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Abraham Ihle, an amateur [[astronomer]] from [[Germany]] was the first to discover a globular cluster when he found the cluster M22 in 1665, however at the time his telescope was not able to resolve the individual stars.  [[Charles Messier]] was the first to identify globular clusters as being made up of individual stars when he observed the cluster M4.  It was [[William Herschel]] though who first coined the term globular cluster in his catalog of deep sky objects in 1789.  Herschel also discovered 37 such clusters alone, as well as fully resolve the stars in 33 previous discovered ones.

Abraham Ihle, an amateur [[astronomer]] from [[Germany]] was the first to discover a globular cluster when he found the cluster M22 in 1665, however at the time his telescope was not able to resolve the individual stars.  [[Charles Messier]] was the first to identify globular clusters as being made up of individual stars when he observed the cluster M4.  It was [[William Herschel]] though who first coined the term globular cluster in his catalog of deep sky objects in 1789.  Herschel also discovered 37 such clusters alone, as well as fully resolve the stars in 33 previous discovered ones.

In 1918, [[Harlow Shapley]] used his studies of globular clusters and their asymmetrical distribution in the galaxy to calculate both the distance of the [[Sun]] to the galactic center, and the overall dimensions of the [[Milky Way]] itself.  Although the measurements he made were off from the actual size of the galaxy, due to not taking into account dust in the Milky Way diminishing light from the various clusters, he did in fact show the galaxy was much larger then previously believed.<ref>http://adsabs.harvard.edu/abs/1918PASP...30...42S</ref>

In 1918, [[Harlow Shapley]] used his studies of globular clusters and their asymmetrical distribution in the galaxy to calculate both the distance of the [[Sun]] to the galactic center, and the overall dimensions of the [[Milky Way]] itself.  Although the measurements he made were off from the actual size of the galaxy, due to not taking into account dust in the Milky Way diminishing light from the various clusters, he did in fact show the galaxy was much larger than previously believed.<ref>http://adsabs.harvard.edu/abs/1918PASP...30...42S</ref>

==Composition==

==Composition==

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 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 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>

[[Category: Astronomy]]

[[Category:Astronomy]]

==References==

==References==

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