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The stars themselves that make up globular clusters are all metal-poor population II stars, older stars similar to those located in the central budge of the Milky Way.  There is also no detectable gas or dust in these clusters suggesting that all such material had long ago been used up to create stars.

The stars themselves that make up globular clusters are all metal-poor population II stars, older stars similar to those located in the central budge of the Milky Way.  There is also no detectable gas or dust in these clusters suggesting that all such material had long ago been used up to create stars.

Globular clusters are future divided into two major groupings known as ''Oosterhoff groups'', the difference between the level of [[metallicity]] found in the stars in the cluster.  Clusters of the type I group are found to have somewhat weak metal absorption line in their spectra, while Type II have very weak metal lines.  As such, Type I clusters are referred to as "metal-rich" and Type II as "metal-poor".  Both types are metal-poor in comparison to younger population I stars found in the [[galactic disk]].<ref>van Albada, T. S.; Baker, Norman (1973). "On the Two Oosterhoff Groups of Globular Clusters". Astrophysical Journal 185: 477–498. do:10.1086/152434.</ref>  In the Milky Way, the more metal-poor type II clusters are located in the outer part of the galactic halo, while the more metal-rich clusters are found near the galactic budge.<ref>http://adsabs.harvard.edu/abs/1976AJ.....81.1095H</ref> Both types of globular cluster populations have been found in several galaxies, being most common in large [[elliptical galaxy|elliptical galaxies]].  What causes the difference between the two types of clusters is not exactly known.  Some scenarios to explain this include galaxy mergers, the absorption of satellite galaxies, and staggered star formation within galaxies.<ref>http://adsabs.harvard.edu/abs/2002astro.ph..7607Y</ref>

Globular clusters are further divided into two major groupings known as ''Oosterhoff groups'', the difference between the level of [[metallicity]] found in the stars in the cluster.  Clusters of the type I group are found to have somewhat weak metal absorption line in their spectra, while Type II have very weak metal lines.  As such, Type I clusters are referred to as "metal-rich" and Type II as "metal-poor".  Both types are metal-poor in comparison to younger population I stars found in the [[galactic disk]].<ref>van Albada, T. S.; Baker, Norman (1973). "On the Two Oosterhoff Groups of Globular Clusters". Astrophysical Journal 185: 477–498. do:10.1086/152434.</ref>  In the Milky Way, the more metal-poor type II clusters are located in the outer part of the galactic halo, while the more metal-rich clusters are found near the galactic budge.<ref>http://adsabs.harvard.edu/abs/1976AJ.....81.1095H</ref> Both types of globular cluster populations have been found in several galaxies, being most common in large [[elliptical galaxy|elliptical galaxies]].  What causes the difference between the two types of clusters is not exactly known.  Some scenarios to explain this include galaxy mergers, the absorption of satellite galaxies, and staggered star formation within galaxies.<ref>http://adsabs.harvard.edu/abs/2002astro.ph..7607Y</ref>

Globular clusters are believed to be around 9 to 13 billion years in age and initially form as a loose collection of stars.  As the cluster passes into "adolescence", the stars near the center of the cluster begin to collapse in towards each other.  This collapse ends when the interaction of [[binary star|binary systems]] prevent any further contraction, at this point the cluster is at "middle age".  Eventually over time, stars in the binary systems are ejected by gravitational disruption as the cluster passes through "old age".  It is believed that virtually all globular clusters are far along in the "old age" portion of their evolution.  However a more recent study of 13 globular clusters suggests though that some of the clusters may actually be much younger then initially believed.  This is due to the fact that three of the clusters were found to still have a large number of [[x-ray]] binaries, suggesting that not enough time has passed to eject many binary companions from the cluster.  If these new observations are confirmed, this would challenge the current theories on the evolution of such clusters.<ref>http://www.astronomynow.com/Oldglobularclusterssurprisinglyyoung.html</ref>

Globular clusters are believed to be around 9 to 13 billion years in age and initially form as a loose collection of stars.  As the cluster passes into "adolescence", the stars near the center of the cluster begin to collapse in towards each other.  This collapse ends when the interaction of [[binary star|binary systems]] prevent any further contraction, at this point the cluster is at "middle age".  Eventually over time, stars in the binary systems are ejected by gravitational disruption as the cluster passes through "old age".  It is believed that virtually all globular clusters are far along in the "old age" portion of their evolution.  However a more recent study of 13 globular clusters suggests though that some of the clusters may actually be much younger then initially believed.  This is due to the fact that three of the clusters were found to still have a large number of [[x-ray]] binaries, suggesting that not enough time has passed to eject many binary companions from the cluster.  If these new observations are confirmed, this would challenge the current theories on the evolution of such clusters.<ref>http://www.astronomynow.com/Oldglobularclusterssurprisinglyyoung.html</ref>