A globular cluster is a dense, roughly spherical swarm of hundreds of thousands to millions of stars, bound tightly by gravity and orbiting in the halo of a galaxy. They are among the oldest structures known, typically ten to thirteen billion years old, and their stars formed early enough to be markedly poor in heavy elements. Their ages once posed a crisis, appearing older than the universe itself until both measurements were refined. The Milky Way hosts roughly 150 of them.