Stars form when clumps of hydrogen and other gases in an H II region contract under their own gravity. As the gas collapses, the central clump grows stronger and the gas heats to extreme temperatures by converting gravitational potential energy to thermal energy. If the temperature gets high enough, nuclear fusion will start, and form a protostar. The protostar is 'born' when it begins to emit enough energy to balance out its gravity and halt gravitational collapse.