= Gravothermal catastrophe
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A collisional self-gravitating stellar core can lose heat to its halo, contract and become hotter because it has <negative heat capacity of a virialized gravitational system>. The increased temperature contrast drives further outward heat flow and further contraction. The classical gravothermal catastrophe is the loss of a finite-density thermodynamic equilibrium at a finite total energy along the confined three-dimensional Newtonian isothermal sequence. It concerns slow <two-body relaxation>, not simply a collisionless instability. The dimensional form of the gravitational interaction and the thermodynamic ensemble matter: a two-dimensional logarithmic gravitational gas need not have the same finite-energy instability.
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