Globular clusters are ancient collections of stars that are gravitationally bound together in a spheroidal shape. Some are almost as old as the universe itself. They can contain anywhere from tens of thousands of stars to millions of stars. Globular clusters can range in size between a few dozen and a few hundred light years. Almost all larger galaxies have globular clusters in orbit around them and the Milky Way is no exception with around 200 of them circling its periphery. The much larger galaxy Messier 87 has as many as 15,000 globular clusters gracing it with their presence.
The stars they contain tend to be very old—globular clusters formed early in the universe's history and have long since exhausted the gas and dust needed for new star formation. The formation of globular clusters is not well understood. Current research leans toward the idea that they formed from very dense molecular clouds in the early universe. Some larger globular clusters may once have been dwarf galaxies whose outer star populations were stripped away by the gravitational influence of larger galaxies.
