Thermal Runaway - Astrophysics

Astrophysics

Runaway thermonuclear reactions can occur in stars when nuclear fusion is ignited in conditions under which the pressure exerted by overlying layers of the star greatly exceeds thermal pressure, a situation that makes possible rapid increases in temperature. Such a scenario may arise in stars containing degenerate matter, in which electron degeneracy pressure rather than normal thermal pressure does most of the work of supporting the star against gravity, and in stars undergoing implosion. In all cases, the imbalance arises prior to fusion ignition; otherwise, the fusion reactions would be naturally regulated to counteract temperature changes and stabilize the star. In a star in which thermal pressure is in equilibrium with overlying pressure, initiation of a new exothermic reaction would cause an increase in temperature and thermal pressure, to which the star would respond by expanding and cooling. A runaway reaction is only possible when this response is inhibited.