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Thermal runaway is the result of positive feedback from some process producing heat, i.e., raising the temperature of some heat-producing process makes it produce more heat. The term is used for astronomical bodies such as planets and stars. An example of such a process is a global-warming scenario: a situation where a higher temperature causes more greenhouse gas to be produced.
In stars, the rate of fusion is often temperature-dependent and fusion produces heat; the ignition of fusion typically induces more to take place, and if no contravening factor immediately limits the rate (e.g., diluting and/or using up the fuel), such a thermal runaway follows. If a star has a degenerate core that becomes hot enough to trigger fusion, the generated heat might raise the temperature without decreasing density through pressure, resulting in a thermal runaway, such as a helium flash.