Editing Globular cluster (section)

===Heavy element content===
Globular clusters normally consist of [[Population&nbsp;II stars]] which, compared with [[Population&nbsp;I stars]] such as the [[Sun]], have a higher proportion of hydrogen and helium and a lower proportion of heavier elements. Astronomers refer to these heavier elements as ''metals'' (distinct from the material concept) and to the proportions of these elements as the metallicity. Produced by [[stellar nucleosynthesis]], the metals are recycled into the [[interstellar medium]] and enter a new generation of stars. The proportion of metals can thus be an indication of the age of a star in simple models, with older stars typically having a lower metallicity.<ref>{{cite book |title=An Introduction to the Sun and Stars |page=240 |publisher=Cambridge University Press |year=2004 |isbn=978-0-521-54622-5 |author1=Green, Simon F. |author2=Jones, Mark H. |author3=Burnell, S. Jocelyn}}</ref>

The Dutch astronomer [[Pieter Oosterhoff]] observed two special populations of globular clusters, which became known as ''Oosterhoff groups''. The second group has a slightly longer period of RR Lyrae variable stars.<ref name="oosterhoff">{{cite journal |title=On the Two Oosterhoff Groups of Globular Clusters |journal=Astrophysical Journal |volume=185 |year=1973 |pages=477–498 |doi=10.1086/152434 |author1=van Albada, T. S. |author2=Baker, Norman |bibcode=1973ApJ...185..477V}}</ref> While both groups have a low proportion of metallic elements as measured by [[Astronomical spectroscopy|spectroscopy]], the metal spectral lines in the stars of Oosterhoff type{{spaces}}I (Oo{{spaces}}I) cluster are not quite as weak as those in type{{spaces}}II (Oo{{spaces}}II),<ref name="oosterhoff" /> and so type{{spaces}}I stars are referred to as ''metal-rich'' (e.g. [[Terzan&nbsp;7]]<ref name="Terzan7">{{cite journal |title=ESO 280-SC06 |journal=Astronomical Journal |volume=109 |page=663 |year=1995 |author1=Buonanno, R. |author2=Corsi, C.E. |author3=Pulone, L. |bibcode=1995AJ....109..663B |doi=10.1086/117309}}</ref>), while type{{spaces}}II stars are ''metal-poor'' (e.g. [[ESO&nbsp;280-SC06]]<ref name="ESO280-6">{{cite web |last=Frommert |first=Hartmut | title=Globular cluster ESO 280-S C06, in Ara | website=Students for the Exploration and Development of Space |url=http://spider.seds.org/spider/MWGC/eso280sc06.html |access-date=April 9, 2021 }}</ref>). These two distinct populations have been observed in many galaxies, especially massive elliptical galaxies. Both groups are nearly as old as the universe itself and are of similar ages. Suggested scenarios to explain these subpopulations include violent gas-rich galaxy mergers, the accretion of dwarf galaxies, and multiple phases of star formation in a single galaxy. In the Milky Way, the metal-poor clusters are associated with the halo and the metal-rich clusters with the bulge.<ref>{{cite journal |last=Harris |first=W.E. |title=Spatial structure of the globular cluster system and the distance to the galactic center |journal=Astronomical Journal |year=1976 |volume=81 |pages=1095–1116 |bibcode=1976AJ.....81.1095H |doi=10.1086/111991}}</ref>

A large majority of the metal-poor clusters in the Milky Way are aligned on a plane in the outer part of the galaxy's halo. This observation supports the view that type{{spaces}}II clusters were captured from a satellite galaxy, rather than being the oldest members of the Milky Way's globular cluster system as was previously thought. The difference between the two cluster types would then be explained by a time delay between when the two galaxies formed their cluster systems.<ref>{{cite journal |author1=Lee, Y.W. |author2=Yoon, S.J. | title=On the Construction of the Heavens |journal=An Aligned Stream of Low-Metallicity Clusters in the Halo of the Milky Way |volume=297 |year=2002 |pages=578–581 |bibcode=2002Sci...297..578Y |doi=10.1126/science.1073090 |pmid=12142530 |issue=5581 |arxiv=astro-ph/0207607|s2cid=9702759 }}</ref>