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A globular cluster (GC) is a group of stars, typically in the shape of a sphere, that is bound to a galaxy, orbiting the galaxy's center. They may be found within, above, or below a disk galaxy's galactic disk, but within its galactic 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 impact (actual impacts are rare), but that 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.
The Milky Way has about 150 globular clusters, the Large Magellanic Cloud (LMC) about 60, and Andromeda (M31), on the order of 500. The Milky-Way globular clusters are generally Population II stars, i.e., less metal and older than the Sun, an indication that they are old. Current aging methods (e.g., using the turn-off point) of Milky-Way globular clusters also show them to be quite old, some on the order of 13 billion years (epoch of reionization), with ~10 billion years common (at or before peak epoch). 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), and with similar metallicity for being born from the same star-forming region, which makes them very useful for study of stellar evolution. H-R diagrams of individual clusters demonstrate this, but some have shown a very slight divisions in their plotted main-sequence lines, 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, but there has been evidence that this is not fully true, i.e., that some older globular clusters show more than one population, stimulating research into how this occurs.
Despite their high density, and the behavior analogous to a gas with gravitationally-interacting stars playing the role of the gas's bouncing molecules, dynamics suggests many globular clusters could shrink further than they have, suggesting 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).
The globular clusters of a galaxy are referred to as its globular cluster system (GC system). Among the characteristics of GC systems is the globular cluster specific frequency (SN,), essentially, the galaxy's number of GCs divided by the galaxy's luminosity. Some GC systems show a globular cluster bimodality, some being metal-rich (termed red) and others metal-poor (termed blue), with few between. The explanation and their formation is an area of research interest: a straight-forward hypothesis is there are two different mechanisms for their formation.
In surveys searching for globular clusters, possibilities are termed globular cluster candidates (GCCs).