Hadean Eon
Overview
Dating
Starting ~4567.30 ± 0.16 MYA (4.6 billion years ago)
Lasting ~536 million years.
Ending ~4031.00 ± 0.16 MYA (4.0 billion years ago)
Etymology
The eon's name comes from Hades, Greek god of the Underworld; it is a poetic reference to the extreme environmental conditions of early Earth.
Other names: Priscoan Period (from priscus, 'ancient'); pre-Archean (deprecated)
Eons are the longest portions of geologic time (eras are the second-longest). Three eons are formally recognized: the Phanerozoic Eon (dating from modern times back to the beginning of the Cambrian Period), the Proterozoic Eon, and the Archean Eon - the Hadeon Eon is an informal predecessor.
Informal?
The eon is characterized by an almost complete absence of preserved rock formations. The oldest known rocks on Earth date back to about 4.0 billion years ago, towards the end of the Hadean. Without significant rock records, it's challenging for geologists to define and study this eon using traditional geological techniques. Geological time scales are primarily based on significant events in Earth's history, often marked by changes in the rock record, such as mass extinctions or major shifts in life forms, sedimentation, or atmospheric composition. The Hadean lacks these clear markers in the rock record, making it harder to define using the criteria applied to later geological periods.
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 (ISM), which is composed of gas (mainly hydrogen, but also helium and trace amounts of heavier elements), plasma, and dust.
Formation of Molecular Hydrogen
The vast majority of the contents of the interstellar medium is gaseous H2, a molecule - a structure consisting of more than one atom - in this case, two hydrogen atoms bound together (H-H). Molecular hydrogen forms when two hydrogen atoms share their electrons through a covalent bond, a chemical process that occurs at the low energies typical in 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. In contrast with a nuclear process (for example, the creation of helium), within the hydrogen molecule each nucleus/electron pair retains its identity as a separate hydrogen atom; they are simply bonded together. The protons (nuclei) do not fuse into a different element with a heavier nucleus, as they would in a higher-energy nuclear reaction.
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 hydrogen gas.
The 'stability' of molecular hydrogen is due to the fact that both hydrogen atoms achieve a full valence shell (for hydrogen: two electrons). Most atoms have a maximum of eight electrons that can occupy their outermost (valence) shell, and they generally seek to stabilize by 'filling' that shell with electrons. For most elements this is called the octet rule - but hydrogen has its own analog, the duplet rule, as its valence shell is naturally more compact, and can hold a maximum of two valence electrons. The behavior of an atom, particularly its ability to form chemical bonds with other atoms, is largely determined by the number of 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 H2 from two hydrogen atoms is energetically favorable and exothermic (it releases energy). However, in space, there's a challenge: conservation of momentum and conservation of energy. The newly formed H2 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 which is rich with atoms, 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 H2 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 a 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 that results 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.
Dust
(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 in reality. Dust is also responsible for nebulae observed in the night sky, as it scatters light from nearby stars or emits light due to its own raised 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. Silicates (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 (H2O), methane (CH4), ammonia (NH3), carbon dioxide (CO2), and other molecules can freeze onto the surfaces. Tiny solid particles of metals such as iron, nickel, and magnesium also contribute to interstellar dust.
Plasma
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 HII clouds (described below; 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
Interstellar clouds are accumulations 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 HI region describes a cloud of neutral atomic hydrogen (HI), 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 (HII is H+ in other sciences) III for doubly-ionized; OIII instead of O++). HI 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 HII 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 (HII, 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 (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 once again). HII regions are known for their bright emission in optical wavelengths - when free electrons recombine with ionized protons (HII), 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). HII 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 HII 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.
Gravitational Collapse
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 add resistance to 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 collapse, a cloud can fragment into smaller pieces, each potentially with its own Jeans radius based on its local 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; a relatively short period 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 the Sun, this phase will last about 10 billion years (which gives it something close to 6 billion years of life left).
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; their remnants became dust in our cloud.
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 even been triggered by shockwaves from one or more nearby supernovas; this is suggested by a high abundance of heavy elements such as gold and uranium 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; though it appears far less so, because of the distortion of distance. This fundamentally illustrates the difference between apparent and absolute magnitude.
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 electrostatic force, but the extreme temperature and pressure of the star's core can push the protons close enough to one another for the strong nuclear force to overcome their electrostatic repulsion (known as the Coulomb barrier).
Fusion - Step 1
Two protons (p+) fuse to form a deuterium nucleus (2H), a positron (e+), and a neutrino (νe). The positron quickly encounters an electron, leading to their mutual annihilation and the release of energy in the form of gamma rays. When two protons come very close, one of the protons can (very rarely) 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 (2H), a hydrogen isotope with one proton and one neutron in its nucleus.
p+ + p+ → 2H + e+ + νe
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, meaning that they follow the Pauli exclusion principle. The transformation from a proton to a neutron changes the type (or flavor) of one of the nucleon's fundamental particles, called quarks. 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, which emits a W+ boson (a carrier particle of the weak nuclear force). The W+ boson then quickly decays into a positron (e+) and an electron neutrino (νe):
p+ →n + e+ + νe
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. 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 newly formed deuterium nucleus then fuses with another proton, producing a helium-3 nucleus (3He) and releasing more energy in the form of gamma radiation (γ; high-energy photons; with short wavelengths / high frequency). In this reaction, the deuterium nucleus (which already has one proton and one neutron) fuses with the incoming proton to form a new nucleus with two protons and one neutron; an isotope of helium known as helium-3.
2H + p+ → 3He + γ
Fusion - Step 3
Two helium-3 nuclei collide and fuse to form helium-4 (4He), 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 star's core. Even at these high temperatures, the 3He 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 a star's core repel each other via the Coulomb barrier (electrostatic force). 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 strong nuclear force to 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 3He nuclei fuse, they form a 4He 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+), which essentially are the 'excess' protons not needed in the formation of the helium-4 nucleus.3He + 3He → 4He + 2p+
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 = mc2. 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 immediately expelled neutrinos is one of the primary proofs that nuclear fusion is the source of the sun's energy. The released gamma rays are a significant source of the Sun'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 a significant amount of time - thousands to millions of years - due to the dense plasma's opacity to electromagnetic radiation. The sunlight you see left the surface eight minutes ago, but it was created in the Sun's core millennia before you were born.
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.
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.
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
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 Sun as a Star
The Sun is by far the brightest object in 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 (dwarf) main-sequence (hydrogen-to-helium fusion-powered) star, the Sun's original chemical composition was inherited from the local interstellar cloud out of which it and its neighbors 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 1/2° 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; but superimposed on this stability is an 11-year cycle of magnetic activity manifested by regions of transient strong magnetic fields.
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=mr2. 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.