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How do fusion reactions change the chemical composition of stars over time?

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laser-activated fusion — nuclear fusion Interior of the U.S. Department of Energy's National Ignition Facility (NIF), located at Lawrence Livermore National Laboratory, Livermore, California. The NIF target chamber uses a high-energy laser to heat fusion fuel to temperatures sufficient for thermonuclear ignition. The facility is used for basic science, fusion energy research, and nuclear weapons testing.

Fusion reactions fundamentally alter a star's chemical composition by converting lighter elements into heavier ones, a process known as nucleosynthesis.1

In main-sequence stars like the Sun, the primary process is the conversion of hydrogen into helium via the proton-proton cycle or, in hotter stars, the CNO cycle. As hydrogen is consumed, the core becomes increasingly enriched with helium "ash".123

As a star evolves and its core contracts and heats up, it may begin fusing helium into heavier elements like carbon through the triple alpha process. In high-mass stars, this cycle continues, with the core reaching temperatures sufficient to fuse increasingly heavy nuclei until the star eventually experiences a supernova explosion. During these catastrophic events, further nuclear reactions—such as neutron capture—produce elements heavier than iron. Conversely, light elements like lithium, beryllium, and boron are typically produced through spallation rather than fusion.34