Editing Messier 2 (section)

==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/>