Editing Hadean Eon (section)

===== Fusion - Step 3 =====
In the final stage of main sequence fusion, two emergent helium-3 nuclei collide and fuse to form the isotope helium-4 (<sup>4</sup>He), releasing two protons in the process. This step does not always follow directly after the first two steps; it requires the accumulation of sufficient helium-3 nuclei from step 2 within the star's core. Even at these high temperatures, the <sup>3</sup>He nuclei carry a positive electrical charge, and thus repel one another. '''Quantum tunneling''' allows them to get close enough for the <u>strong nuclear force</u> to take over at very short ranges, facilitating fusion.  

Quantum tunneling is a [[Quantum mechanics|quantum mechanical]] phenomenon that allows particles to ''pass through'' a potential barrier that they classically shouldn't be able to surmount, due to insufficient energy. Protons in a star's core repel each other via the Coulomb barrier (<u>electrostatic force</u>). At the temperatures and pressures at a star's core, classical physics would suggest that protons do not have enough kinetic energy to overcome this barrier and get close enough to fuse. 

Quantum mechanics allows for the '''probability''' that particles like protons can 'tunnel' through the Coulomb barrier and come within close enough range for the <u>strong nuclear force</u> to take effect in opposition to the laws of classical physics, which significantly increases the rate of fusion reactions in a star's core. When two <sup>3</sup>He nuclei fuse, they form a <sup>4</sup>He nucleus (with 2 protons and 2 neutrons), more stable due to the <u>strong nuclear force</u> binding its nucleons more tightly together. The reaction also releases two protons (2p<sup>+</sup>), which essentially are the 'excess' not needed in the formation of helium-4.

* '''<sup>3</sup>He + <sup>3</sup>He → <sup>4</sup>He + <sup>2</sup>p<sup>+</sup>'''

The energy released during the proton-proton chain reaction comes primarily from the '''mass difference''' between the initial '''reactants''' and the final '''products''', according to [[Einstein]]'s equation, '''E = mc<sup>2</sup>'''. Most of the energy is carried away by gamma rays and neutrinos; the gamma rays make their way to the Sun's surface at a glacial pace and are eventually emitted as sunlight, while neutrinos escape the Sun almost immediately due to their weak interaction with matter (neutrinos respond only to the <u>weak nuclear force</u> and <u>gravitational force</u>).  

The presence of the immediately expelled neutrinos is one of the primary proofs that nuclear fusion is the source of the Sun's energy. Its released gamma rays are a significant source of the star's energy output; but as they make their way out of the Sun's core, they interact with '''solar material''', gradually losing energy and getting converted into '''lower-energy photons''', eventually contributing to the '''sunlight''' we receive on Earth. This process of energy transfer from the core to the surface takes an incredible amount of time - ''thousands to millions of years'' - due to the dense plasma's opacity to electromagnetic radiation. The light you see left the Sun's surface eight minutes ago, but it was created in the Sun's core millennia before you were born. 

The fusion rate in the Sun's core is in a '''self-correcting equilibrium''': a slightly higher rate of fusion would cause the core to heat up more and expand slightly against the weight of the outer layers, reducing the density and hence the fusion rate and correcting the perturbation; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the density and increasing the fusion rate and again reverting it to its present rate.