 |volume=56 |issue=1 |pages=83–136 |doi=10.1146/annurev-astro-081817-051839 |bibcode=2018ARA&A..56...83B|arxiv=1712.01286 |s2cid=59144325 }}</ref> the globular clusters in the [[Large Magellanic Cloud]] (LMC) exhibit a bimodal population, for example. During their youth, these LMC clusters may have encountered giant molecular clouds that triggered a second round of star formation.<ref name="iau258">{{cite conference | last=Piotto | first=Giampaolo |date=June 2009 | title=Observations of multiple populations in star clusters | work=The Ages of Stars, Proceedings of the International Astronomical Union, IAU Symposium | volume=258 | pages=233–244 | doi=10.1017/S1743921309031883 | bibcode=2009IAUS..258..233P | arxiv=0902.1422}}</ref> This star-forming period is relatively brief, compared with the age of many globular clusters.<ref>{{cite news |author1=Weaver, D. |author2=Villard, R. |author3=Christensen, L. L. |author4=Piotto, G. |author5=Bedin, L. | title=Hubble Finds Multiple Stellar 'Baby Booms' in a Globular Cluster | publisher=Hubble News Desk | date=May 2, 2007 | url=http://hubblesite.org/newscenter/archive/releases/2007/18/full/ | access-date=May 1, 2007 }}</ref> It has been proposed that this multiplicity in stellar populations could have a dynamical origin. In the [[Antennae Galaxy]], for example, the Hubble Space Telescope has observed clusters of clusters{{snd}}regions in the galaxy that span hundreds of parsecs, in which many of the clusters will eventually collide and merge. Their overall range of ages and (possibly) metallicities could lead to clusters with a bimodal, or even multimodal, distribution of populations.<ref>{{cite journal |author1=Amaro-Seoane, P. |author2=Konstantinidis, S. |author3=Brem, P. |author4=Catelan, M. | title=Mergers of multimetallic globular clusters: the role of dynamics | journal=[[Monthly Notices of the Royal Astronomical Society]] | date=2013 | volume=435 | issue=1 | pages=809–821 | bibcode=2013MNRAS.435..809A | doi=10.1093/mnras/stt1351 |doi-access=free |arxiv = 1108.5173 |s2cid=54177579 }}</ref>
[[File:The globular star cluster Messier 54.jpg|thumb|right|alt=A small fuzzy white ball in the center of a speckled black backdrop|Globular star cluster [[Messier 54]]<ref>{{cite press release |last1=Mucciarelli |first1=Alessio |last2=Christensen |first2=Lars Lindberg |title=This Star Cluster Is Not What It Seems|url=http://www.eso.org/public/news/eso1428/|publisher=[[European Southern Observatory]]|access-date=April 7, 2021 | date=September 10, 2014 | id=eso1428}}</ref>]]

Observations of globular clusters show that their stars primarily come from regions of more efficient star formation, and from where the interstellar medium is at a higher density, as compared to normal star-forming regions. Globular cluster formation is prevalent in [[Starburst (astronomy)|starburst]] regions and in [[Interacting galaxy|interacting galaxies]].<ref>{{cite journal |author1=Elmegreen, B. G. |author2=Efremov, Y. N. | title=A Universal Formation Mechanism for Open and Globular Clusters in Turbulent Gas | journal=Astrophysical Journal | date=1999 | volume=480 | issue=2 | pages=235–245 | bibcode=1997ApJ...480..235E | doi=10.1086/303966 | doi-access=free }}</ref> Some globular clusters likely formed in dwarf galaxies and were removed by tidal forces to join the Milky Way.<ref name="apj613">{{cite journal|last=Lotz|first=Jennifer M.|author-link=Jennifer Lotz|author2=Miller, Bryan W.|author3=Ferguson, Henry C.|date=September 2004|title=The Colors of Dwarf Elliptical Galaxy Globular Cluster Systems, Nuclei, and Stellar Halos|journal=The Astrophysical Journal|volume=613|issue=1|pages=262–278|arxiv=astro-ph/0406002|bibcode=2004ApJ...613..262L|doi=10.1086/422871|s2cid=10800774}}</ref> In elliptical and [[Lenticular galaxy|lenticular galaxies]] there is a correlation between the mass of the supermassive black holes (SMBHs) at their centers and the extent of their globular cluster systems. The mass of the SMBH in such a galaxy is often close to the combined mass of the galaxy's globular clusters.<ref name="BurkertTremaine2010">{{cite journal |author1=Burkert, Andreas |author2=Tremaine, Scott | title=A correlation between central supermassive black holes and the globular cluster systems of early-type galaxies
 | date=April 1, 2010 | quote=A possible explanation is that both large black-hole masses and large globular cluster populations are associated with recent major mergers.
 | arxiv=1004.0137 | doi=10.1088/0004-637X/720/1/516 | volume=720 |issue=1 | journal=The Astrophysical Journal | pages=516–521 | bibcode=2010ApJ...720..516B|s2cid=118632899 }}</ref>

No known globular clusters display active star formation, consistent with the hypothesis that globular clusters are typically the oldest objects in their galaxy and were among the first collections of stars to form. Very large regions of star formation known as [[super star cluster]]s, such as [[Westerlund&nbsp;1]] in the Milky Way, may be the precursors of globular clusters.<ref>{{cite press release
 |title=Young and Exotic Stellar Zoo: ESO's Telescopes Uncover Super Star Cluster in the Milky Way |last1=Negueruela |first1=Ignacio |last2=Clark |first2=Simon
 |date=March 22, 2005
|publisher=European Southern Observatory
 |url=https://www.eso.org/public/news/eso0510/
 |access-date=April 7, 2021
|url-status=live
 |archive-url=https://web.archive.org/web/20070409105105/http://eso.org/outreach/press-rel/pr-2005/pr-08-05.html
 |archive-date=April 9, 2007 |id=eso0510
}}</ref>

Many of the Milky Way's globular clusters have a [[retrograde orbit]] (meaning that they revolve around the galaxy in the reverse of the direction the galaxy is rotating),<ref>{{cite journal
 | first=V. V.
 | last=Kravtsov
 | title=Globular Clusters and Dwarf Spheroidal Galaxies of the Outer Galactic Halo: on the Putative Scenario of their Formation
 | journal=Astronomical and Astrophysical Transactions
 | volume=20
 | issue=1
 | pages=89–92
 | doi=10.1080/10556790108208191
 | bibcode=2001A&AT...20...89K
 | date=2001
 }}</ref> including the most massive, Omega Centauri. Its retrograde orbit suggests it may be a remnant of a dwarf galaxy captured by the Milky Way.<ref>{{cite journal |last1=Bekki |first1=K. |last2=Freeman |first2=K. C. |title=Formation of ω Centauri from an ancient nucleated dwarf galaxy in the young Galactic disc: Formation of ω Centauri |journal=Monthly Notices of the Royal Astronomical Society |date=2003 |volume=346 |issue=2 |pages=L11–L15 |doi=10.1046/j.1365-2966.2003.07275.x|arxiv=astro-ph/0310348|bibcode=2003MNRAS.346L..11B |doi-access=free }}</ref><ref>{{cite journal |doi=10.3847/1538-3881/ab8819 |title=The Most Metal-poor Stars in Omega Centauri (NGC 5139) |year=2020 |last1=Johnson |first1=Christian I. |last2=Dupree |first2=Andrea K. |last3=Mateo |first3=Mario |last4=Bailey |first4=John I. |last5=Olszewski |first5=Edward W. |last6=Walker |first6=Matthew G. |journal=The Astronomical Journal |volume=159 |issue=6 |page=254 |arxiv=2004.09023 |bibcode=2020AJ....159..254J |s2cid=215827658 |doi-access=free }}</ref>