M3 is an ancient stellar orb. Its stars formed when the Milky Way was still young, making the cluster a relic from the early history of our galaxy.

Over immense stretches of time, gravitational encounters between its stars have gradually smoothed out any original irregularities. Each close pass allows stars to exchange tiny amounts of energy, and after billions of these interactions the cluster has settled into a stable, nearly spherical structure.

There’s no sharp edge marking the end of M3. The density of stars decreases steadily from the brilliant central core toward the cluster’s faint outer regions, known as the tidal boundary. Here the gravitational pull of the Milky Way begins to compete with the cluster’s own gravity, gently tugging at its outermost stars. Over vast timescales some of these stars may escape entirely, drifting away to become part of the Galaxy’s halo.