Galactic tide

The differential gravitational force a galaxy exerts across an object or system within it. It arises because gravity varies with position in the galaxy’s gravitational field.

Tidal tensor: A matrix of spatial derivatives of a gravitational field that describes differential acceleration between nearby points. It expresses the galactic gravitational field’s local stretching and compressing along different directions.
Oort cloud comet: A comet originating in the distant, loosely bound reservoir surrounding the Solar System. Galactic tides can alter its orbit enough to bring it into the planetary region.
Tidal force: The difference in gravitational acceleration across an extended object or between nearby locations. Galactic tide is the large-scale astronomical instance of this general gravitational effect.
Dynamical friction: The deceleration of a massive object moving through a background of particles because of its gravitational wake. It can drive satellites inward, while galactic tides strip their mass along the way.
Jacobi radius: The approximate boundary within which a smaller system’s gravity can retain objects against a larger system’s tidal field. It estimates how far a star cluster’s members can remain gravitationally bound in the Galaxy.
Open cluster: A loosely bound group of stars formed from the same molecular cloud. The Milky Way’s tidal field helps disperse open clusters over time.
Isaac Newton: The English mathematician and physicist who formulated classical mechanics and universal gravitation. Newtonian gravity provides the framework for calculating galactic tidal forces.
Milky Way halo: The extended region surrounding the Milky Way’s disk, containing dark matter, old stars, and hot gas. Tidal debris in the halo constrains its shape and gravitational field.
Hill sphere: The region around an orbiting body where its gravity dominates over a more massive body’s tidal influence. It describes a related gravitational boundary, usually for a body orbiting another body.
Globular cluster: A dense, roughly spherical collection of old stars bound by their mutual gravity. Its outer stars can be stripped by the Galaxy, leaving tidal tails and streams.

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