Accretion (astrophysics)

Accretion is the growth of a celestial body as gravity gathers smaller bodies, gas, or dust. It builds planets, stars, and compact objects, often through a surrounding disk.

Accretion disk: A rotating structure of gas, dust, or plasma that spirals toward a central object. Disks channel matter inward while angular momentum spreads outward.
Core accretion model: A model in which solid planetary cores form first and later capture surrounding gas. It explains how giant planets can grow from solids and acquire massive atmospheres.
Pierre-Simon Laplace: A French mathematician and astronomer who proposed an early nebular account of planetary-system formation. Laplace's nebular model helped frame later ideas about growth from orbiting matter.
Gravitational collapse: The contraction of matter under its own gravity, often initiating star formation. Collapse concentrates a cloud; accretion subsequently adds matter to a central object.
Angular momentum transport: The redistribution of angular momentum that allows rotating matter to move inward or outward. Accretion stalls unless disk material sheds angular momentum.
Nebular hypothesis: The idea that planetary systems form from rotating clouds of gas and dust. Accretion is the growth process within this broad account of planetary origins.
James Jeans: A British physicist and astronomer known for work on stellar structure, radiation, and gravitational instability. His tidal theory offered a competing account of how planetary material originated.
Coagulation: The joining of particles through collisions into progressively larger aggregates. Coagulation describes particle sticking, while accretion also includes gravitational capture.
Gravitational focusing: The enhancement of collision rates when gravity bends incoming trajectories toward a body. A growing body's gravity enlarges its effective collision target.
Stellar formation: The process by which dense regions of molecular clouds collapse into stars. Young stars gain much of their mass from infalling cloud material.

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