Editing Globular cluster (section)

===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>