Nucleosynthesis is the creation of new atomic nuclei, the centers of atoms that are made up of protons and neutrons. Since then, the nuclear reactions in the life and death of stars have formed most of the other nuclei in the universe. Stars can create nuclei through two processes: either by combining two smaller nuclei (called fusion) or breaking a larger nucleus into multiple nuclei (called fission).

Nuclear fusion is the process where the atomic nuclei of lighter elements join together to form the nucleus of a heavier element. When sufficiently light nuclei fuse, the total rest mass of the resulting nucleus is slightly smaller than the combined rest masses of the initial nuclei. This "mass deficit" corresponds to the energy released by the fusion reaction.

“Finally, the central core of the protostar heats up so much that nuclear ‘burning’ is initiated and the star begins its energy production through nuclear fusion. If the initial mass of a star is more than about 8 solar masses further burning phases will take place… These are called advanced burning phases and consist of carbon, neon, oxygen, and silicon burning… In the outermost shell of the star still hydrogen is burnt into helium (hydrogen burning), in the next shell helium to carbon and oxygen (helium burning)… As it has been seen in the preceding sections, stellar burning phases only lead to the production of nuclei up to Fe.”

“The key to producing the heaviest elements on the periodic table is known as the rapid neutron-capture process, or ‘r process,’ and it is thought to be responsible for production of all naturally occurring thorium, uranium and plutonium in the universe. In the scenario Mumpower proposes, a massive star begins to die as its nuclear fuel runs out… Existing atomic nuclei may also be dissolved into individual nucleons, creating more free neutrons to power the r process… the r process may ensue, with heavy elements and isotopes forged and then expelled out into space as the star is ripped apart.”

Stars are furnaces in which light nuclei are fused into heavier nuclei, releasing energy and synthesizing the elements up to iron. The heavier elements are made in later explosive events and neutron-capture processes.

“Stellar nucleosynthesis is the process by which elements are created within stars by combining the protons and neutrons together from the nuclei of lighter elements. All of the atoms in the universe began as hydrogen. Fusion reactions in successive generations of stars convert hydrogen into helium, and then into carbon, oxygen and other heavier elements up to iron.”

Nuclear fusion occurs naturally in stars! Stellar nucleosynthesis – elements synthesised inside the stars – [through] nuclear processes [and] well defined stages of stellar evolution. Because in stars the reactions involve mainly charged particles, stellar nucleosynthesis is a slow process… in stars 12C formation set the stage for the entire nucleosynthesis of heavy elements. The other predominant mechanism for the production of heavy elements is the s-process: nucleosynthesis by means of slow neutron captures occurs in stars during He-burning.

In this video, students learn that stars create new elements by combining lighter nuclei into heavier nuclei in a process called nuclear fusion. Hydrogen nuclei fuse to form helium, and in more massive stars, fusion continues to produce heavier elements such as carbon, oxygen, and iron before the star dies and disperses these elements into space.

The energy of stars comes from nuclear fusion reactions in their cores, where light atomic nuclei combine to make heavier nuclei. For most of a star’s life, four hydrogen nuclei are fused into one helium nucleus. Later, when the core hydrogen is exhausted, helium nuclei fuse to make carbon and oxygen, and in massive stars, fusion proceeds through a series of stages that make still heavier elements up to iron.

“Stellar nucleosynthesis inside stars then fused these lighter elements to form heavier ones like carbon and oxygen. The nuclear fusion reaction inside the star is the process by which nuclear reactions between light elements form heavy elements (up to iron). When most of the hydrogen is used up in the core, the helium begins fusing into carbon (C) at its core.”

“Stellar nucleosynthesis – The formation of heavy elements by the fusion of lighter nuclei in the stars. Stars are hot and dense enough to burn hydrogen… Heavier elements were later produced during stellar nucleosynthesis inside stars through nuclear fusion reactions like the carbon-nitrogen-oxygen cycle and triple alpha process.”

The theory of stellar nucleosynthesis is a cornerstone of astrophysics explaining how elements are created within the cores of stars. Through nuclear fusion, stars produce lighter elements like hydrogen and helium and over time they create heavier elements such as carbon, oxygen, and iron. At the core of stellar nucleosynthesis lies nuclear fusion, where atomic nuclei collide and combine to form heavier nuclei, releasing vast amounts of energy that powers stars.

At around 2:08–2:13, the narrator explains: “Stellar nucleosynthesis is the process by which elements are formed within the stars as a result of nuclear fusion. Nuclear fusion is the process by which multiple nuclei join together to form a heavier nucleus… More and more alpha particles are fused to create heavier elements all the way to iron.”