A globular cluster is a spherical grouping of stars that share a common origin and orbits the galactic core of a galaxy as a satellite. They are very tightly bound by gravity and contain anywhere from tens of thousands to million of stars in an area that is only some 300 light years across or less, and are generally oblate spheroids in shape. The density of the clusters are on average around 0.4 stars per cubic parsec, but increases toward to the center of the cluster, reaching as high as 100 or even a 1000 stars per cubic parsec.
Unlike open clusters, which contain population I stars and reside in the galactic disk, globular clusters are made up of population II stars and are found in either the galactic bulge or galactic halo. Some globular clusters are found as far out as 131,000 light years from the core of the galaxy. Current, there are 158 known globular clusters around the Milky Way, with several more perhaps yet to be discovered, all moving in highly eccentric orbits. Beyond the Milky Way, most other galaxies in the Local Group and beyond have globular clusters. Andromeda is known to have some 500.
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 than previously believed.
The stars themselves that make up globular clusters are all metal-poor population II stars, similar to those located in the central budge of the Milky Way. There is also no detectable gas or dust in these clusters.
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. 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. Both types of globular cluster populations have been found in several galaxies, being most common in large 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.
- van Albada, T. S.; Baker, Norman (1973). "On the Two Oosterhoff Groups of Globular Clusters". Astrophysical Journal 185: 477–498. do:10.1086/152434.