Why do stars build an iron core over time?? Please explain with real life examples

Nuclear fusion takes a lot of enegy.In any normal stars there always is enough energy to fuse H into He,He and H into Li and so on.The heat or energy liberated during fusion helps to keep the star maintain its temperature,shape.As higher mass number particles are generated,more heat is needed to fuse them.So not all stars,but the bigger ones,having mass 4 times the mass of the sun,have the amount of heat to fuse silicon into iron.But at this stage,the heat needed for fusion becomes greater than the heat energy liberated during fusion.So the enrgy of the star decreases and it dies.