Messier 15 — Globular Cluster in Pegasus

M15 is one of the densest and most luminous globular clusters in the Milky Way, located about 33,600 light-years from Earth in the constellation Pegasus. It contains roughly 100,000 stars and spans about 175 light-years in diameter. M15 has undergone core collapse — its central density has increased dramatically as stars have sunk toward the center over billions of years, producing an extremely compact core. At magnitude 6.2, it is one of the brightest globular clusters visible from northern latitudes. The cluster is estimated to be about 12.5 billion years old.

Located about 4 degrees northwest of the star Enif (Epsilon Pegasi). At magnitude 6.2, it is faintly visible to the naked eye under dark skies. Binoculars show a bright, concentrated fuzzy star. A 4-inch telescope at 100x reveals a blazing core surrounded by a hazy envelope. Larger apertures of 8 inches and above at 150-200x resolve individual stars in the outer regions, though the core remains an intense pinpoint of light. Best observed from August through November.

Discovered by Jean-Dominique Maraldi on September 7, 1746. Charles Messier cataloged it in 1764. In 1928, the first planetary nebula found inside a globular cluster — Pease 1 — was discovered within M15. It was also the first globular cluster in which an X-ray source was detected, in 1974.