A globular cluster (GC) is a group of stars, typically in the shape of a sphere, that is bound to a galaxy and orbits its center. They may be found above or below the galactic plane, but within the galaxy's presumed dark matter halo. They have as many as a million stars, and are very compact, and can have a core of hundreds of thousands of stars: a star within such a core can have tens of thousands of neighbor stars nearer than Alpha Centauri is to us. They are considered collisional, not meaning that stars frequently collide (actual collisions are rare), but there are frequent strong gravitational interactions between stars, i.e., encounters that drastically change their direction of motion. There are even more weak gravitational interactions, slight changes in direction when passing somewhat close to another star.
They are generally Population II stars, i.e., less metal and older than the Sun, indicating they are old. Current aging methods (e.g., using the turn-off point) of Milky-Way globular clusters show them to be quite old, some on the order of 13 billion years, with ~10 billion years common. Examples of younger globular clusters can be found in the Magellanic Clouds.
Globular clusters and open clusters are generally thought to be stars formed over a short period of time, making the stars approximately the same age (coeval). However, clusters have shown a very slight division in the plotted main-sequence line on globular cluster H-R diagrams, and the observation of spectral signs of aluminum and sodium suggest a second generation of stars as these are elements likely to be synthesized by short-lived first-generation stars (O-type star and A-type star) in their giant phases. Over time, evidence has mounted of multiple populations in older globular clusters, and theories as to how this comes about have challenges.
Despite their high density, and the behavior analogous to a gas, of gravitationally-interacting stars playing the role of the gas's bouncing molecules, dynamics suggests many globular clusters could shrink further than they have, evidence of some energy source adding to the kinetic energy of the stars. A possible source is binary stars: strong interactions with a third star can increase their orbit hardness, which releases potential energy from the pair in the form of the third star leaving with a higher velocity. The largest sources of such energy are the most massive binaries, which likely continue to have such encounters after they have evolved into black holes or neutron stars. This source of energy has been termed binary burning, in analogy to the energy source of stars, their "burning" (fusion). Dense clusters and clusters that are small tend to eject more of the massive objects, thus lose much of this source of energy.
Some clusters have a central region in which surface brightness is uniform, while others show an intensity increasing all the way to the center, as if the cluster has undergone a core collapse. The term gravithermal catastrophe refers to such a core collapse in which the kinetic energy "heat" (i.e., movement of the stars) has been removed from the central region through the more-likely escape of stars with the highest velocities, possibly leaving the whole globular cluster, allowing the region to become denser since energy is removed. An oscillation can form from rises in density, and subsequent falls as some of the stars escape, the escape referred to as evaporation (in analogy to molecules escaping from a liquid surface).