Nearly all matter is composed of "star stuff". According to the Big Bang theory, the early universe was hot enough to allow the nucleosynthesis of hydrogen, helium, and small amounts of lithium and beryllium. Deuterium, a common isotope of hydrogen, was also important as a reactant in many of the reactions required to form helium. The universe expanded and cooled too rapidly to form the heavier elements.
So where did all of the heavier stuff come from? It came from the cores of stars that formed later. The fusion processes in the cores of stars start with hydrogen fusion via the proton-proton chain and carbon-nitrogen-oxygen (CNO) cycle. This second process only occurs in modern stars enriched from the nucleosynthesis of earlier stars. Once the core "burns" up its fuel, there is no outward force to balance the pull of gravity and the star begins to collapse. This collapse raises the temperature in the core until it is hot enough to burn the "ash" from the previous reactions. With each stage, the ash gets heavier as new elements are forged. In the most massive stars, this chain leads to nickel-56, which later decays to iron-56 and does not release binding energy from fusion (in fact it absorbs energy); there are then no new fuel sources available. The star undergoes a violent collapse, giving birth to a supernova that spreads the newly formed elements throughout space to enrich the medium in which later stars will form.
This Demonstration shows paths taken by the primary fusion processes. Additional chains may occur that are not shown here. Individual arrows represent chains of reactions. Some processes involve multiple chains. Each arrow begins at the beginning of a chain and passes through end products generated in that chain. Protons and alpha particles (helium nuclei) are often reactants and are not shown for most processes.
Snapshot 1: stars like our Sun obtain most of their energy from the proton-proton chain, fusing hydrogen in helium
Snapshot 2: stars with slightly hotter cores continue fusing hydrogen, but through reactions that involve carbon, nitrogen, and oxygen (the CNO cycle) created in earlier generations of stars
Snapshot 3: Once the hydrogen in the core is used up, the core collapses until it heats enough to ignite the helium ash from earlier reactions. The primary reactions involve three helium nuclei (alpha particles) and result in the formation of carbon and oxygen in the core. Stars begin to swell into red giants at this phase, due to the increased radiation pressure pushing outward from the core.
Snapshot 4: additional alpha reactions, starting with carbon, can also take place and can create neon and magnesium
Snapshot 5: once the helium is used up, the core collapses and heats up again until the carbon ignites; this results in the formation of oxygen, magnesium, sodium, and neon
Snapshot 6: after carbon fusion and additional core collapse, the neon is the next fuel to ignite and results in more oxygen and magnesium formation
Snapshot 7: oxygen fusion processes result in the formation of yet heavier elements such as phosphorus, sulfur, and silicon
Snapshot 8: Finally, the silicon fusion processes come at the end of a very massive star's lifetime and last for only a couple of weeks. This chain rapidly proceeds through the formation of elements such as sulfur, argon, calcium, titanium, chromium, iron, and nickel.
Snapshot 9: Once nickel and iron (from the decay of nickel) are formed in the core, there are no further fusion reactions possible. These elements have the highest binding energy per nucleon of any other elements that are formed, and lie at the peak of this plot. Additional fusion would absorb energy, and so energy generation in the core essentially halts. The core collapses and results in a supernova explosion.
Although stars cannot form elements heavier than iron by standard core fusion, neutron capture during supernova explosions as well as other processes can lead to the heaviest naturally formed elements. These are known as the s- and r-processes, referring to slow and rapid neutron capture. The s-process occurs primarily in old red giants, while the flood of neutrons from a supernova event results in rapid neutron capture events in the r-process.