Messier 15

Messier 15 (M15 or NGC 7078) is a globular cluster, unusual for the dense packing of stars in the center (the profile of the density rising all the way to the center), apparently having experienced a core collapse, a collapse of the middle region of a globular cluster. (Globular clusters typically have a central region of a high but relatively constant density of stars.) It includes 100,000+ stars, including a known planetary nebula and some known pulsars and variable stars. Its estimated age is 12 gigayears and it has a diameter of about 175 ly.