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The tendency for a small perturbation in the density or equilibrium of a system to result in further disruption of the initial state, owing to the attractive nature of the gravitational force. In gas clouds, for example, a slight compression that raises the gravitational force locally causes more matter to be sucked in, which, in turn, raises the local field further. Instabilities of this sort, perhaps started by compression in the spiral arms of galaxies, are probably the means by which star formation is initiated in giant molecular clouds.
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