 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 ignite and form a protostar . The protostar is ' born ' when it begins to emit enough radiative energy to balance out its gravity and halt gravitational collapse . 
