The NASA/ESA Hubble Space Telescope offers this wonderful view of the crowded astral encampment called Messier 68, a spherical, star-filled region of space known as a circular cluster. Mutual gravitational attraction amongst a cluster’s hundreds of thousands or even millions of stars keeps stellar members in check, allowing globular clusters to hang together for many billions of years. More than 150 of these objects surround our Milky Way galaxy.
On a galactic scale, globular clusters are indeed not all that big. In Messier 68's case, its constituent stars span a volume of space with a diameter of little more than a hundred light-years. The disc of the Milky Way, on the other hand, extends over some 100,000 light-years or more. Astronomers can measure the ages of globular clusters by looking at the light of their constituent stars.