Stellar Nucleosynthesis: How Stars Forge the Elements

The calcium in your bones, the iron in your blood, and the oxygen you just breathed were all manufactured inside stars that died before the Sun was born. Astronomer Carl Sagan called us "star stuff," and he meant it literally. Nucleosynthesis is the set of nuclear reactions that build heavy elements from lighter ones, and it is the reason the periodic table has more than just hydrogen.

- Explain how stars fuse hydrogen into helium and release energy - Trace the fusion chain up to iron in massive stars - Describe why elements heavier than iron require a supernova or neutron-star merger - Connect stellar fusion to the origin of atoms in your own body

A star spends most of its life fusing hydrogen into helium in its core. In the Sun, this happens through the proton-proton chain: four hydrogen nuclei (protons) combine, through several steps, into one helium-4 nucleus. The helium nucleus weighs slightly less than the four protons that made it. That missing 0.7 percent of mass is converted to energy via Einstein's E = mc squared, which is why the Sun shines. The Sun fuses about 600 million tons of hydrogen every second.

When a massive star exhausts its hydrogen, its core contracts and heats, igniting helium fusion into carbon and oxygen. Larger stars keep going, fusing successively heavier elements: carbon to neon, neon to oxygen, oxygen to silicon, and silicon to iron, forming an onion-like shell structure. Fusion stops at iron. Iron-56 has the most tightly bound nucleus, so fusing it would absorb energy rather than release it. With no energy to hold itself up, the core collapses.