Editing Globular cluster (section)

===Mass segregation, luminosity and core collapse===
In most Milky Way clusters, the surface brightness of a globular cluster as a function of decreasing distance to the core first increases, then levels off at a distance typically 1–2 parsecs from the core. About 20% of the globular clusters have undergone a process termed "core collapse". The luminosity in such a cluster increases steadily all the way to the core region.<ref>{{cite journal |author1=Djorgovski, S. |author2=King, I. R. |date=1986 |title=A preliminary survey of collapsed cores in globular clusters |journal=Astrophysical Journal |volume=305 |pages=L61–L65 |bibcode=1986ApJ...305L..61D |doi=10.1086/184685 |s2cid=122668507 |url=https://authors.library.caltech.edu/97564/ |archive-url=https://web.archive.org/web/20200915023244/https://authors.library.caltech.edu/97564/ |url-status=dead |archive-date=September 15, 2020 }}</ref><ref>{{cite journal |last1=Bianchini |first1=P |last2=Webb |first2=J J |last3=Sills |first3=A |last4=Vesperini |first4=E |title=Kinematic fingerprint of core-collapsed globular clusters |journal=Monthly Notices of the Royal Astronomical Society: Letters |date=March 21, 2018 |volume=475 |issue=1 |pages=L96–L100 |doi=10.1093/mnrasl/sly013 |doi-access=free | bibcode=2018MNRAS.475L..96B | arxiv=1801.07781}}</ref>
[[File:Globular_cluster_47_Tucanae.jpg|thumb|right|alt=Thousands of white-ish dots scattered on a black background, strongly concentrated towards the center|[[47 Tucanae]] is the second most luminous globular cluster in the Milky Way, after Omega Centauri.]]

Models of globular clusters predict that core collapse occurs when the more massive stars in a globular cluster encounter their less massive counterparts. Over time, dynamic processes cause individual stars to migrate from the center of the cluster to the outside, resulting in a net loss of [[kinetic energy]] from the core region and leading the region's remaining stars to occupy a more compact volume. When this gravothermal instability occurs, the central region of the cluster becomes densely crowded with stars, and the [[surface brightness]] of the cluster forms a [[power-law]] cusp.<ref name=ashman_zepf1998>{{cite book
 |author1=Ashman, Keith M. |author2=Zepf, Stephen E. | title=Globular Cluster Systems | date=1998 | volume=30
 | series=Cambridge astrophysics series | page=29
 | publisher=Cambridge University Press | isbn=978-0-521-55057-4 }}</ref> A massive black hole at the core could also result in a luminosity cusp.<ref name=binney_merrifield1998>{{cite book
 |author1=Binney, James |author2=Merrifield, Michael | title=Galactic astronomy | date=1998 | page=371
 | series=Princeton series in astrophysics
 | publisher=Princeton University Press
 | isbn=978-0-691-02565-0 }}</ref> Over a long time, this leads to a concentration of massive stars near the core, a phenomenon called [[mass segregation]].<ref name=lymanspitzer1984>{{cite journal |last1=Spitzer |first1=Lyman |title=Dynamics of Globular Clusters |journal=Science |date=1984 |volume=225 |issue=4661 |pages=465–472 |doi=10.1126/science.225.4661.465 |jstor=1693970 |pmid=17750830 |bibcode=1984Sci...225..465S |s2cid=30929160 |issn=0036-8075}}</ref>

The dynamical heating effect of binary star systems works to prevent an initial core collapse of the cluster. When a star passes near a binary system, the orbit of the latter pair tends to contract, releasing energy. Only after this primordial supply of energy is exhausted can a deeper core collapse proceed.<ref name=vanbeveren2001>{{cite book
 | first=D. | last=Vanbeveren | date=2001
 | title=The Influence of Binaries on Stellar Population Studies | volume=264
 | series=Astrophysics and space science library
 | publisher=Springer | page=397 | isbn=978-0-7923-7104-5 }}</ref><ref name=spitzer1986>{{cite conference
 | title=Dynamical Evolution of Globular Clusters
 | last=Spitzer | first=L. Jr. | work=The Use of Supercomputers in Stellar Dynamics, Proceedings of a Workshop Held at the Institute for Advanced Study
 | series=Lecture Notes in Physics | location=Princeton, USA | date=June 2–4, 1986
 | volume=267 |editor1=P. Hut |editor2=S. McMillan | publisher=Springer-Verlag, Berlin Heidelberg New York
 | page=3 | doi=10.1007/BFb0116388
 | bibcode=1986LNP...267....3S | isbn=978-3-540-17196-6 }}</ref> In contrast, the effect of [[tidal shock]]s as a globular cluster repeatedly passes through the plane of a spiral galaxy tends to significantly accelerate core collapse.<ref name=apj522_2_935>{{cite journal
 | title=Effects of Tidal Shocks on the Evolution of Globular Clusters
 |author1=Gnedin, Oleg Y. |author2=Lee, Hyung Mok |author3=Ostriker, Jeremiah P. | journal=The Astrophysical Journal | volume=522
 | issue=2 | pages=935–949 |date=September 1999
 | doi=10.1086/307659 | bibcode=1999ApJ...522..935G
 | arxiv=astro-ph/9806245 |s2cid=11143134 }}</ref>

Core collapse may be divided into three phases. During a cluster's adolescence, core collapse begins with stars nearest the core. Interactions between [[binary star]] systems prevents further collapse as the cluster approaches middle age. The central binaries are either disrupted or ejected, resulting in a tighter concentration at the core.<ref name=pnas107_16_7164>{{cite journal | title=Effects of Tidal Shocks on the Evolution of Globular Clusters |author1=Pooley, David | journal=Proceedings of the National Academy of Sciences | volume=107 | issue= 16 | pages= 7164–7167 |date=April 2010 | doi= 10.1073/pnas.0913903107 |pmid=20404204 |pmc=2867700 | bibcode=2010PNAS..107.7164P| s2cid= 15402180 |doi-access=free }}</ref> The interaction of stars in the collapsed core region causes tight binary systems to form. As other stars interact with these tight binaries they increase the energy at the core, causing the cluster to re-expand. As the average time for a core collapse is typically less than the age of the galaxy, many of a galaxy's globular clusters may have passed through a core collapse stage, then re-expanded.<ref name=bahcall_piran_weinberg2004>{{cite book
 |author1=Bahcall, John N. |author2=Piran, Tsvi |author3=Weinberg, Steven | title=Dark Matter in the Universe | page=51 | edition=2nd
 | publisher=World Scientific | date=2004 | isbn=978-981-238-841-4 }}</ref>
[[File:The stars of the Large Magellanic Cloud.jpg|thumb|alt=Hundreds of white-ish dots scattered on a black background, concentrated towards the center|Globular cluster [[NGC 1854]] is located in the Large Magellanic Cloud.<ref>{{cite web |title=The stars of the Large Magellanic Cloud |url=https://esahubble.org/images/potw1625a/ |website=European Space Agency/Hubble |language=en |date=June 20, 2016 | id=potw1625a | access-date=April 7, 2021}}</ref>]]

The HST has provided convincing observational evidence of this stellar mass-sorting process in globular clusters. Heavier stars slow down and crowd at the cluster's core, while lighter stars pick up speed and tend to spend more time at the cluster's periphery. The cluster [[47&nbsp;Tucanae]], made up of about one million stars, is one of the densest globular clusters in the Southern Hemisphere. This cluster was subjected to an intensive photographic survey that obtained precise velocities for nearly fifteen thousand stars in this cluster.<ref>{{cite press release
 | title=Stellar Sorting in Globular Cluster 47
 | publisher=Hubble News Desk | date=October 4, 2006
| url=https://hubblesite.org/contents/news-releases/2006/news-2006-33.html
 | access-date=April 9, 2021 | id= 2006-33 }}</ref>

The overall luminosities of the globular clusters within the Milky Way and the Andromeda Galaxy each have a roughly [[Gaussian curve|Gaussian distribution]], with an average magnitude M<sub>v</sub> and a variance σ<sup>2</sup>. This distribution of globular cluster luminosities is called the Globular Cluster Luminosity Function (GCLF). For the Milky Way, M<sub>v</sub>&nbsp;= {{nowrap|−7.29 ± 0.13}}, σ&nbsp;= {{nowrap|1.1 ± 0.1}}. The GCLF has been used as a "[[standard candle]]" for measuring the distance to other galaxies, under the assumption that globular clusters in remote galaxies behave similarly to those in the Milky Way.<ref>{{cite journal
 | last = Secker | first = Jeff | date= 1992
 | title = A Statistical Investigation into the Shape of the Globular cluster Luminosity Distribution
 | journal = Astronomical Journal | volume = 104
 | issue = 4 | pages = 1472–1481
 | bibcode = 1992AJ....104.1472S | doi = 10.1086/116332 }}</ref>