Gravitational instability

Gravitational instability is the growth of density variations when self-gravity overcomes pressure, rotation, or motion that would otherwise disperse matter.

Jeans instability: The gravitational instability of a gas when a density perturbation exceeds a critical scale set by pressure and gravity. It gives the classic criterion for when gas-density fluctuations grow instead of being pressure-supported.
Star formation: The process by which dense regions of interstellar gas collapse and form stars. Gravitational instability initiates collapse within sufficiently dense molecular-cloud regions.
Self-gravity: The gravitational attraction exerted by a system's own mass on its constituent matter. It is the collective force that amplifies overdense regions.
Thermal pressure: Pressure arising from the random thermal motion of particles in a material. It resists compression and can keep a region below its gravitational instability threshold.
Jeans length: The critical wavelength above which pressure cannot prevent a gas perturbation from collapsing under self-gravity. It sets the characteristic size threshold in the Jeans instability criterion.
Molecular cloud: A cold, dense interstellar cloud composed primarily of molecular hydrogen and dust. These clouds provide the gas in which pressure, turbulence, and gravity compete during star formation.
Density perturbation: A local deviation in matter density from a chosen background value. Instability begins with such departures and concerns whether they grow with time.
Turbulence: Irregular fluid motion characterized by interacting eddies across a range of scales. Turbulent motions can support a cloud on large scales while promoting dense structures locally.
Virial theorem: A relation connecting the time-averaged kinetic energy and potential energy of a bound system. It helps assess whether a gravitating cloud has enough internal motion to resist collapse.
Protoplanetary disk: A rotating disk of gas and dust surrounding a young star, from which planets can form. Strong disk self-gravity can fragment the disk into bound clumps.

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