Editing Messier 2

{{short description|Globular cluster in the constellation Aquarius}}
{{Infobox globular cluster | <!-- put name or coordinates into the following URL -->
| name = Messier 2
| image = [[Image:Messier2 - HST - Potw1913a.jpg|250px]]
| caption = Messier 2 by [[Hubble Space Telescope]]; 2.5{{prime}} view
| credit = 
| epoch = [[Epoch (astronomy)#Julian years and J2000|J2000]]
| class = II<ref name=hcob849_11/>
| constellation = [[Aquarius (constellation)|Aquarius]]
| ra = {{RA|21|33|27.02}}<ref name=aj140_6_1830/>
| dec = {{DEC|–00|49|23.7}}<ref name=aj140_6_1830/>
| dist_ly = {{Convert|55000|ly|kpc|abbr=on|lk=on}}<ref name=helmi2018/>
| appmag_v = 6.5<ref>{{cite web |url=http://www.messier.seds.org/m/m002.html |title=Messier 2 |access-date=21 July 2024 |website=SEDS Messier Catalog}}</ref>
| size_v = 16.0{{prime}}
| mass_msol = {{Val|1.04|e=5}}<ref name=apj742_1_51/>
| radius_ly = 87.3&nbsp;ly<ref>distance × sin( diameter_angle / 2 ) = 87.3 ly. radius</ref>
| metal_fe = –1.65<ref name=apj742_1_51/>
| v_hb = 
| age = 12.5&nbsp;[[Gigayear|Gyr]]<ref name=mf2009/>
| notes = 
| names = [[New General Catalogue|NGC]] 7089<ref name="simbad" />
}}
'''Messier 2''' or '''M2''' (also designated '''NGC 7089''') is a [[globular cluster]] in the [[constellation]] [[Aquarius (constellation)|Aquarius]], five degrees north of the [[star]] [[Beta Aquarii]]. It was discovered by [[Jean-Dominique Maraldi]] in 1746, and is one of the largest known globular clusters.

==Discovery and Visibility==
M2 was discovered by the French astronomer [[Jean-Dominique Maraldi]] in 1746<ref name=omeara2014/> while observing a comet with [[Jacques Cassini]].<ref>{{Cite web |title=THE GLOBULAR CLUSTER M2 – Astronomy Magazine – Interactive Star Charts, Planets, Meteors, Comets, Telescopes |url=https://cs.astronomy.com/asy/m/starclusters/493473.aspx |access-date=2023-04-29 |website=cs.astronomy.com}}</ref> [[Charles Messier]] rediscovered it in 1760, but thought that it is a [[nebula]] without any stars associated with it. [[William Herschel]], in 1783, was the first to resolve individual stars in the cluster.<ref>{{Cite web |last=admin |date=2015-01-17 |title=Messier 2 |url=https://www.messier-objects.com/messier-2/ |access-date=2023-04-29 |website=Messier Objects |language=en-US}}</ref>

M2 is, under extremely good conditions, just visible to the naked eye. [[Binoculars]] or a [[small telescope]] will identify this cluster as non-stellar, while larger telescopes will resolve individual stars, of which the brightest are of [[apparent magnitude]] 6.5.<ref name="myeong2018" />

==Characteristics==
[[File:M2map.png|thumb|left|Chart showing location of M2]]
M2 is about 55,000 [[light-year]]s distant from [[Earth]]. At 175 light-years in diameter, it is one of the larger globular clusters known. The cluster is rich, compact, and significantly [[Ellipse|elliptical]]. It is 12.5 billion years old and one of the older globular clusters associated with the [[Milky Way]] galaxy.<ref name="mf2009" />

M2 contains about 150,000 stars, including 21 known [[variable stars]].  Its brightest stars are [[red giant|red]] and [[yellow giant]] stars.  The overall [[stellar classification|spectral type]] is F4.<ref name="simbad" />
M2 is part of the [[Gaia Sausage]], the hypothesized remains of a merged dwarf galaxy.<ref name=myeong2018/>

Data from ''[[Gaia (spacecraft)|Gaia]]'' has led to the discovery of an extended tidal [[stellar stream]], about 45 degrees long and 300 light-years (100 pc) wide, that is likely associated with M2. It was possibly perturbed due to the presence of the [[Large Magellanic Cloud]].<ref name=grillmair2022/>

Messier 2 is located within our Milky Way galaxy, and is one of the oldest clusters of stars designated to the Milky Way. Like most globular clusters, M2 is found within the [[galactic halo]], specifically in the southern galactic cap. This places it right below the southern pole of the Milky Way.<ref>{{Cite web |title=M2, NGC 7089 |url=https://www.noirlab.edu/public/images/noao-m2/ |access-date=2023-04-29 |website=noirlab.edu |language=en}}</ref>

==Oosterhoff Classification==
[[File:Curva RRab.png|thumb|325x325px|Light curve of an RRab variable star]]
M2 is defined as an Oosterhoff type II globular cluster. Oosterhoff type is a classification system of globular clusters originally observed by [[Pieter Oosterhoff]] in where globular clusters are generally separated into two types. Oosterhoff type is determined by [[metallicity]], age, and average pulsation period of type ab [[RR Lyrae variable|RR Lyrae variable stars]] of the cluster. A cluster metallicity below −1.6, an age above 13 billion years,<ref name="vdb2011" /> and an average RRab Lyrae pulsation period around .64 days indicates a type II cluster.<ref name="stobie1971" /> This .64 day value, coupled with a metallicity of −1.65, provides evidence that M2 follows the Oosterhoff Gap phenomena. This is an observed gap in the grouping of type I and type II clusters in the Milky Way on a metallicity vs average RRab pulsation period plot.<ref>{{Cite arXiv |last1=Kuehn |first1=Charles A. |last2=Smith |first2=Horace A. |last3=Catelan |first3=Marcio |last4=Jeon |first4=Young-Beom |last5=Nemec |first5=James M. |last6=Walker |first6=Alistair R. |last7=Kunder |first7=Andrea |last8=Dame |first8=Kyra |last9=Pritzl |first9=Barton J. |last10=De Lee |first10=Nathan |last11=Borissova |first11=Jura |date=2013-10-01 |title=RR Lyrae in the LMC: Insights Into the Oosterhoff Phenomenon |class=astro-ph.SR |eprint=1310.0553 }}</ref>

M2 is a bit of an anomaly in reference to Oosterhoff type. While it satisfies the metallicity and RRab Lyrae pulsation period conditions, it actually has an age of 12.5 Gyr, well below the cutoff age of 13 Gyr normal for a Oosterhoff type II cluster. This is unexpected because age of a cluster is generally determined from metallicity. However, this abnormality is explained in an article by Marín-Franch.<ref name=mf2009/>

==References==
{{reflist|30em|refs=

<ref name=grillmair2022>{{cite journal|doi=10.3847/1538-4357/ac5bd7|title=The Extended Tidal Tails of NGC 7089 (M2) |year=2022 |last1=Grillmair |first1=Carl J. |journal=The Astrophysical Journal |volume=929 |issue=1 |page=89 |arxiv=2203.04425 |bibcode=2022ApJ...929...89G |s2cid=247318732 |doi-access=free }}</ref>

<ref name=myeong2018>{{cite journal |arxiv=1805.00453 |last1=Myeong |first1=G. C |title=The Sausage Globular Clusters |journal=The Astrophysical Journal |volume=863 |issue=2 |pages=L28 |last2=Evans |first2=N. W |last3=Belokurov |first3=V |last4=Sanders |first4=J. L |last5=Koposov |first5=S. E |year=2018|bibcode=2018ApJ...863L..28M |doi=10.3847/2041-8213/aad7f7 |s2cid=67791285 |doi-access=free }}</ref>

<ref name=omeara2014>{{cite book | author = Stephen James O'Meara | date = 7 April 2014 | title = Deep-Sky Companions: The Messier Objects | publisher = Cambridge University Press | pages = 47– | isbn = 978-1-107-01837-2 | url = https://books.google.com/books?id=fDQZAwAAQBAJ&pg=PA47}}</ref>

<ref name=helmi2018>{{cite journal |arxiv=1804.09381 |title=Gaia Data Release 2: Kinematics of globular clusters and dwarf galaxies around the Milky Way |journal=Astronomy and Astrophysics |volume=616 |pages=A12 |last1=Helmi |first1=A |last2=van Leeuwen |first2=F |last3=McMillan |first3=P J |last4=Massari |first4=D |last5=Antoja |first5=T |last6=Robin |first6=A |last7=Lindegren |first7=L |last8=Bastian |first8=U |others=Gaia Collaboration|year=2018|bibcode=2018A&A...616A..12G |doi=10.1051/0004-6361/201832698 |s2cid=260496632 }}</ref>

<ref name=stobie1971>{{cite journal| bibcode=1971ApJ...168..381S | title=On the Difference Between the Oosterhoff Types i and II Globular Clusters | last1=Stobie | first1=R. S. | journal=The Astrophysical Journal | year=1971 | volume=168 | page=381 | doi=10.1086/151094 }}</ref>

<ref name=mf2009>{{cite journal| bibcode=2009ApJ...694.1498M | title=The ACS Survey of Galactic Globular Clusters. VII. Relative Ages | last1=Marín-Franch | first1=Antonio | last2=Aparicio | first2=Antonio | last3=Piotto | first3=Giampaolo | last4=Rosenberg | first4=Alfred | last5=Chaboyer | first5=Brian | last6=Sarajedini | first6=Ata | last7=Siegel | first7=Michael | last8=Anderson | first8=Jay | last9=Bedin | first9=Luigi R. | last10=Dotter | first10=Aaron | last11=Hempel | first11=Maren | last12=King | first12=Ivan | last13=Majewski | first13=Steven | last14=Milone | first14=Antonino P. | last15=Paust | first15=Nathaniel | last16=Reid | first16=I. Neill | journal=The Astrophysical Journal | year=2009 | volume=694 | issue=2 | page=1498 | doi=10.1088/0004-637X/694/2/1498 | arxiv=0812.4541 | s2cid=118407466 }}</ref>

<ref name=vdb2011>{{Cite journal |last=van den Bergh |first=Sidney |date=2011 |title=Some Systematics of Galactic Globular Clusters |journal=Publications of the Astronomical Society of the Pacific |volume=123 |issue=907 |pages=1044–1053 |bibcode=2011PASP..123.1044V |doi=10.1086/662132 |arxiv=1108.0708 |doi-access=free |issn=0004-6280}}</ref>

<ref name=aj140_6_1830>{{cite journal | display-authors=1 | last1=Goldsbury | first1=Ryan | last2=Richer | first2=Harvey B. | last3=Anderson | first3=Jay | last4=Dotter | first4=Aaron | last5=Sarajedini | first5=Ata | last6=Woodley | first6=Kristin | title=The ACS Survey of Galactic Globular Clusters. X. New Determinations of Centers for 65 Clusters | journal=The Astronomical Journal | volume=140 | issue=6 | pages=1830–1837 |date=December 2010 | doi=10.1088/0004-6256/140/6/1830 | bibcode=2010AJ....140.1830G |arxiv = 1008.2755 | s2cid=119183070 }}</ref>

<ref name=hcob849_11>{{cite journal | last1=Shapley | first1=Harlow | last2=Sawyer | first2=Helen B. | title=A Classification of Globular Clusters | journal=Harvard College Observatory Bulletin | volume=849 | issue=849 | pages=11–14 |date=August 1927 | bibcode=1927BHarO.849...11S }}</ref>
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<ref name=apj670_1_363>{{citation | display-authors=1 | last1=Hessels | first1=J. W. T. | last2=Ransom | first2=S. M. | last3=Stairs | first3=I. H. | last4=Kaspi | first4=V. M. | last5=Freire | first5=P. C. C. | title=A 1.4 GHz Arecibo Survey for Pulsars in Globular Clusters | journal=The Astrophysical Journal | volume=670 | issue=1 | pages=363–378 |date=November 2007 | doi=10.1086/521780 | bibcode=2007ApJ...670..363H |arxiv = 0707.1602 }}</ref>
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<ref name=apj742_1_51>{{cite journal | display-authors=1 | last1=Boyles | first1=J. | last2=Lorimer | first2=D. R. | last3=Turk | first3=P. J. | last4=Mnatsakanov | first4=R. | last5=Lynch | first5=R. S. | last6=Ransom | first6=S. M. | last7=Freire | first7=P. C. | last8=Belczynski | first8=K. | title=Young Radio Pulsars in Galactic Globular Clusters | journal=The Astrophysical Journal | volume=742 | issue=1 | page=51 |date=November 2011 | doi=10.1088/0004-637X/742/1/51 | bibcode=2011ApJ...742...51B |arxiv = 1108.4402 | s2cid=118649860 }}</ref>

<ref name="simbad">{{cite simbad | title=M 2 | access-date=2006-11-15 }}</ref>

}}

== See also ==
* [[Messier object#Messier_objects|List of Messier objects]]

== External links ==
{{Commons category|Messier 2}}
* [http://www.messier.seds.org/m/m002.html M2,SEDS Messier pages]
* [http://gclusters.altervista.org/cluster_4.php?ggc=M+2 M2, Galactic Globular Clusters Database page]
* {{WikiSky}}

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{{DEFAULTSORT:Messier 002}}
[[Category:Globular clusters]]
[[Category:NGC objects]]
[[Category:Messier objects|002]]
[[Category: Gaia-Enceladus]]
[[Category:Aquarius (constellation)]]
[[Category:Astronomical objects discovered in 1746|17460911]]