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Nuclear transmutation is the conversion of one chemical element or an isotope into another chemical element. Nuclear transmutation occurs in any process where the number of protons or neutrons in the nucleus of an atom is changed.
A transmutation can be achieved either by nuclear reactions or by radioactive decay, where no outside cause is needed.
Natural transmutation by stellar nucleosynthesis in the past created most of the heavier chemical elements in the known existing universe, and continues to take place to this day, creating the vast majority of the most common elements in the universe, including helium, oxygen and carbon. Most stars carry out transmutation through fusion reactions involving hydrogen and helium, while much larger stars are also capable of fusing heavier elements up to iron late in their evolution.
Elements heavier than iron, such as gold or lead, are created through elemental transmutations that can naturally occur in supernova. One goal of alchemy, the transmutation of base substances into gold, is now known to be impossible by chemical means but possible by physical means. As stars begin to fuse heavier elements, substantially less energy is released from each fusion reaction. This continues until it reaches iron which is produced by an endothermic reaction consuming energy. No heavier element can be produced in such conditions.