What fuels the sun's nuclear reactions?

The Sun's nuclear reactions are primarily fuelled by hydrogen atoms undergoing fusion to form helium.

The Sun, like other stars, is a massive sphere of hot, glowing gas. This gas is primarily composed of hydrogen, which makes up about 75% of its mass. The Sun's immense gravitational force creates extreme pressure and temperature at its core, reaching over 15 million degrees Celsius. These conditions are ideal for nuclear fusion, the process that fuels the Sun's energy output.

Nuclear fusion in the Sun primarily involves the fusion of hydrogen atoms to form helium, a process known as the proton-proton chain. This process begins when two protons collide and fuse together, creating a deuterium atom (a type of heavy hydrogen), a positron (a type of antimatter), and a neutrino. The deuterium atom then collides with another proton, forming a helium-3 atom and releasing a gamma-ray photon. When two helium-3 atoms collide, they fuse to create a helium-4 atom and two protons. This entire process releases a tremendous amount of energy, which we perceive as sunlight.

The energy produced by nuclear fusion in the Sun's core takes a long time to reach the surface. The photons of light produced in the core bounce around within the Sun, in a process known as the 'random walk'. It can take a photon up to 170,000 years to finally reach the surface and escape into space. Once it does, it travels at the speed of light, reaching Earth in just over 8 minutes.

The Sun has been burning hydrogen for about 4.6 billion years and it's estimated that it has enough hydrogen left to continue this process for another 5 billion years. After this, it will start to burn helium, causing it to expand into a red giant and eventually shed its outer layers, leaving behind a white dwarf. This is the typical life cycle of a star like our Sun.