What are the stages of thermonuclear fusion that take place in the Sun?

In the Sun, almost all of the fusion energy is released by the fusion of hydrogen into helium. The process, which requires high density and temperature, produces energy through the loss of mass. Through a three-step process (each creating energy), a pair of hydrogen atoms combine to form a single helium atom.

This is called the proton-proton chain reaction.

The first stage involves the combination of two separate atoms of hydrogen into deuterium, an isotope having one proton and one neutron in its nucleus, instead of just a proton. Energy is released when one proton forms a neutron, a neutrino, and a positron. More energy is released when the positron annihilates with an electron.

In the second stage, the deuterium atom combines with a single-proton hydrogen atom to form a single-neutron isotope of helium (helium-3). This also releases energy.

In the most common third stage, two of these helium-3 atoms fuse to form a single atom of helium-4, the common form of helium, throwing off the two extra protons as hydrogen nuclei.

(Other reactions can temporarily form beryllium or lithium, which decay to form helium.)

The final mass of the helium atoms is slightly less than the mass of the hydrogen from which it was formed. This missing mass has been converted into energy, in the form of gamma ray photons and neutrinos. The photons' energy is transferred to the surrounding matter and moves outward from the core to the Sun's surface, where it is emitted as solar radiation.

(The neutrinos, however, easily pass through the Sun and escape into space at nearly the speed of light. They also pass through the Earth and other planets, where only a tiny fraction ever collide with an atom.)