Star cluster

Globular cluster Messier 2 (or M2, or NGC 7089) is a globular cluster in the constellation Aquarius

A star cluster is a group of stars. They are held together by mutual gravitation. They are related by common origin, and have similar or identical ages.

In contrast, constellations and asterisms are just line-of-sight visual groups as seen from the Earth.

There are two major types of clusters:

1. open star clusters. Open clusters are loose, and they have less than a few hundred stars, which are often very young. A famous star cluster is the Pleiades, which is an open cluster.
2. globular clusters. Globular clusters are tight groups of hundreds and thousands of stars. Every large galaxy has hundreds or thousands of globulars revolving round it at great distances, usually outside the disk of the galaxy.

Star clusters visible to the naked eye include the Pleiades, the Hyades, and 47 Tucanae.

Super star clusters

Super star clusters of very young large stars are known. They are thought to be precursors of globular clusters. The short-lived huge blue stars emit lots of UV radiation which ionises the surrounding gas. In our galaxy, examples are Westerlund 1 and Westerlund 2 in the Milky Way. In Andromeda there is an even bigger one: Mayall II, nicknamed Globular One, has a greater luminosity than any other known globular cluster in the Local Group of galaxies.