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An endothermic reaction is a reaction that absorbs heat, the inverse of an exothermic reaction, i.e., that emits heat. (Fire and explosions are examples of exothermic reactions.) Some nuclear reactions require kinetic energy of the involved particles (i.e., heat), and absorb that heat, converting the energy into the (rest) mass of the resulting particles. The presence of such reactions affects the net heat generated by fusion (e.g., within stars), and also contributes to core collapses, such as that causing core collapse supernovae: the absorption of heat results in less pressure to support the core. The iron peak results from the fact that beyond iron, nucleosynthesis reactions of heavier nuclei are generally endothermic.