A globular cluster is a spherical collection of stars that orbits a galaxy core as a satellite. They are tightly bound by gravity, which gives them their spherical shape and relatively high stellar density towards their core. There are more than 150 known globular clusters in the Milky Way, with perhaps many more undiscovered. Large galaxies can have more: Andromeda, for instance, may have as many as 500. Some giant elliptical galaxies, such as M87, may have as many as 10,000 globular clusters. These globular clusters orbit the galaxy out to large radii, 40,000 parsecs or more. Every galaxy of sufficient mass in the Local Group and almost every large galaxy surveyed has an associated system of globular clusters. The Sagittarius Dwarf Spheroidal Galaxy and Canis Major Dwarf Galaxy both appear to be in the process of donating their associated globular clusters to the Milky Way, such as Palomar 12. (Full article...)