A large star burns hotter and faster, fusing all the hydrogen in its core to helium in less than 1 billion years. The star then becomes a red supergiant. They fuse helium into carbon, carbon and helium into oxygen, and two carbon atoms into magnesium. Through a combination of such processes, successively heavier elements, up to iron, are formed.

Heavy elements are produced by nucleosysthesis - the fusion of nuclei deep within the cores of stars. ... within their cores the fusion process created heavier and heavier elements; the most massive stars produced nuclei as heavy as iron. Red Giant: A phase in the evolution of a star after nuclear fusion reactions that convert hydrogen to helium have consumed all the hydrogen in the core of the star, and energy generated by hydrogen fusion in the shell causes the star’s diameter to greatly expand and cool.

Massive stars can fuse progressively heavier elements in their cores after hydrogen and helium burning. These later stages can produce elements up to iron, which marks the endpoint of energy-generating fusion in stellar interiors.

“The iron group elements show an approximation to nuclear statistical equilibrium at a temperature of several ×10^9 K or several tenths of an MeV leading to the iron peak… The iron peak is thought to result from explosive nucleosynthesis which may occur in one or other of two typical situations. One of these involves the shock that emerges from the core of a massive star that has collapsed into a neutron star… Another possible cause is the sudden ignition of carbon in a white dwarf… (supernova Type Ia).” “Calculations and observations both indicate that the dominant product is actually 56Ni… which later decays into 56Fe.”

“Prior to core collapse, the most massive stars have developed an onionlike structure with concentric shells of hydrogen, helium, carbon, neon, oxygen, and silicon burning. Silicon burning in the innermost region produces matter in quasi–nuclear statistical equilibrium dominated by iron-peak nuclei.” “We find that the bulk of the iron-peak elements in the ejecta originates from explosive silicon burning during the supernova explosion rather than from hydrostatic burning during the red-supergiant phase.”

High mass stars can have many successive stages of fusion of an element in a core and lighter elements in shells around the core, exhaustion of the element, core collapse and heating, fusion of higher mass elements, etc. Over time the internal structure of a high mass star has an "onion-skin" character with layers of elements layered over each other, with highest mass elements at the centre. Final structure has inert iron core, outer shells of lighter elements undergoing nuclear fusion. Under normal circumstances iron doesn't fuse in a star… These [fusion] reactions are exothermic (release energy) as long as the reactants are lighter than iron.

“Low-mass stars, which become red giants, only fuse their way up to carbon and oxygen before they shed their outer layers and leave behind a white dwarf. They cannot reach the core temperatures needed to fuse elements all the way to iron.” “Massive stars evolve into red supergiants and experience successive burning stages that build up an iron core. Since no energy can be gained by fusing iron into heavier elements, the core collapses and the star explodes as a supernova, during which many of the elements heavier than iron are formed.”

Astronomers now realize that better places for the creation of heavy elements are the stars themselves. In the interiors of stars, both the lighter elements and the heavier elements up to iron are synthesized by nuclear fusion.

Supergiants and giants with M > 4 M☉ become hot enough to fuse carbon into heavier elements. Once the central temperature reaches T > 600,000,000 Kelvin, carbon and oxygen can fuse into heavier elements, such as silicon, sulfur, and iron. The star's core is now pure iron: end of the line as far as fusion is concerned.

As a massive star evolves, its core successively burns hydrogen, helium, carbon, neon, oxygen, and silicon. Each new fuel begins burning after the previous one is exhausted and the core contracts and heats up. Silicon burning produces a core composed mostly of iron-group elements. Once an iron core has formed, no further energy can be generated by fusion in the core, because fusing iron into heavier elements requires an input of energy rather than releasing energy.

When the Sun leaves the main sequence, it will become a red giant and start burning helium in its core via the triple-alpha process, making carbon and some oxygen. The Sun is not massive enough to ignite carbon burning in its core. Low-mass red giants like the Sun will never reach the temperatures required to fuse elements heavier than carbon and oxygen; their cores eventually become carbon–oxygen white dwarfs.

After core helium burning, a sufficiently massive star expands to a red supergiant and begins to burn carbon in its core. As the core contracts and heats, it successively ignites neon, oxygen, and silicon burning. These advanced burning stages build up an iron core at the center of the star. The red supergiant phase is thus associated with the onset of advanced nuclear burning that ultimately produces an iron core in the most massive stars.

In massive stars, nuclear synthesis continues to create even heavier elements until an iron core forms, which cannot undergo further fusion without energy input. Less massive stars, like the Sun, end their nucleosynthesis with the production of carbon and oxygen and never reach the conditions necessary to synthesize elements as heavy as iron.

Learn how stars create elements in this video adapted from NOVA. The resource explains that massive stars fuse successively heavier elements in later stages of life, with the process ending at iron.

In standard stellar evolution theory, low- and intermediate-mass stars (≈0.8–8 solar masses) become red giants and undergo hydrogen and helium fusion, sometimes igniting limited carbon burning, but they do not reach the core temperatures required for sustained fusion of elements up to iron. Their nucleosynthesis effectively stops at carbon–oxygen (or, for somewhat more massive ones, at an oxygen–neon–magnesium core). By contrast, high-mass stars (>~8 solar masses) evolve into red supergiants and proceed through successive core and shell burning stages (H, He, C, Ne, O, Si) that build an iron core; the bulk of the iron-peak elements, however, is synthesized during the brief explosive silicon burning and nuclear statistical equilibrium in the ensuing core-collapse supernova, not during the long-lived red-supergiant fusion phases themselves.

The video explains: “Then you have carbon to oxygen… Then oxygen to silicon… And then silicon to iron which is a day. And then once iron is fused, stars cannot produce enough energy to fight against gravity. So the core collapses… and in the explosion process… that's when all the other elements on the periodic table are created.” It summarizes that “fusion always stops at iron in massive stars, often leading to a supernova,” indicating that iron is the upper limit of energy-producing fusion and that elements beyond iron are produced largely in the supernova explosion rather than in the prior red giant/supergiant fusion phases.