A pair-instability supernova is believed to result from runaway oxygen fusion in the core of a massive, 130–250 solar mass, low to moderate metallicity star. According to theory, in such a star, a large but relatively low density core of nonfusing oxygen builds up, with its weight supported by the pressure of gamma rays produced by the extreme temperature. As the core heats further, the gamma rays eventually begin to pass the energy threshold needed for collision-induced decay into electron-positron pairs, a process called pair production. This causes a drop in the pressure within the core, leading it to contract and heat further, causing more pair production, a further pressure drop, and so on. The core starts to undergo gravitational collapse. At some point this ignites runaway oxygen fusion, releasing enough energy to obliterate the star. These explosions are rare, perhaps about one per 100,000 supernovae.