Editing Globular cluster (section)

==History of observations==

The first known globular cluster, now called [[Messier 22|M 22]], was discovered in 1665 by [[Johann Abraham Ihle|Abraham Ihle]], a German amateur astronomer.<ref>Kirch, Gottfried (1682) ''Annus II.  Ephemeridum Motuum Coelestium Ad Annum Aerae Christianae M. DC. LXXXII. …'' [Second year.  Ephemerides of the celestial motions for the year of the Christian era 1682.]  Leipzig, (Germany):  Heirs of Friedrich Lanckisch. (in Latin) 54 pages.  The pages of this book are not numbered.  However, in the Appendix, section ''III. Stella nebulosa prope pedem borealem Ganymedis observata, Lipsia, die 1. Sept. 1681.'' (III. Nebula near the northern foot of Ganymede observed, Leipzig, 1. September 1681.), first paragraph, Kirch enumerated recently discovered nebulae: ''" […] & tertia in Sagittaris, quam Dn. Joh. Abrah. Ihle Anno 1665. deprehendit; […] "'' ([…] and the third [nebula] in Sagittarius, which Mr. Johann Abraham Ihle discovered in the year 1665; […])  Downloadable at: [https://opendata2.uni-halle.de/handle/1516514412012/32738 Digitale Sammlungen der Universitäts- und Landesbibliothek Sachsen-Anhalt] (Digital collections of the university- and state library of Sachsen-Anhalt)</ref><ref name=M22>{{cite journal |last=Lynn |first=W.T. |date=April 1886 |title=The discovery of the star-cluster 22&nbsp;Messier in Sagittarius |journal=The Observatory |volume=9 |pages=163–164 |bibcode=1886Obs.....9..163L |url=https://books.google.com/books?id=XHEKAAAAIAAJ&pg=PA163}}</ref><ref>{{cite web |last=Sharp |first=N.A. |title=M22, NGC&nbsp;6656 |publisher=[[NOIRLab]] |url=https://noirlab.edu/public/images/noao-m22/ |access-date=August 23, 2021}}</ref> The cluster [[Omega Centauri]], easily visible in the southern sky with the naked eye, was known to ancient astronomers like [[Ptolemy]] as a star, but was reclassified as a nebula by [[Edmond Halley]] in 1677,<ref>{{cite book |last1=Halley |first1=Edmond |title=Catalogus Stellarum Australium … |trans-title= Catalog of southern stars … |date=1679 |publisher=Thomas James |location=London, England |url=https://books.google.com/books?id=QVg4AAAAMAAJ&pg=PP26}} 
This book's pages are not numbered.  However in the "Centaurus" section, one entry is labeled "''in dorso equino nebula''" (nebula in the horse's back); the position of this nebula is consistent with Omega Centauri.</ref> then finally as a globular cluster in the early 19th century by [[John Herschel]].<ref>{{cite book |last1=Herschel |first1=John F. W. |title=Results of astronomical observations made during the years 1834, 5, 6, 7, 8, at the Cape of Good Hope; being the completion of a telescopic survey of the whole surface of the visible heavens, commenced in 1825 |date=1847 |publisher=Smith, Elder and Co. |location=London, England |page=105 |bibcode=1847raom.book.....H |url=https://archive.org/details/resultsofastrono00hers/page/104/mode/2up}}  See entry:  🜨 [symbol for globular cluster]; ω Centauri</ref><ref>{{cite book |last=O'Meara |first=Stephen James |year=2012 |title=Deep-Sky Companions: Southern gems |publisher=Cambridge University Press |location=Cambridge |isbn=978-1-107-01501-2 |pages=243–245 |url=https://books.google.com/books?id=S5QIEKns33sC&pg=PA244 |access-date=September 24, 2021}}</ref><ref>{{cite web |title=Omega Centauri |website=eso.org |language=en |publisher=[[European Southern Observatory]] |url=https://www.eso.org/public/france/images/b09/?lang |access-date=September 24, 2021}}</ref> The French astronomer [[Nicolas Louis de Lacaille|Abbé Lacaille]] listed [[47 Tucanae|NGC&nbsp;104]], {{nobr|[[NGC 4833]]}}, [[Messier 55|M 55]], [[Messier 69|M 69]], and {{nobr|[[NGC 6397]]}} in his 1751–1752 catalogue.<ref group="lower-alpha">The label '''M''' before a number refers to Charles {{underline|M}}essier's [[Messier object|catalogue]], while '''NGC''' is from ''the [[New General Catalogue|{{underline|N}}ew {{underline|G}}eneral {{underline|C}}atalogue]]'' by [[John Louis Emil Dreyer|John Dreyer]].</ref> The low resolution of early [[telescope]]s prevented individual stars in a cluster from being [[Angular resolution|visually separate]]d until [[Charles Messier]] observed [[Messier 4|M 4]] in 1764.<ref name=Messier-1771>{{cite journal |last=Messier |first=Charles |author-link=Charles Messier |year=1771 |title=Catalogue des Nébuleuses & des amas d'Étoiles, que l'on découvre parmi les Étoiles fixes sur l'horizon de Paris; observées à l'Observatoire de la Marine, avec differens instruments |trans-title=Catalog of nebulas and star clusters, that one discovers among the fixed stars on the horizon of Paris; observed at the Naval Observatory, with various instruments |language=fr |journal=Histoire de l'Académie Royale des Sciences ... Avec les Mémoires de Mathématique & de Physique, pour la même Année, ... [History of the Royal Academy of Sciences ... with the Mathematical and Physical Memoirs, for the same year, ...] |pages=435–461 |url=https://gallica.bnf.fr/ark:/12148/bpt6k35697/f613.image}}</ref>{{efn|
From page&nbsp;437: ''Le 8&nbsp;Mai 1764, j'ai découvert une nébuleuse ... de 25<sup>d</sup> 55′ 40″ méridionale.''<br/>
"On 8&nbsp;May 1764, I discovered a nebula near [[Antares]], and on its parallel; it is a [source of] light which has little extension, which is dim, and which is seen with difficulty; by using a good telescope to see it, one perceives very small stars in it. Its right ascension was determined to be 242° 16′ 56″, and its declination, 25° 55′ 40″ south."<ref name=Messier-1771/>{{rp|style=ama|page= 437}}
}}<ref name=boyd2008>{{cite book |last=Boyd |first=Richard N. |year=2008 |title=An Introduction to Nuclear Astrophysics |publisher=University of Chicago Press |isbn=978-0-226-06971-5 |page=376 |url=https://books.google.com/books?id=uoIyc538X9kC}}</ref>
{| class="wikitable" style="margin-top: 0px; margin-left: 0.5em;" align="right"
|+ Early globular cluster discoveries
!Cluster name
!Discovered by
!Year
|-
|style="text-align: center;"|[[Messier 22|M 22]]<ref name=M22/>
|[[Abraham Ihle]]
|1665
|-
|style="text-align: center;"|[[Omega Centauri|ω Cen]]{{efn|[[Omega Centauri]] was known in antiquity, but Halley discovered its nature as a nebula.}}<ref>{{cite journal |last=Halley |first=Edmond |author-link=Edmond Halley |year=1716 |title=An account of several nebualæ or lucid spots like clouds, lately discovered among the fixt stars by help of the telescope |journal=[[Philosophical Transactions of the Royal Society of London]] |volume=29 |issue=347 |pages=390–392 |doi=10.1098/rstl.1714.0046 |doi-access=free }}</ref>
|[[Edmond Halley]]
|1677
|-
|style="text-align: center;"|[[Messier 5|M 5]]<ref name=Moore-2003>{{cite book |last=Moore |first=Patrick |author-link=Patrick Moore |year=2003 |title=Atlas of the Universe |publisher=Firefly Books |isbn=978-0-681-61459-8 |url=https://archive.org/details/fireflyatlasofun0000moor_t2l5/page/n5/mode/2up |url-access=registration}}</ref>{{rp|style=ama|page= [https://archive.org/details/fireflyatlasofun0000moor_t2l5/page/n5/mode/2up 237]}}<ref>{{cite web |last1=Frommert |first1=Hartmut |last2=Kronberg |first2=Christine |title=Gottfried Kirch (1639–1710) |publisher=[[Students for the Exploration and Development of Space]] (SEDS) |url=http://www.messier.seds.org/xtra/Bios/kirch.html |access-date=August 9, 2021}}</ref>
|[[Gottfried Kirch]]
|1702
|-
|style="text-align: center;"|[[Messier 13|M 13]]<ref name=Moore-2003/>{{rp|style=ama|page= [https://archive.org/details/fireflyatlasofun0000moor_t2l5/page/n5/mode/2up 235]}}
|Edmond Halley
|1714
|-
|style="text-align: center;"|[[Messier 71|M 71]]<ref name=cudnik2012>{{cite book |last=Cudnik |first=Brian |year=2012 |title=Faint Objects and How to Observe Them |publisher=Springer Science & Business Media |isbn=978-1-4419-6756-5 |page=8 |url=https://books.google.com/books?id=ZsY5NuwjdTEC&pg=PA8}}</ref>
|[[Philippe Loys de Chéseaux]]
|1745
|-
|style="text-align: center;"|[[Messier 4|M 4]]<ref name=cudnik2012/>
|Philippe Loys de Chéseaux
|1746
|-
|style="text-align: center;"|[[Messier 15|M 15]]<ref name=chen2015>{{cite book |last=Chen |first=James L. |year=2015 |title=A Guide to Hubble Space Telescope Objects: Their selection, location, and significance |publisher=Springer |isbn=978-3-319-18872-0 |page=110 |url=https://books.google.com/books?id=qj0wCgAAQBAJ&pg=PA110 |others=Illustrated by Adam Chen}}</ref>
|[[Jean-Dominique Maraldi]]
|1746
|-
|style="text-align: center;"|[[Messier 2|M 2]]<ref name=chen2015/>
|Jean-Dominique Maraldi
|1746
|}

When [[William Herschel]] began his comprehensive survey of the sky using large telescopes in 1782, there were 34&nbsp;known globular clusters. Herschel discovered another&nbsp;36 and was the first to resolve virtually all of them into stars. He coined the term ''globular cluster'' in his [[Catalogue of Nebulae and Clusters of Stars#History|''Catalogue of a Second Thousand New Nebulae and Clusters of Stars'']] (1789).<ref name=Herschel-1789>{{cite journal |last=Herschel |first=William |author-link=William Herschel |year=1789 |title=Catalogue of a second thousand of new nebulæ and clusters of stars, with a few introductory remarks on the construction of the heavens |journal=Philosophical Transactions of the Royal Society of London |volume=79 |pages=212–255 |bibcode=1789RSPT...79..212H |url=https://babel.hathitrust.org/cgi/pt?id=pst.000008680679;view=1up;seq=248 |access-date=April 28, 2021}}</ref>{{efn|
From page&nbsp;218, discussing the shapes of star clusters, [[William Herschel|Herschel]] wrote:<br/>
"And thus, from the above-mentioned appearances, we come to know that there are globular clusters of stars nearly equal in size, which are scattered evenly at equal distances from the middle, but with an encreasing [sic] accumulation towards the center."<ref name=Herschel-1789/>{{rp|style=ama|page= 218}}
}}<ref name=SEDS>{{cite web |last1=Frommert |first1=Hartmut |last2=Kronberg |first2=Christine |title=Globular Star Clusters |website=The Messier Catalog |publisher=Students for the Exploration and Development of Space |url=http://messier.seds.org/glob.html |access-date=June 19, 2015 |archive-url=https://web.archive.org/web/20150430163307/http://messier.seds.org/glob.html |archive-date=April 30, 2015  }}</ref> In 1914, [[Harlow Shapley]] began a series of studies of globular clusters, published across about forty scientific papers. He examined the clusters' [[RR&nbsp;Lyrae variable]]s (stars which he assumed were [[Cepheid variable]]s) and used their [[Period-luminosity relation|luminosity and period of variability]] to estimate the distances to the clusters. RR&nbsp;Lyrae variables were later found to be fainter than Cepheid variables, causing Shapley to overestimate the distances.<ref name=ashman_zepf_1998>{{cite book |last1=Ashman |first1=Keith M. |author1-link=Keith M. Ashman |last2=Zepf |first2=Stephen E. |year=1998 |title=Globular Cluster Systems |series=Cambridge Astrophysics Series |volume=30 |page=2 |publisher=Cambridge, UK University Press |isbn=978-0-521-55057-4 |url=https://books.google.com/books?id=sXwXfSH6g90C}}</ref>
[[File:NGC 7006 (HST).jpg|thumb|left|alt=Thousands of white-ish dots scattered on a black background, strongly concentrated towards the center|[[NGC 7006]] is a highly concentrated, Class&nbsp;I globular cluster.]]

A large majority of the Milky Way's globular clusters are found in the halo around the galactic core. In 1918, Shapley used this strongly asymmetrical distribution to determine the overall dimensions of the galaxy. Assuming a roughly spherical distribution of globular clusters around the galaxy's center, he used the positions of the clusters to estimate the position of the Sun relative to the [[Galactic Center]].<ref>{{cite journal |last=Shapley |first=Harlow |author-link=Harlow Shapley |year=1918 |title=Globular clusters and the structure of the galactic system |journal=[[Publications of the Astronomical Society of the Pacific]] |volume=30 |issue=173 |pages=42–54 |bibcode=1918PASP...30...42S |doi=10.1086/122686 |doi-access=free }}</ref> He correctly concluded that the Milky Way's center is in the [[Sagittarius constellation]] and not near the Earth. He overestimated the distance, finding typical globular cluster distances of {{convert|10-30|kpc|ly}};<ref>{{cite journal |last=Trimble |first=V.L. |author-link=Virginia Louise Trimble |date=December 1995 |title=The 1920 Shapley-Curtis Discussion: Background, issues, and aftermath |journal=[[Publications of the Astronomical Society of the Pacific]] |volume=107 |page=1133 |doi=10.1086/133671|bibcode=1995PASP..107.1133T |s2cid=122365368 |url=https://escholarship.org/uc/item/1q68k7m0 }}</ref> the modern distance to the [[Galactic Center]] is roughly {{convert|8.5|kpc|ly}}.{{efn|
Harlow Shapley's error was aggravated by [[interstellar dust]] in the Milky Way, which absorbs and diminishes the amount of light from distant objects (such as globular clusters), thus making them appear to be farther away.
}}<ref name=bennett>{{cite book |last1=Bennett |first1=Jeffrey O. |last2=Donahue |first2=Megan |last3=Schneider |first3=Nicholas |last4=Voit |first4=Mark |year=2020 |title=The Cosmic Perspective |publisher=Pearson |isbn=978-0-134-87436-4 |edition=9th}}</ref><ref>{{cite book |last1=Zeilik |first1=Michael |last2=Gregory |first2=Stephen A |title=Introductory Astronomy & Astrophysics |year=1998 |publisher=Brooks/Cole, Cengage Learning |location=Belmont Drive, CA |isbn=978-0-03-006228-5 |page=277 |edition=4th}}</ref><ref>{{cite book |last1=Ryden |first1=Barbara Sue |last2=Peterson |first2=Bradley M. |year=2010 |title=Foundations of Astrophysics |location=San Francisco, CA |isbn=978-0-321-59558-4 |page=436}}</ref> Shapley's measurements indicated the Sun is relatively far from the center of the galaxy, contrary to what had been inferred from the observed uniform distribution of ordinary stars. In reality most ordinary stars lie within the galaxy's disk and are thus obscured by gas and dust in the disk, whereas globular clusters lie outside the disk and can be seen at much greater distances.<ref name=ashman_zepf_1998/>[[File:A Swarm of Ancient Stars - GPN-2000-000930.jpg|thumb|right|upright=1.4|alt=Thousands of white-ish dots scattered on a black background, strongly concentrated towards the center|The [[Messier 80]] globular cluster in the constellation [[Scorpius]] is located about 30,000 [[light-year]]s from the Sun and contains hundreds of thousands of stars.<ref>{{cite web |title=Hubble images a swarm of ancient stars |url=https://esahubble.org/images/opo9926a/ |publisher=[[European Space Agency]] (ESA) |access-date=August 23, 2021}}</ref>]]

The count of known globular clusters in the Milky Way has continued to increase, reaching 83 in 1915, 93 in 1930, 97 by 1947,<ref name=SEDS/> and 157 in 2010.<!--
<ref name=harris_catalog>{{cite journal |last=Harris |first=William E. |date=October 1996 |title=A catalog of parameters for globular clusters in the Milky Way |journal=[[The Astronomical Journal]] |volume=112 |page=1487 |doi=10.1086/118116 |bibcode=1996AJ....112.1487H}} [https://www.physics.mcmaster.ca/~harris/mwgc.dat 2010 edition].</ref>
--><ref>{{cite web |last=Frommert |first=Hartmut |date=August 2007 |title=Milky Way Globular Clusters |publisher=[[Students for the Exploration and Development of Space]] | url=http://spider.seds.org/spider/MWGC/mwgc.html |access-date=February 26, 2008 }}</ref><ref>{{cite book |last=Carroll |first=Bradley W. |year=2017 |title=An introduction to modern astrophysics |publisher=[[Cambridge University Press]] |location=Cambridge, United Kingdom |isbn=978-1-108-42216-1 |page=894 |edition=2nd |url=https://books.google.com/books?id=PY0wDwAAQBAJ&pg=PA894 |access-date=September 24, 2021}}</ref> The number of known globular clusters in the Milky Way reached 158 by the end of 2010, according to the [[European Southern Observatory]], before two new globular clusters were discovered as part of the ESO’s VISTA ([[VISTA (telescope)|Visible and Infrared Survey Telescope for Astronomy]]) infrared survey, known as Variables in the Vía Láctea (VVV) survey, bringing the total to 160 known globular clusters.<ref>{{Cite web |last=information@eso.org |title=VISTA Finds New Globular Star Clusters - and sees right through the heart of the Milky Way |url=https://www.eso.org/public/news/eso1141/ |access-date=2025-02-09 |website=www.eso.org |language=en}}</ref> The two discovered by VISTA in 2011 are named VVV CL001 and VVV CL002.<ref>{{Cite journal |last=Minniti |first=D. |last2=Hempel |first2=M. |last3=Toledo |first3=I. |last4=Ivanov |first4=V. D. |last5=Alonso-García |first5=J. |last6=Saito |first6=R. K. |last7=Catelan |first7=M. |last8=Geisler |first8=D. |last9=Jordán |first9=A. |last10=Borissova |first10=J. |last11=Zoccali |first11=M. |last12=Kurtev |first12=R. |last13=Carraro |first13=G. |last14=Barbuy |first14=B. |last15=Clariá |first15=J. |date=2011-03-01 |title=Discovery of VVV CL001 - A low-mass globular cluster next to UKS 1 in the direction of the Galactic bulge |url=https://www.aanda.org/articles/aa/full_html/2011/03/aa15795-10/aa15795-10.html |journal=Astronomy & Astrophysics |language=en |volume=527 |pages=A81 |doi=10.1051/0004-6361/201015795 |issn=0004-6361|arxiv=1012.2450 }}</ref><ref>{{Cite journal |last=Bidin |first=C. Moni |last2=Mauro |first2=F. |last3=Geisler |first3=D. |last4=Minniti |first4=D. |last5=Catelan |first5=M. |last6=Hempel |first6=M. |last7=Valenti |first7=E. |last8=Valcarce |first8=A. a. R. |last9=Alonso-García |first9=J. |last10=Borissova |first10=J. |last11=Carraro |first11=G. |last12=Lucas |first12=P. |last13=Chené |first13=A.-N. |last14=Zoccali |first14=M. |last15=Kurtev |first15=R. G. |date=2011-11-01 |title=Three Galactic globular cluster candidates |url=https://www.aanda.org/articles/aa/abs/2011/11/aa17488-11/aa17488-11.html |journal=Astronomy & Astrophysics |language=en |volume=535 |pages=A33 |doi=10.1051/0004-6361/201117488 |issn=0004-6361|arxiv=1109.1854 }}</ref>

Additional, undiscovered globular clusters are believed to be in the [[galactic bulge]]<ref>{{cite journal |last1=Camargo |first1=D. |last2=Minniti |first2=D. |year=2019 |title=Three candidate globular clusters discovered in the Galactic bulge |journal=Monthly Notices of the Royal Astronomical Society: Letters |volume=484 |pages=L90–L94 |arxiv=1901.08574 |doi=10.1093/mnrasl/slz010|doi-access=free }}</ref> or hidden by the gas and dust of the Milky Way.<ref name="milky way">{{cite journal  |last1=Ashman |first1=Keith M. |author-link1=Keith M. Ashman |last2=Zepf |first2=Stephen E. |year=1992 |title=The formation of globular clusters in merging and interacting galaxies, Part&nbsp;1 |journal=[[Astrophysical Journal]] |volume=384 |pages=50–61 |bibcode=1992ApJ...384...50A |doi=10.1086/170850 }}</ref> For example, most of the [[Palomar Globular Clusters]] have only been discovered in the 1950s, with some located relatively close-by yet obscured by dust, while others reside in the very far reaches of the Milky Way halo. The [[Andromeda Galaxy]], which is comparable in size to the Milky Way, may have as many as five hundred globulars.<ref>{{cite journal |last1=Barmby |first1=P. |author-link1=Pauline Barmby |last2=Huchra |first2=J.P. |author-link2=John Huchra |title=M31 globular clusters in the Hubble Space Telescope Archive. I. Cluster detection and completeleness |journal=[[The Astronomical Journal]] |year=2001 |volume=122 |issue=5 |pages=2458–2468 |doi = 10.1086/323457 | bibcode=2001AJ....122.2458B|arxiv = astro-ph/0107401 |s2cid=117895577 }}</ref> Every galaxy of sufficient mass in the [[Local Group]] has an associated system of globular clusters, as does almost every large galaxy surveyed.<ref>{{cite journal | last = Harris | first = William E. | year=1991 | title=Globular cluster systems in galaxies beyond the Local Group | journal=[[Annual Review of Astronomy and Astrophysics]] | volume=29 | issue = 1 | pages=543–579 | bibcode=1991ARA&A..29..543H | doi=10.1146/annurev.aa.29.090191.002551 }}</ref> Some giant [[Elliptical galaxy|elliptical galaxies]] (particularly those at the centers of [[galaxy cluster]]s), such as [[Messier 87|M 87]], have as many as 13,000&nbsp;globular clusters.<ref>{{cite journal |author1=McLaughlin, Dean E. |author2=Harris, William E. |author3=Hanes, David A. | year=1994 | title=The spatial structure of the M87 globular cluster system | journal=[[Astrophysical Journal]] | volume=422 | issue=2 | pages=486–507 | bibcode=1994ApJ...422..486M | doi=10.1086/173744 }}</ref>

===Classification===
{{Main|Shapley–Sawyer Concentration Class}}

Shapley was later assisted in his studies of clusters by [[Henrietta Swope]] and [[Helen Sawyer Hogg]]. In 1927–1929, Shapley and Sawyer categorized clusters by the degree of concentration of stars toward each core. Their system, known as the [[Shapley–Sawyer Concentration Class]], identifies the most concentrated clusters as Class&nbsp;I and ranges to the most diffuse Class&nbsp;XII.<ref group="lower-alpha">The [[Shapley–Sawyer Concentration Class|Concentration Class]] is sometimes given with Arabic numerals (Classes&nbsp;1–12) rather than [[Roman numeral]]s.</ref><ref name=Hogg1965>{{cite journal |last=Hogg |first=Helen Battles Sawyer |year=1965 |title=Harlow Shapley and globular glusters |journal=[[Publications of the Astronomical Society of the Pacific]] |volume=77 |issue=458 |pages=336–346 |bibcode=1965PASP...77..336S |doi=10.1086/128229 |doi-access=free }}</ref> Astronomers from the [[Pontifical Catholic University of Chile]] proposed a new type of globular cluster on the basis of observational data in 2015: [[Dark globular cluster]]s.<ref>{{cite news |title=The Very Large Telescope discovers new kind of globular star cluster |date=May 13, 2015 |magazine=[[Astronomy (magazine)|Astronomy]] |url=http://www.astronomy.com/news/2015/05/the-very-large-telescope-discovers-new-kind-of-globular-star-cluster |access-date=May 14, 2015 }}</ref>
{{clear}}