What reactions occur within a star in stellar equilibrium?

A star is in stellar equilibrium, also referred to as hydrostatic equilibrium, when the internal pressures, which are thermal pressure brought about by fusion reactions occurring in the star's core, equalize the force of gravity.

A star remains in equilibrium until its core runs out of hydrogen. At that point, the star begins to collapse and heats up until more fusion reactions take place.

When a star reaches a state of stellar equilibrium, its internal reactions are mostly nuclear fusion reactions; these reactions release a great deal of energy in the form of photons as hydrogen undergoes a series of fusion reactions to become helium. The most frequent type of nuclear fusion reaction in stars similar to our Sun is the proton-proton chain reaction, in which protons fuse to form helium nuclei, releasing energy in the form of gamma rays and neutrinos.

In stellar equilibrium, the primary reactions occurring within a star involve nuclear fusion processes. These reactions are responsible for the release of energy that maintains the star's stability and counterbalances the gravitational forces trying to collapse it. The primary reactions depend on the mass and composition of the star.

In the core of a main sequence star like our Sun, the primary reaction is the fusion of hydrogen nuclei (protons) into helium nuclei. This process, known as the proton-proton chain, involves several steps where hydrogen nuclei fuse to form helium-4 nuclei, releasing energy in the form of gamma rays and neutrinos.

In more massive stars, particularly those with higher temperatures and pressures in their cores, a different fusion process known as the CNO cycle becomes dominant. In this cycle, carbon, nitrogen, and oxygen nuclei act as catalysts to convert hydrogen into helium, releasing energy in the process.

Additionally, in the later stages of a star's life cycle, when it exhausts its hydrogen fuel in the core, fusion reactions involving heavier elements may occur. This can include the fusion of helium into carbon and oxygen, and in more massive stars, the fusion of heavier elements like carbon, oxygen, and silicon into even heavier elements like iron.

Throughout these reactions, the energy released from nuclear fusion balances the gravitational forces, maintaining the star's stability and allowing it to shine steadily over millions to billions of years.