Once protogalaxies began to form and contract, the first halo stars, called Population III stars, appeared within them. These were composed of primordial gas, almost entirely of hydrogen and helium. Emission from the first stars heats the remaining gas helping to trigger additional star formation; the ultraviolet light emission from the first generation of stars re-ionized the surrounding neutral hydrogen in expanding spheres eventually reaching the entire universe, an event called reionization. The most massive stars collapse in violent supernova explosions releasing heavy elements ("metals") into the interstellar medium. This metal content is incorporated into population II stars.