In addition to sporting the heavy mass of 130 to 250 times that of our Sun, candidate progenitor stars for pair-instability supernovae must also have slow to moderate rotation rates, as well as a very low metallicity. Low metallicity means that the candidate progenitor star for such special, catastrophic, and weird supernovae must be made up almost entirely of hydrogen and helium.
During the strange catastrophic–and completely fatal–pair-instability supernova, the doomed and dying giant star’s core grows so extremely hyper-energetic that atomic nuclei and gamma-rays violently crash into one another. This smash-up results in the formation of electron-positron pairs. A positron is an electron’s antimatter twin that possesses a positive charge as opposed to the negative charge of an electron. This process sucks up a large amount of the available thermal energy, which triggers a dramatic fall in pressure. In turn, the rapid drop in pressure triggers the doomed giant star’s catastrophic collapse, when it falls victim to the relentless pull of its own gravity.
The regions of stellar gravitational collapse are quickly heated up to extremely high temperatures and pressures, resulting in the rapid fusion of atomic nuclei–and a terrible beauty is born in the form of an enormous and powerful blast of energy. Indeed, the resulting quantity of thermal energy is so huge that it causes the star to be blasted out of existence–vanishing without a trace. Nothing at all is left behind as a souvenir to the Universe that once the exploded gigantic star ever existed. There is no relic black hole. There is absolutely nothing. All other supernovae leave a tattle-tale stellar-mass black hole behind or–if the progenitor star was a little less massive–a neutron star. Neutron stars are extremely dense city-size spheres that are essentially one big atomic nucleus. One teaspoon of neutron star material can weigh as much as a river full of sun-bathing hippos.
Pair-instability supernovae are probably rare in the modern Universe. Currently, most of the stars in the Cosmos are too light, and too heavily laden with metals, to perish in such a monumental explosion. But, when our Universe was very young, such terrible stellar blasts were more common than they are now. The ancient Universe’s stars had lower metallicities, and were much more massive, than the stars we are familiar with. This is because only hydrogen, helium, and traces of lithium formed during the Big Bang birth of the Universe almost 14 billion years ago (Big Bang nucleosynthesis). All of the atomic elements heavier than helium (metals) were cooked up by way of the process of nuclear fusion in the cores of the Universe’s myriad stars (stellar nucleosynthesis). However, the heaviest atomic elements of all, such as gold, formed in the supernova blast itself.