Gravitationally bound system

A collection of objects held together by their mutual gravitational attraction strongly enough to resist separating with cosmic expansion.

Gravitational binding energy: The energy required to disperse a collection of masses against their mutual gravitational attraction. A system is bound when its total energy lies below the threshold for dispersal.
Gravity: The interaction by which mass and energy attract and influence one another. Mutual gravity is the binding influence at the heart of this concept.
Solar System: The Sun and the objects gravitationally orbiting it, including planets, dwarf planets, and smaller bodies. Its planets and smaller members remain bound to the Sun despite universal expansion.
Cosmic web: The universe’s large-scale network of filaments, sheets, and voids formed by matter under gravity. Bound systems occupy dense nodes and filaments within a much larger expanding structure.
Virial theorem: A relation between time-averaged kinetic and potential energies in a stable, bounded system. It connects the motions of a system’s components to the gravity holding them together.
Gravitational potential: Potential energy per unit mass associated with a gravitational field. It describes how the system’s mass distribution shapes the energy landscape for its members.
Milky Way: The barred spiral galaxy containing the Solar System and billions of other stars. Its stars and dark matter form a galaxy-scale structure bound by gravity.
Galaxy cluster: A massive gravitationally bound collection of galaxies, hot gas, and dark matter. Clusters are among the largest known structures whose members are gravitationally bound.
Escape velocity: The minimum speed needed for an object to escape a gravitational field without further propulsion. It measures whether an object’s motion can carry it beyond a system’s gravitational hold.
Orbital mechanics: The study of motion under gravity, especially the trajectories of orbiting bodies. Many bound systems persist through recurring orbits rather than static arrangements.

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