Clusters of stars come in two flavors. Open clusters, where the stars eventually drift apart; and globular clusters, where the stars are gravitationally bound. Our galaxy has about 150 globular clusters orbiting in its halo. At an age of 8 billion years, Messier 3 is a youngster as far as globular clusters go. At a distance of 33k light-years it's apparent diameter, 18 archmin, corresponds to a physical diameter of about 180 light-years.
M3 is an excellent target for relatively small telescopes, and can even be glimpsed by the unaided eye under darks skies. With a telescope of at least 8" in diameter, the stars become well resolved.
Nikon 300mm MF ED f/4.5, @ f/5.6
Regim - Sig18
cropped - 50% reduced