Editing Hadean Eon

'''Dating'''


Starting 4567.30 ± 0.16 MYA ('''4.5 billion years ago) to 4.031 billion years ago'''

Other names: Priscoan Period (from ''priscus'', 'ancient'); pre-Archean (deprecated)


'''Etymology'''


The eon's name comes from '''Hades''', Greek god of the Underworld; it is a poetic reference to the extreme environmental conditions of the early Earth.


'''The Molecular Cloud'''


Evidence suggests that the Solar System began to form ~4.6 GYA (GYA means 'gigayears ago'/ 'billion years ago'), when a giant '''molecular cloud''' experienced '''gravitational collapse'''. Molecular clouds are sometimes referred to as stellar nurseries, as star formation often occurs within them; they are a distinct type of structure within the interstellar medium. The '''interstellar medium''' (ISM) is composed of gas (mainly '''hydrogen''', but also '''helium''' and trace amounts of heavier elements), plasma, and dust.


The vast majority of the contents of the interstellar medium is gaseous '''H'''2. '''H'''2 ''(at right)'', is a molecule (more than one atom) consisting of two '''hydrogen atoms''' bound together ('''H-H''').  Molecular '''hydrogen''' forms when two '''hydrogen''' atoms share their '''electrons''' through a '''covalent bond'''. This is a '''chemical''' process that occurs at relatively low energies, typically in the cold, dense regions of the interstellar medium. The resulting molecule consists of two '''hydrogen''' nuclei ('''protons''') and two '''electrons'''. It's the most basic molecular structure possible and is a stable, neutral molecule. As opposed to a '''nuclear''' change (as in the creation of '''helium'''), in molecular '''hydrogen''' ('''H'''2), each atom retains its identity as a '''hydrogen''' atom; they are simply bonded together; the protons (nuclei) do not fuse into a different element.


Electrons do not have fixed '''orbits''' in the way planets orbit a star; their positions are described by a '''probability distribution''' within '''electron clouds'''. When two hydrogen atoms come close together, their electron clouds merge to form a region of shared electron density (the covalent bond, when the 1s orbitals of each '''hydrogen''' atom overlap). This combination of electron density in the space between the nuclei holds the atoms together into a molecule of gas. The shared electron density between the two hydrogen atoms also contributes to the molecule's stability and the bond's energy state.


'Chemical' in this context refers to the formation, breaking, or rearranging of bonds between atoms that result from interactions between the electrons of those atoms, not changes in the atomic nuclei (which would be a '''nuclear''' process). The 'stability' of molecular hydrogen is due to the fact that both hydrogen atoms achieve a full '''valence shell''' (two electrons) by sharing their electrons, satisfying the octet rule (or, in the case of hydrogen, the duplet rule).


The valence shell is the outermost shell of an atom that contains its '''valence electrons'''. The behavior of an atom, particularly its ability to form chemical bonds with other atoms, is largely determined by the electrons in this '''outer shell'''. Hydrogen is the simplest element, with just one proton and one electron; its electron resides in the 1s orbital, the closest orbital to the nucleus. Because hydrogen is in the first row of the periodic table, its 1s orbital can hold a maximum of two electrons. By sharing its single electron with another atom that also needs additional electrons to complete its own valence shell, hydrogen can effectively achieve a stable electronic arrangement resembling that of the '''noble gases''', which are naturally stable and nonreactive due to their full valence shells.


The formation of '''H'''2 from two hydrogen atoms is energetically favorable and '''exothermic''' (it releases energy). However, in space, there's a challenge: conservation of momentum and energy. The newly formed '''H'''2 molecule must find a way to release the excess energy from the bond formation to become stable. On Earth, this energy would be dissipated quickly into the surrounding environment, but in the low-density environment of space, there's often nothing nearby to absorb it. Without a third body to take away the energy, the two '''hydrogen''' atoms would simply bounce off each other without forming a stable molecule (this is why dust often acts as a suitable 'surface' upon which these reactions can occur).


When a chemical bond forms, such as an '''H-H''' bond in an '''H'''2 molecule, energy is released because the system goes from a higher '''energy state''' (separate hydrogen atoms) to a lower energy state (the '''bonded''' molecule). This is a fundamental principle of chemistry - systems tend to move toward lower energy states, which are more stable. Separated hydrogen atoms have '''potential energy''' due to their mutual '''electrostatic''' potential and the potential of their electrons in the field of another nucleus. When they come close enough to share their electrons and form a covalent bond, they create a molecule with lower potential energy than the sum of the two atoms taken separately.


The bond formation process releases energy, primarily in the form of '''heat''', which can be absorbed by the dust grain, causing it to vibrate more intensely. This process not only allows the dust to act as a '''heat sink''' but also to catalyze the formation of molecular hydrogen. As these interactions occur on a vast scale, they collectively facilitate the formation and thermal regulation of molecular clouds, rich in molecular hydrogen and dust, precursors to star formation. The energy transfer to dust grains and subsequent infrared radiation contribute to the cloud's '''thermal balance''', essential for its evolution and the birth of new stars. This dynamic interplay of physical and chemical processes underpins the formation of molecular clouds in the interstellar medium, marking them as fertile grounds for the complex chemistry that leads to star and planet formation


'''(Star) dust''' refers to tiny solid particles composed of elements like '''carbon''', '''oxygen''', '''silicon''', '''magnesium''', and '''iron''', often found in the form of compounds or minerals similar to those seen on Earth (silicates, graphite, water ice, and iron oxides). These particles are remnants of previous astrophysical processes, including '''outflows''' from aging stars (when stars at the ends of their lives begin to 'leak' matter and energy), '''supernovas''' (when dying stars explode), and the collision and fragmentation of '''asteroids''' and '''comets''' with other bodies. Dust particles can range in size from a few molecules to micrometers of diameter.


Dust particles act as cooling agents in the ISM by absorbing energy from surrounding gas and re-emitting it at '''infrared''' wavelengths, facilitating the collapse of gas into denser clouds that can eventually form stars (as described above). Dust grains also provide ''surfaces'' on which chemical reactions can occur, which can lead to the formation of complex organic molecules, including those essential for life. Dust absorbs and scatters visible and ultraviolet light, affecting the appearance of astronomical objects in a process known as '''interstellar extinction'''. This makes distant stars appear dimmer and redder than they are. Dust is also responsible for the beautiful nebulae observed in the night sky, as it scatters light from nearby stars or emits light due to its own temperature.


Star dust can include '''graphite''' and complex organic molecules, some of which are similar to soot or '''polycyclic aromatic hydrocarbons''' (PAHs); others form large, interconnected structures. Sillicates (particles primarily composed of '''silicon''', '''oxygen''', '''magnesium''', and '''iron''') are also common. These are similar in composition to many of the minerals found in Earth's crust but are in the form of very small grains in space. In colder regions of the ISM, dust grains can be coated with ices - water ('''H'''2'''O'''), methane ('''CH'''4), ammonia ('''NH'''3), carbon dioxide ('''CO'''2), and other molecules can freeze onto the surface of dust grains. Tiny solid particles of metals such as '''iron''', '''nickel''', and '''magnesium''' also contribute to interstellar dust.


'''Plasma''', often referred to as the fourth state of matter, is a gas in which a certain proportion of the particles are '''ionized''' (meaning electrons have been separated from atoms) resulting in a mix of '''free electrons''' and '''ions'''. In the context of the ISM, plasma exists primarily in ionized regions such as '''H'''II clouds (around hot stars where UV radiation is strong enough to ionize hydrogen). Plasma often has unique properties compared to neutral gases, such as being highly '''conductive''' to electricity and responsive to magnetic fields due to its '''charged''' particles.


'''Interstellar clouds''' are accummulations of gas, plasma, and dust within a galaxy. They are simply regions of space where this material is denser-than-average compared to the wider interstellar medium. The ISM is not uniform; it exists in various configurations and structures due to variations in gravity, pressure, and the remnants of earlier stellar processes.

* An '''H'''I region describes a cloud of '''neutral''' atomic '''hydrogen''' ('''H'''I), in addition to the local abundance of '''helium''' and other elements already present. '''H''' is the chemical symbol for '''hydrogen''', and I is the Roman numeral for     'one'. It is customary in astronomy to use the Roman numeral I for neutral atoms, II for singly-ionized ('''H'''II is H+ in other sciences) III for doubly-ionized; '''O'''III instead of O++). '''H'''I regions form     where temperatures are relatively low, allowing '''hydrogen''' atoms to remain neutral rather than becoming ionized by     stellar radiation. These conditions are conducive to the large-scale     accumulation of '''hydrogen'''. The movement and interaction of     galaxies can compress interstellar gas, leading to the formation of HI     regions. Over time, these regions can become the birthplaces of new stars,     but of themselves they do not emit detectable visible light.
* An '''H'''II region is a vast cosmic cloud of '''ionized     hydrogen''' gas ('''H'''+), where new stars are born and illuminate their     surroundings. These regions are named after the ionized state of '''hydrogen''' ('''H'''II, using the astronomical convention where 'II'     indicates ionization; H+ in chemical notation). HII regions represent     areas within the ISM where '''hydrogen''' gas has been     ionized, meaning its atoms have lost their electrons due to high energy,     typically from '''ultraviolet''' (UV) radiation emitted by nearby young, hot stars. The     theoretical model describing the ionized region around a hot star is     called a '''Strömgren sphere''', a spherical volume where the radiation from the star     has ionized hydrogen gas within its boundaries. The horizon of this sphere     marks where the rate of ionization balances with the rate of '''recombination''' (electrons     recombining with ions to form neutral atoms). HII regions are known for     their bright emission in optical wavelengths, as when free electrons     recombine with ionized protons ('''H'''II), they emit     '''photons'''. The specific wavelengths of light emitted during these     transitions produce the characteristic emission lines observed in the     spectra of HII regions. The most prominent of these lines is the '''Balmer     series''' of '''hydrogen''', especially the '''H-alpha     line'''     (Hα). '''H'''II regions are not randomly distributed within galaxies     but are often found in the arms of '''spiral galaxies''' and the     central regions of '''irregular galaxies'''. The Orion Nebula (M42) is one     of the closest and most studied '''H'''II regions,     providing a wealth of information about star formation and the interaction     of newly formed stars with their natal environments. The Carina Nebula and the Eagle Nebula (M16), famous     for the '''Pillars of Creation''', are other notable examples.


The accumulation of gas, dust, and plasma in the ISM can reach a threshold where gravitational forces overcome internal pressure, leading to collapse and the formation of new stars; this is called '''gravitational collapse'''. This process is influenced by the density and temperature of the cloud, as well as external factors such as shock waves from nearby supernovas; it relates to the Jeans instability criterion, which describes the conditions under which a cloud will begin to collapse under its own gravity.

* Jeans Mass: The minimum mass that a cloud must have for     gravitational forces to overcome internal pressure and initiate collapse.     If a cloud's mass is greater, it is considered gravitationally unstable and likely to collapse to form     stars. The internal '''thermal energy''' of a cloud provides the outward pressure needed to     resist gravitational collapse. When a cloud's mass increases (holding     temperature constant), its gravitational attraction becomes stronger,     making it more difficult for the thermal pressure to resist collapse.     Conversely, if the cloud can lose thermal energy (cool down), the reduced     pressure makes it easier for gravity to dominate, leading to collapse.     During the collapse, gravitational potential energy is converted into     kinetic energy (heat) as particles accelerate toward the cloud's center.     This heat must be radiated away for the collapse to proceed; otherwise, it     would increase the internal pressure and resist further collapse.
* Jeans Radius (or Jeans Length): The critical     radius of a cloud at which it becomes gravitationally unstable, and prone     to collapse.  It essentially marks     the minimum size that a cloud must have, given its mass and temperature,     for gravity to overcome the internal gas pressure and initiate collapse.     Efficient cooling mechanisms (such as radiation from dust and stable     molecules) lower the temperature, thereby decreasing the Jeans radius.     During the collapse, a cloud can fragment into smaller pieces, each     potentially with its own Jeans radius     based on its specific conditions (mass, temperature, density). This '''fragmentation''' can lead to     the formation of multiple stars or a star system, rather than a single     monolithic star. This also enhances the cloud's instability by making it     easier for any portions that exceed this reduced Jeans radius to collapse     into star formation.


When a molecular cloud reaches the Jeans instability criterion, several key processes unfold, leading to star formation. The specific timeframe for these processes can vary widely depending on the initial conditions of the cloud, such as its mass, temperature, and composition. During the Free-fall Phase, the cloud contracts, and the density increases, particularly at the center. For a '''solar-mass''' star, the initial collapse from a diffuse cloud to a denser core might take about 100,000 years. This phase is relatively short compared to the total lifetime of a star.


As the cloud collapses, it may fragment into smaller clumps due to variations in density. Each clump, containing a few solar masses of material, can potentially form a star or a star system; this fragmentation is critical in determining the initial mass function of stars (the distribution of stellar masses at birth). The collapsing material forms a '''protostar''' at the center of each clump; the material from the surrounding cloud continues to '''accrete''' onto it, increasing its mass and temperature. For a solar-mass star, the accretion phase might last around 100,000 to several million years.


Protostars often exhibit powerful outflows and '''jets''', which help to remove '''angular momentum''' and allow accretion to continue. These outflows can impact the surrounding cloud, influencing further star formation. As the core of the protostar becomes increasingly dense and hot (reaching about 10 million Kelvin for a star like Sol), '''hydrogen''' nuclei begin to fuse into '''helium,''' releasing energy through nuclear '''fusion'''. This marks the birth of a '''star''' and its entry into the '''main sequence''' phase of its life. The outward pressure from nuclear fusion balances the inward pull of gravity, stabilizing the star; it will remain in the main sequence phase for the majority of its lifetime, converting '''hydrogen''' to '''helium''' in its core. For Sol, this phase will last about 10 billion years.


Each second, the Sun fuses approximately 600 million tons of '''hydrogen''' into '''helium''' in a process known as the '''proton–proton chain''' (4 '''hydrogen''' form 1 '''helium'''). This process occurs in the core of the Sun, where temperatures exceed 10 million Kelvin, providing the necessary conditions for nuclear '''fusion'''. The proton-proton chain comprises several steps, involving the fusion of protons (hydrogen nuclei) into helium nuclei ('''alpha particles'''). Protons (which are positively charged and normally repel each other due to the '''electrostatic force''') can come close enough for the '''strong nuclear force''' to overcome their electrostatic repulsion. This environment is necessary for the protons to have enough kinetic energy to overcome the '''Coulomb barrier''' of electrostatic repulsion.


* '''Step 1''': Two '''protons''' (p<sup>+</sup>) fuse to form a '''deuterium''' nucleus (2'''H'''), a '''positron''' (e<sup>+</sup>), and a '''neutrino''' (νe). The     positron quickly encounters an electron, leading to their annihilation and     the release of energy in the form of gamma rays. When two protons come     very close, one of the protons can undergo a weak nuclear reaction, where     it transforms into a '''neutron'''. This     transformation is mediated by the '''weak nuclear force''', which     operates at very short ranges and is responsible for processes like beta     decay. The newly produced neutron from the proton's transformation 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.


'''p<sup>+</sup> + p<sup>+</sup> → <sup>2</sup>H + e<sup>+</sup> + ν<sub>e</sub>'''


Both protons and neutrons are '''nucleons''' (components of atomic nuclei); a proton is positively charged, while a neutron has no electric charge. Despite their charge differences, protons and neutrons are quite similar in mass and are both '''fermions''', following the '''Pauli exclusion principle'''. The transformation from a proton to a neutron involves the weak nuclear force, which changes the type (or '''flavor''') of '''quarks''' (the fundamental particles that make up protons and neutrons). Specifically, a 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. The conversion of a proton into a neutron inside the nucleus is an example of beta-plus decay ('''β+ decay'''). In this type of radioactive decay, one of the proton's up quarks changes into a down quark, emitting a '''W+ boson''' (a carrier particle of the weak force). The W+ boson then quickly decays into a positron (e<sup>+</sup>) and an electron neutrino (ν<sub>e</sub>):


'''p<sup>+</sup> →n + e<sup>+</sup> + ν<sub>e</sub>'''


The transformation is accompanied by the release of energy, as the mass-energy of the initial proton is slightly higher than that of the resulting neutron, positron, and neutrino. This energy difference accounts for the kinetic energy of the emitted particles along with the gamma radiation produced. 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 the same.


* '''Step 2''': The newly formed '''deuterium''' nucleus then     fuses with another '''proton''', producing a '''helium'''-3 nucleus (<sup>3</sup>'''He''') and releasing more energy in the form of '''gamma     radiation''' (γ; high-energy '''photons'''; short     wavelengths with a high frequency). In this reaction, the '''deuterium''' nucleus (which already has one proton and one neutron)     combines with the incoming proton to form a new nucleus with two protons     and one neutron; an isotope of '''helium''' known as '''helium'''-3.


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

* '''Step 3''': Two '''helium'''-3 nuclei     collide and fuse to form '''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 in the Sun's core. Even at     these high temperatures, the <sup>3</sup>'''He''' nuclei are positively charged and repel one another. '''Quantum     tunneling''' allows them to get close enough for the '''strong     nuclear force''' to take over at very short ranges, facilitating fusion.     Quantum tunneling is a 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 the Sun's core     repel each other due to the electrostatic force (the '''Coulomb     barrier'''). At the temperatures and pressures at the Sun'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 a very close range where the     strong nuclear force can take effect, despite not having enough energy to     overcome the barrier in a classical sense. This quantum tunneling     significantly increases the rate of fusion reactions in the Sun. When two <sup>3</sup>'''He''' nuclei fuse, they form a <sup>4</sup>'''He''' nucleus (with 2 protons and 2 neutrons) which is more     stable due to the strong nuclear force binding its nucleons more tightly     together. The reaction also releases two protons (2p<sup>+</sup>), which essentially are the "excess" protons     not needed in the formation of the helium-4 nucleus.


'''<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 gradually make their way to the Sun's surface and are emitted as sunlight, while neutrinos escape the Sun almost immediately due to their weak interaction with matter. The proton-proton chain reaction is the primary source of the Sun's energy and, by extension, the primary source of energy for the Solar System. It sustains the Sun's luminosity and supports life on Earth by providing light and warmth. The presence of these expelled neutrinos are one of the primary proofs that nuclear fusion is indeed the source of the sun's energy. The released gamma rays are a significant source of the Sun's energy output. 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 a significant amount of time - ''thousands to millions of years'' - due to the dense plasma's opacity to electromagnetic radiation.


The fusion rate in the 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.


'''Formation of the Sun'''


Originally the Sun would have been about 71.1% '''hydrogen''', 27.4% '''helium''', and 1.5% heavier elements (now 74.9% '''hydrogen''', 23.8% '''helium''', 1.3% heavier elements). The '''hydrogen''' and most of the '''helium''' were produced by Big Bang '''nucleosynthesis''' in the first moments of the universe, and the heavier elements were produced by previous generations of stars and spread into the interstellar medium during the final stages of their stellar lives.


Most of the collapsing cloud (99.86% of the total mass) gathered in the center, whereas the rest flattened into an orbiting disk that became the solar system. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core, which formed the star we call the Sun. It has an '''absolute magnitude''' of +4.83, estimated to be brighter than about 85% of the stars in the Milky Way galaxy (most of which are '''red dwarfs'''). The Sun is a Population I, or '''heavy-element-rich''', star. Its formation may have been triggered by shockwaves from one or more nearby supernovas, suggested by a high abundance of heavy elements such as gold and uranium in the solar system relative to the abundances of these elements in so-called Population II, '''heavy-element-poor''', stars.


Apparent magnitude measures how bright a star or celestial object ''appears from Earth''. It is a '''scale''', traditionally inversely logarithmic, where lower (or more negative) values denote brighter objects. This scale is rooted in an ancient system devised by the Greek astronomer '''Hipparchus'''. Absolute magnitude is a measure of the ''intrinsic brightness'' of a celestial object. It is defined as the apparent magnitude the object would have if it were placed at a standard distance of 10 parsecs (about 32.6 light-years) from Earth. This allows astronomers to compare the true luminosities of celestial objects without the distortion caused by distance. The Sun's absolute magnitude is +4.83. Although it appears very bright in the sky due to its proximity, in terms of the vast array of stars, the Sun is of average brightness. Sirius has an absolute magnitude of +1.4, indicating it is intrinsically brighter than the Sun if both were observed from the same distance. Sirius is indeed more luminous than our Sun, being about 25 times more luminous - though it appears far less so, because of the distortion of distance. This fundamentally illustrates the difference between apparent and aboslute magnitude.


'''Parts of the Sun'''

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. The core formed as the center of the collapsing molecular cloud reached sufficiently high temperatures and pressures for nuclear fusion to commence. This happened once the core's temperature surpassed about 10 million Kelvin.

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.


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 '''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 opaque to light, making it effectively the 'surface' from which sunlight is radiated. The photosphere is where the plasma is dense enough that photons are readily emitted towards space, making it the layer that shines brightly and defines the Sun's appearance. This emission of light defines the Sun's visible diameter and features that can be observed, such as sunspots; hence it acts as a visible surface, even though it too is plasma; anything below it is not visible.


Just above the photosphere, the '''chromosphere''' is a layer of the Sun's atmosphere 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.

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.

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. 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. The Sun rotates faster at its equator than at its poles, due to '''convective motion''' from heat transport and the '''Coriolis force''' from 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 10-kilometer difference mentioned).


The Sun exhibits this '''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.


As the cloud collapsed under its own gravity to form the Sun and the solar system, it conserved its '''angular momentum'''. Angular momentum is a physical quantity that measures the amount of rotation of an object, and it is conserved in a system unless acted upon by an external '''torque'''. Since there's no external force to provide such a torque in the vacuum of space, the collapsing cloud's rotation speed increased as it shrank in size, much like a figure skater spins faster when pulling in their arms.


'''L = Iω''', where:

* I is the     moment of inertia (a measure of an object's resistance to changes in its     rotation rate), and
* ω is the     angular velocity (the rate of rotation). The moment of inertia is     dependent on the mass distribution relative to the axis of rotation; for a     given mass, spreading it further from the rotation axis increases


The cloud's radius decreases, significantly reducing its moment of inertia (I) by I=mr<sup>2</sup>. According to the conservation of angular momentum, if I decreases and L is conserved, then ω (angular velocity) must increase. This means the cloud spins faster as it collapses.


The outcome of this conservation is that as the molecular cloud that became the solar system collapsed, it spun faster, leading to the formation of a rotating protostar - the young Sun. Over time, the Sun reached its current rotation period, which is about 25 days at the equator and longer at higher latitudes, due to its gaseous nature allowing for differential rotation. Over time, the distribution of the Sun's mass and interactions between its magnetic field and the solar wind (a stream of charged particles emitted by the Sun) have likely influenced its rotation rate, a process known as magnetic braking. This process can gradually slow down the rotation rate of stars, including the Sun.


The Sun


The Sun is by far the brightest object in the Earth's sky, with an '''apparent magnitude''' of −26.74. This is about 13 billion times brighter than the next brightest star, Sirius, which has an apparent magnitude of −1.46. A G-type main-sequence star (spectral type: G-V), the Sun's original chemical composition was inherited from the interstellar medium out of which it formed.


The radius of the Sun (R☉) is 109 times that of Earth, but its distance from Earth is 215 R☉, so it '''subtends''' an angle of only ° in the sky, roughly the same as that of the Moon. The mass of the Sun, M☉, is 743 times the total mass of all the planets in the solar system and 330,000 times that of Earth.


The Sun is a very stable source of energy; its radiative output, called the solar constant, is 1.366 kilowatts per square metre at Earth and varies by no more than 0.1 percent. Superposed on this stable star, however, is an interesting 11-year cycle of magnetic activity manifested by regions of transient strong magnetic fields called sunspots.