The fire on the Sun, or what appears as fire, is actually a result of nuclear fusion reactions occurring in its core. The Sun generates its energy through a process called thermonuclear fusion, specifically a reaction known as the proton-proton chain.
In the Sun's core, where temperatures and pressures are extremely high, hydrogen nuclei (protons) undergo a series of fusion reactions to form helium. The process begins with two protons fusing together to form a deuterium nucleus (a proton and a neutron). This reaction releases a positron and a neutrino. The deuterium nucleus then combines with another proton to form a helium-3 nucleus. Finally, two helium-3 nuclei fuse to produce a helium-4 nucleus and release two protons.
During these fusion reactions, a tremendous amount of energy is released in the form of gamma rays. This energy gradually travels outwards through the layers of the Sun, undergoing a process of absorption and re-emission, until it reaches the Sun's surface. At the surface, this energy is emitted as light and heat, which gives the Sun its fiery appearance.
It's important to note that the Sun does not actually contain fire as we understand it on Earth. The "fire" on the Sun is a result of the immense energy released by nuclear fusion reactions occurring in its core.