Globular clusters are tightly bound groups of stars which orbit galaxies. The Latin word globulus, from which these clusters take their name, means a small sphere. A large mass in the rich stellar centre of a globular cluster pulls the stars inward to form a ball of stars.
Globular clusters are generally very ancient objects that form around the same time as their host galaxy. Thus far, no new star formation has been observed within amy globular cluster. The lack of young stars explains the abundance of aging yellow stars in this image, most of them containing very few heavy elements.
Image Credit: NASA