A globular cluster is a spherical collection of stars. Globular clusters are very tightly bound by gravity, giving them their spherical shapes and high concentrations of stars toward their centers. Their name is derived from Latin globulus—a small sphere. Globular clusters are occasionally known simply as globulars.

Although one globular cluster, Omega Centauri, was observed and thought to be a star in antiquity, recognition of the clusters' true nature came with the advent of telescopes in the 17th century. In early telescopic observations, globular clusters appeared as fuzzy blobs, leading French astronomer Charles Messier to include many of them in his catalog of astronomical objects that he thought could be mistaken for comets. Using larger telescopes, 18th-century astronomers recognized that globular clusters are groups of many individual stars. Early in the 20th century, the distribution of globular clusters in the sky was some of the first evidence that the Sun is far from the center of the Milky Way.

Globular clusters are found in nearly all galaxies. In spiral galaxies like the Milky Way, they are mostly found in the outer, spheroidal part of the galaxy—the galactic halo. They are the largest and most massive type of star cluster, tending to be older, denser, and composed of fewer heavy elements than open clusters, which are generally found in the disks of spiral galaxies. The Milky Way has over 150 known globulars and possibly many more undiscovered.

The origin of globular clusters and their role in galactic evolution remain unclear. Some are among the oldest objects in their galaxies and even the universe, constraining estimates of the universe's age. Star clusters are often assumed to consist of stars that all formed at the same time from one star-forming nebula, but nearly all globular clusters contain stars that formed at different times, or that have differing compositions. Some clusters may have had multiple episodes of star formation, and some may be remnants of smaller galaxies captured by larger galaxies.

 

Note:   The above text is excerpted from the Wikipedia article Globular cluster, which has been released under the GNU Free Documentation License.