Editing Hadean Eon (section)

== Parts of the Sun ==
The formation of these layers and features was a gradual process that occurred over millions of years as the collapsing molecular cloud reached nuclear fusion temperatures at its core and began to differentiate based on temperature, density, and rotational dynamics.

The Sun does not have a definite boundary, but its density decreases exponentially with increasing height above the '''photosphere'''. For the purpose of measurement, the Sun's radius is considered to be the distance from its center to the edge of the photosphere (its apparent visible surface). It is a near-perfect '''sphere''', with its polar diameter differing from its equatorial diameter by only 10 kilometers. 

=== Core ===
The '''core''' is the central region of the Sun, where nuclear fusion occurs. Hydrogen nuclei fuse to form helium, releasing enormous amounts of energy that powers the star. Each second within the core of the Sun, ~600 million tons of hydrogen atoms transform into helium in a fusion process known as the '''proton–proton chain''' (4 hydrogen atoms form 1 helium atom). The proton-proton chain comprises several steps, involving the fusion of solitary protons (hydrogen nuclei) into helium nuclei. Protons are positively charged and normally repel each other due to the <u>electrostatic force</u>, but the extreme temperature and pressure of the star's core can push protons (hydrogen nuclei) close enough to one another for the <u>strong nuclear force</u> to overcome their electrostatic repulsion (known as the '''Coulomb barrier'''<u>)</u>. 

===== Fusion - Step 1 =====
When two protons come very close, one of the protons can (very rarely; one in a billion such interactions) undergo a weak nuclear reaction, where it transforms into a '''neutron'''. This transformation is mediated by the <u>weak nuclear force</u>, which operates at very short ranges and is responsible for processes like '''radioactive decay'''. The emergent neutron can then pair with the remaining proton to form '''deuterium''' (<sup>2</sup>H), a hydrogen '''isotope''' with one proton and one neutron in its nucleus. The remaining '''proton''' (p<sup>+</sup>) fuses with its decayed neutron partner, forming a new '''deuterium''' nucleus (<sup>2</sup>H), but also releasing a '''positron''' (e<sup>+</sup>), and a '''neutrino''' (ν<sub>e</sub>).   

* '''1p<sup>+</sup> + [1p<sup>+</sup>→n + (W+→e<sup>+</sup> + ν<sub>e</sub>)]'''<sub>The proton becomes a neutron, releasing a W+ boson (which quickly becomes a positron and a neutrino).</sub> 
* '''p<sup>+</sup> + n → <sup>2</sup>H + e<sup>+</sup> + ν<sub>e</sub>'''<sub>The proton and the neutron fuse, creating a new deuterium nucleus; the positron and neutrino remain.</sub>

Both protons and neutrons are '''nucleons''' (components of atomic nuclei); a proton has a positive electric charge, while a neutron has no '''electric charge'''. Despite this difference, protons and neutrons are quite similar in mass and are both '''fermions''', meaning that they both follow the <u>Pauli exclusion principle</u>. 

The transformation from a proton to a neutron changes the type (or '''flavor''') of one of the nucleon's fundamental particles, called '''quarks'''. The proton (composed of two '''up quarks''' and one '''down quark''') transforms into a neutron (composed of one up quark and two down quarks) through this process, an example of beta-plus decay ('''β+ decay'''). This transformation emits a '''W+ boson''' (a carrier particle of the <u>weak nuclear force</u>), which then quickly decays into a positron (e<sup>+</sup>) and an electron neutrino (ν<sub>e</sub>). In the transformation, '''charge conservation''' is maintained; the initial positive charge of the proton is balanced by the creation of a '''positron''', which carries a positive charge, ensuring that the total charge before and after the reaction remains constant.

===== Fusion - Step 2 =====
The emergent positron quickly encounters another electron, leading to their mutual '''annihilation''' and an energy release as '''gamma rays'''. Electrons naturally carry a negative '''electrical charge'''; positrons have the same properties as electrons but an exactly analogous negative charge and are considered ''antielectrons'' (their '''antiparticle''' equivalent). Particles and their antiparticles annihilate one another on contact, often (as in this case) producing new particles as a result of the energy release. The newly formed deuterium nucleus then fuses with yet another proton (hydrogen nucleus), producing a helium-3 nucleus (<sup>3</sup>He) and releasing more energy in the form of gamma radiation (γ; high-energy '''photons'''; with short wavelengths / high frequency). 

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

===== 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.

=== Radiative Zone ===
Surrounding the core, the '''radiative zone''' is where energy produced in the core is transported outward by '''photons''' through radiative diffusion. As the collapsed cloud's central region heated up and began nuclear fusion, the surrounding material, still collapsing, became stratified due to temperature and density gradients, forming the radiative zone.

=== Convective Zone ===
Above the radiative zone, the '''convective zone''' is where energy is transported by convection. Hot plasma rises, cools as it loses energy to the outer layers, and then sinks to be reheated and rise again. The convective zone formed because the outer layers, being cooler than the core, became inefficient at radiative energy transfer, turning instead to convection. 

The Sun rotates faster at its equator than at its poles, due to '''convective motion''' from heat transport, and the '''Coriolis force''' of the Sun's rotation. Convective motion refers to the movement of plasma within the Sun driven by the transfer of heat from the interior towards the surface. Heat from the core, generated via nuclear fusion, is initially transferred through the radiative zone via the emission and absorption of photons. Closer to the surface, in the convective zone, the plasma becomes cooler and less efficient at radiative heat transfer. The temperature gradient causes the plasma to become buoyant and rise toward the photosphere. As the plasma reaches the photosphere, it releases its heat into space and cools down, becomes denser, and sinks back toward the interior, creating a continuous cycle of convective motion. This process is similar to boiling water, where hot water rises, cools, and then sinks in a convection cell. 

The Coriolis force affects the motion of the plasma in the Sun, particularly in the convective zone. As plasma moves up and down in the convective zone, the Coriolis force acts on it, deflecting its path. In the northern hemisphere, the motion is deflected to the right, and in the southern hemisphere, to the left. This effect is due to the Sun's rotation and affects how heat and magnetic fields are distributed across it. The combination of '''centrifugal force''' from the Sun's rotation and the Coriolis force slightly flattens the Sun at the poles and bulges it at the equator, although the difference is very small.

The Sun exhibits '''differential rotation''' because it is a fluid body, not a solid one. The Coriolis effect introduces some asymmetries in solar activities (like sunspot patterns and flows within the convective zone) but doesn't significantly alter the overall spherical shape due to the dominant balancing act of '''hydrostatic equilibrium'''. The Sun's rotation has its origins in the process of star formation from a rotating molecular cloud in the interstellar medium, which was not perfectly stationary but had some initial rotation. This rotation could have been due to gravitational interactions with nearby objects, the impact of galactic rotation, or the remnants of motion from previous supernovae explosions in the area.

=== Photosphere ===
The '''photosphere''' is the visible surface of the Sun from which light is emitted. It's where '''sunspots''', regions cooler than their surroundings, are seen. The photosphere formed as the outermost layer of the Sun cooled and became transparent to light, making it effectively the 'surface' from which sunlight is radiated. This emission of light defines the Sun's visible diameter and features that can be observed.

=== Chromosphere ===
Just above the photosphere, the '''chromosphere''' is a layer of the Sun characterized by the reddish glow seen during solar eclipses. It contains '''spicules''' and '''filaments''' driven by the Sun's magnetic field. The chromosphere developed above the photosphere as the temperature began to increase with altitude, influenced by the Sun's magnetic field dynamics.

=== Corona ===
The '''corona''' is the outermost layer of the Sun's atmosphere, extending millions of kilometers into space. It is surprisingly hot, much hotter than the surface, and is visible during a total solar eclipse. The corona formed as the Sun's magnetic field lines extend outward into space, carrying with them solar wind and plasma. The heating mechanism of the corona is a topic of active research, with '''magnetic reconnection''' and '''wave heating''' being key theories.

=== Solar Wind ===
Beyond a distance of 5''R''<sub>☉</sub> from the Sun, the corona flows outward at a speed (near Earth) of 400 kilometres per second (km/s); this flow of charged particles is called the solar wind. The '''solar wind''' is a stream of charged particles (plasma) released from the upper atmosphere of the Sun (the corona) into space. The solar wind forms as hot coronal gas expands into space, accelerated by the Sun's intense heat and magnetic field.