How does gravity hold the stars of a star cluster together?
Most of the brightest stars are older yellow stars, but a few young blue stars are also visible. This picture is a composite of three images taken by the Hubble Space Telescope.
Star clusters are stellar assemblages held together by the mutual gravitational attraction of their member stars. These stars are physically related, typically having formed together within a molecular cloud where they accumulated sufficient mass to be pulled toward one another by gravity.12
The structural integrity of a cluster is governed by its internal dynamics. Astronomers use dynamical models to understand how these stars interact, accounting for their individual orbits and encounters with one another. While gravity acts to bind the cluster, its stability is also influenced by external factors, such as the disruptive tidal forces of the Galaxy.34
For example, globular clusters are massive enough to remain bound against these tidal forces, whereas many open clusters are less massive and may eventually disperse as they are pulled apart by the Galaxy's gravitational influence. The balance between a cluster's internal gravitational binding and these external forces determines its long-term survival and structure.453