Editing Globular cluster (section)

==Composition==
[[File:Djorgovski 1.jpg|thumb|alt=A loose scattering of small dull white dots on a black background with a few brighter coloured stars|[[Djorgovski 1]]'s stars contain hydrogen and helium, but not much else. In astronomical terms they are ''metal-poor''.<ref>{{cite web |publisher=[[European Space Agency|European Space Agency (ESA)]] |title=Engulfed by Stars Near the Milky Way's Heart |url=http://www.spacetelescope.org/images/potw1126a/ |access-date=June 28, 2011 }}</ref>]]

Globular clusters are generally composed of hundreds of thousands of [[low-metal]], old stars. The stars found in a globular cluster are similar to those in the bulge of a [[spiral galaxy]] but confined to a spheroid in which [[half-light radius|half the light]] is emitted within a radius of only a few to a few tens of [[parsec]]s.<ref name="gratton" /> They are free of gas and dust,<ref>{{cite journal |last1=Bastian |first1=N. |last2=Strader |first2=J. |title=Constraining globular cluster formation through studies of young massive clusters – III. A lack of gas and dust in massive stellar clusters in the LMC and SMC |journal=Monthly Notices of the Royal Astronomical Society |date=October 1, 2014 |volume=443 |issue=4 |pages=3594–3600 |doi=10.1093/mnras/stu1407|doi-access=free |arxiv=1407.2726 }}</ref> and it is presumed that all the gas and dust was long ago either turned into stars or blown out of the cluster by the massive first-generation stars.<ref name="gratton" />

Globular clusters can contain a high density of stars; on average about 0.4{{spaces}}stars per cubic parsec, increasing to 100 or 1000{{spaces}}stars/pc{{Sup|3}} in the core of the cluster.<ref>{{cite web |last=Talpur |first=Jon |year=1997 |url=http://www.astro.keele.ac.uk/workx/globulars/globulars.html |title=A Guide to Globular Clusters |publisher=Keele University |access-date=April 25, 2007 | archive-url=https://web.archive.org/web/20210404045753/https://www.astro.keele.ac.uk/workx/globulars/globulars.html | archive-date=April 4, 2021 | url-status=live }}</ref> In comparison, the stellar density around the Sun is roughly 0.1&nbsp;stars/pc{{Sup|3}}.<ref name="ROCH">{{cite web |last1=Mamajek |first1=Eric |title=Number Densities of Stars of Different Types in the Solar Vicinity |url=http://www.pas.rochester.edu/~emamajek/memo_star_dens.html |department=Department of Physics and Astronomy |publisher=University of Rochester |access-date=September 5, 2021}}</ref> The typical distance between stars in a globular cluster is about one light year,<ref>{{cite web |url=http://www.dur.ac.uk/ian.smail/gcCm/gcCm_intro.html |last=Smail|first=Ian| access-date=September 5, 2021 |publisher=University of Durham |department=Department of Physics |title=The Hertzsprung-Russell Diagram of a Globular Cluster}}</ref> but at its core the separation between stars averages about a third of a light year{{snd}}thirteen times closer than the Sun is to its nearest neighbor, [[Proxima Centauri]].<ref>{{cite web |url=https://www.nasa.gov/mission_pages/hubble/multimedia/ero/ero_omega_centauri.html |publisher=[[NASA]] |date=September 9, 2009 |title=Colorful Stars Galore Inside Globular Star Cluster Omega Centauri |access-date=April 28, 2021 |archive-date=January 26, 2021 |archive-url=https://web.archive.org/web/20210126021258/https://www.nasa.gov/mission_pages/hubble/multimedia/ero/ero_omega_centauri.html |url-status=dead }}</ref>

Globular clusters are thought to be unfavorable locations for planetary systems. Planetary orbits are dynamically unstable within the cores of dense clusters because of the gravitational perturbations of passing stars. A planet orbiting at one [[astronomical unit]] around a star that is within the core of a dense cluster, such as [[47&nbsp;Tucanae]], would survive only on the order of a hundred million years.<ref>{{cite journal |last=Sigurdsson |first=Steinn |title=Planets in globular clusters? |journal=Astrophysical Journal |year=1992 |volume=399 |issue=1 |pages=L95–L97 |bibcode=1992ApJ...399L..95S |doi=10.1086/186615}}</ref> There is a planetary system orbiting a [[pulsar]] ([[PSR B1620-26|PSR{{spaces}}B1620−26]]) that belongs to the globular cluster [[Messier 4|M4]], but these planets likely formed after the event that created the pulsar.<ref>{{cite journal |title=Orbital Parameters of the PSR B1620-26 Triple System |journal=Proceedings of the 160th Colloquium of the International Astronomical Union |year=1999 |volume=105 |bibcode=1996ASPC..105..525A |arxiv=astro-ph/9605141 |author1=Arzoumanian, Z. |author2=Joshi, K. |author3=Rasio, F. A. |author4=Thorsett, S.E. |page=525 }}</ref>

Some globular clusters, like Omega Centauri in the Milky Way and [[Mayall&nbsp;II]] in the Andromeda Galaxy, are extraordinarily massive, measuring several million [[solar mass]]es ({{Solar mass}}) and having multiple stellar populations. Both are evidence that supermassive globular clusters formed from the cores of [[Dwarf galaxy|dwarf galaxies]] that have been consumed by larger galaxies.<ref>{{cite journal |author1=Bekki, K. |author2=Freeman, K.C. |author-link2=Ken Freeman (astronomer) |title=Formation of ω Centauri from an ancient nucleated dwarf galaxy in the young Galactic disc |journal=Monthly Notices of the Royal Astronomical Society |volume=346 |issue=2 |pages=L11–L15 |date=December 2003 |doi=10.1046/j.1365-2966.2003.07275.x| arxiv=astro-ph/0310348 |bibcode=2003MNRAS.346L..11B |s2cid=119466098 }}</ref> About a quarter of the globular cluster population in the Milky Way may have been accreted this way,<ref>{{cite journal |author1=Forbes, Duncan A. |author2=Bridges, Terry |title=Accreted versus in situ Milky Way globular clusters |date=January 25, 2010 |arxiv=1001.4289 |doi=10.1111/j.1365-2966.2010.16373.x |journal=Monthly Notices of the Royal Astronomical Society |volume=404 |issue=3 |page=1203 |doi-access=free |bibcode=2010MNRAS.404.1203F |s2cid=51825384 }}</ref> as with more than 60% of the globular clusters in the outer halo of Andromeda.<ref name="forbes_formation">{{cite journal |last1=Forbes |first1=Duncan A. |last2=Bastian |first2=Nate |last3=Gieles |first3=Mark |last4=Crain |first4=Robert A. |last5=Kruijssen |first5=J. M. Diederik |last6=Larsen |first6=Søren S. |last7=Ploeckinger |first7=Sylvia |last8=Agertz |first8=Oscar |last9=Trenti |first9=Michele |last10=Ferguson |first10=Annette M. N. |last11=Pfeffer |first11=Joel |last12=Gnedin |first12=Oleg Y. |title=Globular cluster formation and evolution in the context of cosmological galaxy assembly: open questions |journal=Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences |date=February 2018 |volume=474 |issue=2210 |page=20170616 |doi=10.1098/rspa.2017.0616|pmid=29507511 |pmc=5832832 |arxiv=1801.05818 |bibcode=2018RSPSA.47470616F |doi-access=free }}</ref>

===Heavy element content===
Globular clusters normally consist of [[Population&nbsp;II stars]] which, compared with [[Population&nbsp;I stars]] such as the [[Sun]], have a higher proportion of hydrogen and helium and a lower proportion of heavier elements. Astronomers refer to these heavier elements as ''metals'' (distinct from the material concept) and to the proportions of these elements as the metallicity. Produced by [[stellar nucleosynthesis]], the metals are recycled into the [[interstellar medium]] and enter a new generation of stars. The proportion of metals can thus be an indication of the age of a star in simple models, with older stars typically having a lower metallicity.<ref>{{cite book |title=An Introduction to the Sun and Stars |page=240 |publisher=Cambridge University Press |year=2004 |isbn=978-0-521-54622-5 |author1=Green, Simon F. |author2=Jones, Mark H. |author3=Burnell, S. Jocelyn}}</ref>

The Dutch astronomer [[Pieter Oosterhoff]] observed two special populations of globular clusters, which became known as ''Oosterhoff groups''. The second group has a slightly longer period of RR Lyrae variable stars.<ref name="oosterhoff">{{cite journal |title=On the Two Oosterhoff Groups of Globular Clusters |journal=Astrophysical Journal |volume=185 |year=1973 |pages=477–498 |doi=10.1086/152434 |author1=van Albada, T. S. |author2=Baker, Norman |bibcode=1973ApJ...185..477V}}</ref> While both groups have a low proportion of metallic elements as measured by [[Astronomical spectroscopy|spectroscopy]], the metal spectral lines in the stars of Oosterhoff type{{spaces}}I (Oo{{spaces}}I) cluster are not quite as weak as those in type{{spaces}}II (Oo{{spaces}}II),<ref name="oosterhoff" /> and so type{{spaces}}I stars are referred to as ''metal-rich'' (e.g. [[Terzan&nbsp;7]]<ref name="Terzan7">{{cite journal |title=ESO 280-SC06 |journal=Astronomical Journal |volume=109 |page=663 |year=1995 |author1=Buonanno, R. |author2=Corsi, C.E. |author3=Pulone, L. |bibcode=1995AJ....109..663B |doi=10.1086/117309}}</ref>), while type{{spaces}}II stars are ''metal-poor'' (e.g. [[ESO&nbsp;280-SC06]]<ref name="ESO280-6">{{cite web |last=Frommert |first=Hartmut | title=Globular cluster ESO 280-S C06, in Ara | website=Students for the Exploration and Development of Space |url=http://spider.seds.org/spider/MWGC/eso280sc06.html |access-date=April 9, 2021 }}</ref>). These two distinct populations have been observed in many galaxies, especially massive elliptical galaxies. Both groups are nearly as old as the universe itself and are of similar ages. Suggested scenarios to explain these subpopulations include violent gas-rich galaxy mergers, the accretion of dwarf galaxies, and multiple phases of star formation in a single galaxy. In the Milky Way, the metal-poor clusters are associated with the halo and the metal-rich clusters with the bulge.<ref>{{cite journal |last=Harris |first=W.E. |title=Spatial structure of the globular cluster system and the distance to the galactic center |journal=Astronomical Journal |year=1976 |volume=81 |pages=1095–1116 |bibcode=1976AJ.....81.1095H |doi=10.1086/111991}}</ref>

A large majority of the metal-poor clusters in the Milky Way are aligned on a plane in the outer part of the galaxy's halo. This observation supports the view that type{{spaces}}II clusters were captured from a satellite galaxy, rather than being the oldest members of the Milky Way's globular cluster system as was previously thought. The difference between the two cluster types would then be explained by a time delay between when the two galaxies formed their cluster systems.<ref>{{cite journal |author1=Lee, Y.W. |author2=Yoon, S.J. | title=On the Construction of the Heavens |journal=An Aligned Stream of Low-Metallicity Clusters in the Halo of the Milky Way |volume=297 |year=2002 |pages=578–581 |bibcode=2002Sci...297..578Y |doi=10.1126/science.1073090 |pmid=12142530 |issue=5581 |arxiv=astro-ph/0207607|s2cid=9702759 }}</ref>

===Exotic components===
[[File:Messier 53 HST.jpg|thumb|alt=Thousands of white-ish dots scattered on a black background, strongly concentrated towards the center|[[Messier 53]] contains an unusually large number of a type of star called ''blue stragglers''.<ref>{{cite web |title=Spot the Difference – Hubble spies another globular cluster, but with a secret |url=http://spacetelescope.org/images/potw1140a/ |work=Picture of the Week |publisher=ESA/Hubble |access-date=October 5, 2011}}</ref><ref>{{cite web |title=APOD: 2021 February 7 – Blue Straggler Stars in Globular Cluster M53 |url=https://apod.nasa.gov/apod/ap210207.html |website=[[Astronomy Picture of the Day]] |access-date=February 28, 2021}}</ref>]]

Close interactions and near-collisions of stars occur relatively often in globular clusters because of their high star density. These chance encounters give rise to some exotic classes of stars{{snd}}such as [[blue straggler]]s, [[millisecond pulsar]]s, and [[low-mass X-ray binaries]]{{snd}}which are much more common in globular clusters. How blue stragglers form remains unclear, but most models attribute them to interactions between stars, such as [[stellar merger]]s, the transfer of material from one star to another, or even an encounter between two binary systems.<ref name="leonard">{{cite journal
 | author=Leonard, Peter J. T. | date= 1989
 | title=Stellar collisions in globular clusters and the blue straggler problem
 | journal=The Astronomical Journal | volume=98
 | pages=217–226 | doi=10.1086/115138
 | bibcode=1989AJ.....98..217L }}</ref><ref name=":0">{{cite journal|last1=Ferraro|first1=F. R.|last2=Lanzoni|first2=B.|last3=Raso|first3=S.|last4=Nardiello|first4=D.|last5=Dalessandro|first5=E.|last6=Vesperini|first6=E.|last7=Piotto|first7=G.|last8=Pallanca|first8=C.|last9=Beccari|first9=G.|last10=Bellini|first10=A.|last11=Libralato|first11=M.|date=June 8, 2018|title=The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. XV. The Dynamical Clock: Reading Cluster Dynamical Evolution from the Segregation Level of Blue Straggler Stars|journal=The Astrophysical Journal|volume=860|issue=1|page=36|arxiv=1805.00968|bibcode=2018ApJ...860...36F|doi=10.3847/1538-4357/aac01c|first15=S.|last15=Cassisi|last12=Anderson|first12=J.|first13=A.|last14=Bedin|first14=L. R.|first20=R. P.|last16=Milone|last20=van der Marel|first19=M.|last19=Salaris|first18=A.|last18=Renzini|first17=S.|last17=Ortolani|first16=A. P.|last13=Aparicio|s2cid=119435307 |doi-access=free }}</ref> The resulting star has a higher temperature than other stars in the cluster with comparable luminosity and thus differs from the [[main-sequence]] stars formed early in the cluster's existence.<ref name="murphy">{{cite journal |author1=Rubin, V.C. |author2=Ford, W.K.J. |author-link1=Vera Rubin |title=A Thousand Blazing Suns: The Inner Life of Globular Clusters |journal=Mercury |year=1999 |volume=28 |issue=4 |page=26 |url=http://www.astrosociety.org/pubs/mercury/9904/murphy.html |access-date=June 2, 2006 |bibcode=1999Mercu..28d..26M |archive-date=May 21, 2006 |archive-url=https://web.archive.org/web/20060521044631/http://www.astrosociety.org/pubs/mercury/9904/murphy.html  }}</ref> Some clusters have two distinct sequences of blue stragglers, one bluer than the other.<ref name=":0" />
[[File:STSci-2002-18.jpg|thumb|right|alt=Hundreds of white-ish dots scattered on a black background, concentrated towards the center|Globular cluster M15 may have an [[intermediate-mass black hole]] at its core,<ref>{{cite press release |title=Hubble Discovers Black Holes in Unexpected Places |url=https://hubblesite.org/contents/news-releases/2002/news-2002-18.html |publisher=Space Telescope Science Institute |language=en | id=2002-18 | date=September 17, 2002}}</ref> but this claim is contested.<ref name="baumgardt_m15" />]]
[[File:STScI-01H0MY22SC4HPS3SYE78B4EZ4F AdobeExpress.gif|thumb|alt=Simulation of stellar motions in Messier 4|Simulation of stellar motions in [[Messier 4]], where astronomers suspect that an [[intermediate-mass black hole]] could be present.<ref name="Vitral+23">{{cite journal|last=Vitral|first=E.|display-authors=etal|title=An elusive dark central mass in the globular cluster M4|journal=Monthly Notices of the Royal Astronomical Society|date=2023|volume=522|issue=4 |pages=5740–5757|doi=10.1093/mnras/stad1068|doi-access=free |arxiv =2305.12702 |bibcode = 2023MNRAS.522.5740V }}</ref><ref name="NASAV23">{{cite news |date=23 May 2023|title=NASA's Hubble Hunts for Intermediate-Sized Black Hole Close to Home | url=https://www.nasa.gov/feature/goddard/2023/nasas-hubble-hunts-for-intermediate-sized-black-hole-close-to-home/ |work=NASA|access-date=23 May 2023}}</ref> If confirmed, the black hole would be in the center of the cluster, and would have a [[sphere of influence (black hole)]] limited by the red circle.]]

Astronomers have searched for [[black hole]]s within globular clusters since the 1970s. The required resolution for this task is exacting; it is only with the [[Hubble Space Telescope]] (HST) that the first claimed discoveries were made, in 2002 and 2003. Based on HST observations, other researchers suggested the existence of a {{Solar mass|4,000}}(solar masses) [[intermediate-mass black hole]] in the globular cluster [[Messier 15|M15]] and a {{Solar mass|20,000}} black hole in the [[Mayall&nbsp;II]] cluster of the Andromeda Galaxy.<ref>{{cite news|author1=Savage, D.|author2=Neal, N.|author3=Villard, R.|author4=Johnson, R.|author5=Lebo, H.|date=September 17, 2002|title=Hubble discovers black holes in unexpected places|publisher=Space Telescope Science Institute|url=http://hubblesite.org/newscenter/newsdesk/archive/releases/2002/18/text/|access-date=May 25, 2006|archive-url=https://web.archive.org/web/20031119083627/http://hubblesite.org/newscenter/newsdesk/archive/releases/2002/18/text/|archive-date=November 19, 2003}}</ref> Both [[X-ray]] and [[radio]] emissions from Mayall{{spaces}}II appear consistent with an intermediate-mass black hole;<ref>{{cite news |first=Dave |last=Finley |title=Star cluster holds midweight black hole, VLA indicates |publisher=NRAO |date=May 28, 2007 |url=http://www.nrao.edu/pr/2007/globularbh/ |access-date=May 29, 2007 }}</ref> however, these claimed detections are controversial.<ref name=greene2007>{{cite journal |last1=Greene |first1=Jenny E. |last2=Strader |first2=Jay |last3=Ho |first3=Luis C. |title=Intermediate-Mass Black Holes |journal=Annual Review of Astronomy and Astrophysics |date=August 18, 2020 |volume=58 |issue=1 |pages=257–312 |doi=10.1146/annurev-astro-032620-021835 |bibcode=2020ARA&A..58..257G|arxiv=1911.09678 |s2cid=208202069 }}</ref>

The heaviest objects in globular clusters are expected to migrate to the cluster center due to [[mass segregation]]. One research group pointed out that the mass-to-light ratio should rise sharply towards the center of the cluster, even without a black hole, in both M15<ref name="baumgardt_m15">{{cite journal |author1=Baumgardt, Holger |author2=Hut, Piet |author3=Makino, Junichiro |author4=McMillan, Steve |author5=Portegies Zwart, Simon |title=On the Central Structure of M15 |journal=Astrophysical Journal Letters |year=2003 |volume=582 |issue=1 |page=21 |bibcode=2003ApJ...582L..21B |doi=10.1086/367537 |arxiv=astro-ph/0210133|s2cid=16216186 }}</ref> and Mayall II.<ref>{{Cite journal |author1=Baumgardt, Holger |author2=Hut, Piet |author3=Makino, Junichiro |author4=McMillan, Steve |author5=Portegies Zwart, Simon | title=A dynamical model for the globular cluster G1 |journal=Astrophysical Journal Letters |year=2003 |volume=589 |issue=1 |page=25 |doi=10.1086/375802 |bibcode=2003ApJ...589L..25B |arxiv=astro-ph/0301469 |s2cid=119464795 }}</ref> Observations from 2018 find no evidence for an intermediate-mass black hole in any globular cluster, including M15, but cannot definitively rule out one with a mass of {{Solar mass|500–1000}}.<ref>{{cite journal |last1=Tremou |first1=Evangelia |last2=Strader |first2=Jay |last3=Chomiuk |first3=Laura |last4=Shishkovsky |first4=Laura |last5=Maccarone |first5=Thomas J. |last6=Miller-Jones |first6=James C. A. |last7=Tudor |first7=Vlad |last8=Heinke |first8=Craig O. |last9=Sivakoff |first9=Gregory R. |last10=Seth |first10=Anil C. |last11=Noyola |first11=Eva |title=The MAVERIC Survey: Still No Evidence for Accreting Intermediate-mass Black Holes in Globular Clusters |journal=The Astrophysical Journal |date=July 18, 2018 |volume=862 |issue=1 |page=16 |doi=10.3847/1538-4357/aac9b9 |bibcode=2018ApJ...862...16T | arxiv=1806.00259|s2cid=119367485 |doi-access=free }}</ref> Finally, in 2023, an analysis of HST and the Gaia spacecraft data from the closest globular cluster, [[Messier 4]], revealed an excess mass of roughly {{Solar mass|800}} in the center of this cluster, which appears to not be extended. This could thus be considered as kinematic evidence for an intermediate-mass black hole<ref name="Vitral+23"/><ref name="NASAV23"/> (even if an unusually compact cluster of compact objects like [[white dwarfs]], [[neutron stars]] or stellar-mass [[black holes]] cannot be completely discounted).

The confirmation of intermediate-mass black holes in globular clusters would have important ramifications for theories of galaxy development as being possible sources for the [[supermassive black hole]]s at their centers. The mass of these supposed intermediate-mass black holes is proportional to the mass of their surrounding clusters, following a pattern previously discovered between supermassive black holes and their surrounding galaxies.<ref name=greene2007/><ref name=baumgardt2019>{{cite journal |bibcode=2019MNRAS.488.5340B |title=No evidence for intermediate-mass black holes in the globular clusters ω Cen and NGC 6624 |last1=Baumgardt |first1=H. |last2=He |first2=C. |last3=Sweet |first3=S. M. |last4=Drinkwater |first4=M. |last5=Sollima |first5=A. |last6=Hurley |first6=J. |last7=Usher |first7=C. |last8=Kamann |first8=S. |last9=Dalgleish |first9=H. |last10=Dreizler |first10=S. |last11=Husser |first11=T. -O. |journal=Monthly Notices of the Royal Astronomical Society |year=2019 |volume=488 |issue=4 |page=5340 |doi=10.1093/mnras/stz2060 |doi-access=free |arxiv=1907.10845 }}</ref>