Superman has never died for good. His death is best known for happening when he went down in the fight with the intergalactic monster Doomsday. He eventually revived as the laws of death do not apply to him like they do to regular humans. He has also died in episodes of old cartoons where he was overexposed to kryptonite- but there always seemed to be a way to bring him back. He is considered to be ageless from Earth's yellow sun and therefore will remain alive until it turns red.