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140-457: The Sun 's corona lies above the chromosphere and extends millions of kilometres into outer space. Coronal light is typically obscured by diffuse sky radiation and glare from the solar disk, but can be easily seen by the naked eye during a total solar eclipse or with a specialized coronagraph . Spectroscopic measurements indicate strong ionization in the corona and a plasma temperature in excess of 1 000 000 kelvins , much hotter than

280-483: A central subject for astronomical research since antiquity . The Sun orbits the Galactic Center at a distance of 24,000 to 28,000 light-years . From Earth, it is 1  astronomical unit ( 1.496 × 10  km ) or about 8 light-minutes away. Its diameter is about 1,391,400 km ( 864,600 mi ), 109 times that of Earth. Its mass is about 330,000 times that of Earth, making up about 99.86% of

420-531: A combination of the thermal Doppler broadening and the impact pressure broadening yields a Voigt profile . However, the different line broadening mechanisms are not always independent. For example, the collisional effects and the motional Doppler shifts can act in a coherent manner, resulting under some conditions even in a collisional narrowing , known as the Dicke effect . The phrase "spectral lines", when not qualified, usually refers to lines having wavelengths in

560-564: A distance of one astronomical unit (AU) from the Sun (that is, at or near Earth's orbit). Sunlight on the surface of Earth is attenuated by Earth's atmosphere , so that less power arrives at the surface (closer to 1,000 W/m ) in clear conditions when the Sun is near the zenith . Sunlight at the top of Earth's atmosphere is composed (by total energy) of about 50% infrared light, 40% visible light, and 10% ultraviolet light. The atmosphere filters out over 70% of solar ultraviolet, especially at

700-403: A fairly small amount of power being generated per cubic metre . Theoretical models of the Sun's interior indicate a maximum power density, or energy production, of approximately 276.5 watts per cubic metre at the center of the core, which, according to Karl Kruszelnicki , is about the same power density inside a compost pile . The fusion rate in the core is in a self-correcting equilibrium:

840-414: A few millimeters. Re-emission happens in a random direction and usually at slightly lower energy. With this sequence of emissions and absorptions, it takes a long time for radiation to reach the Sun's surface. Estimates of the photon travel time range between 10,000 and 170,000 years. In contrast, it takes only 2.3 seconds for neutrinos , which account for about 2% of the total energy production of

980-416: A finite line-of-sight velocity projection. If different parts of the emitting body have different velocities (along the line of sight), the resulting line will be broadened, with the line width proportional to the width of the velocity distribution. For example, radiation emitted from a distant rotating body, such as a star , will be broadened due to the line-of-sight variations in velocity on opposite sides of

1120-401: A granular appearance called the solar granulation at the smallest scale and supergranulation at larger scales. Turbulent convection in this outer part of the solar interior sustains "small-scale" dynamo action over the near-surface volume of the Sun. The Sun's thermal columns are Bénard cells and take the shape of roughly hexagonal prisms. The visible surface of the Sun, the photosphere,

1260-445: A hot material are detected, perhaps in the presence of a broad spectrum from a cooler source. The intensity of light, over a narrow frequency range, is increased due to emission by the hot material. Spectral lines are highly atom-specific, and can be used to identify the chemical composition of any medium. Several elements, including helium , thallium , and caesium , were discovered by spectroscopic means. Spectral lines also depend on

1400-520: A period known as the Maunder minimum . This coincided in time with the era of the Little Ice Age , when Europe experienced unusually cold temperatures. Earlier extended minima have been discovered through analysis of tree rings and appear to have coincided with lower-than-average global temperatures. The temperature of the photosphere is approximately 6,000 K, whereas the temperature of

1540-485: A phenomenon described by Hale's law . During the solar cycle's declining phase, energy shifts from the internal toroidal magnetic field to the external poloidal field, and sunspots diminish in number and size. At solar-cycle minimum, the toroidal field is, correspondingly, at minimum strength, sunspots are relatively rare, and the poloidal field is at its maximum strength. With the rise of the next 11-year sunspot cycle, differential rotation shifts magnetic energy back from

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1680-473: A result, the outward-flowing solar wind stretches the interplanetary magnetic field outward, forcing it into a roughly radial structure. For a simple dipolar solar magnetic field, with opposite hemispherical polarities on either side of the solar magnetic equator, a thin current sheet is formed in the solar wind. At great distances, the rotation of the Sun twists the dipolar magnetic field and corresponding current sheet into an Archimedean spiral structure called

1820-410: 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. The radiative zone

1960-410: A sudden increase of the radiative flux emitted from small regions of the corona. They are very complex phenomena, visible at different wavelengths; they involve several zones of the solar atmosphere and many physical effects, thermal and not thermal, and sometimes wide reconnections of the magnetic field lines with material expulsion. Flares are impulsive phenomena, of average duration of 15 minutes, and

2100-423: A tiny spectral band with a nonzero range of frequencies, not a single frequency (i.e., a nonzero spectral width ). In addition, its center may be shifted from its nominal central wavelength. There are several reasons for this broadening and shift. These reasons may be divided into two general categories – broadening due to local conditions and broadening due to extended conditions. Broadening due to local conditions

2240-406: A transition layer, the tachocline . This is a region where the sharp regime change between the uniform rotation of the radiative zone and the differential rotation of the convection zone results in a large shear between the two—a condition where successive horizontal layers slide past one another. Presently, it is hypothesized that a magnetic dynamo, or solar dynamo , within this layer generates

2380-400: Is a weaker or stronger region in an otherwise uniform and continuous spectrum . It may result from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to identify atoms and molecules . These "fingerprints" can be compared to the previously collected ones of atoms and molecules, and are thus used to identify

2520-429: Is broadened because the photons at the line center have a greater reabsorption probability than the photons at the line wings. Indeed, the reabsorption near the line center may be so great as to cause a self reversal in which the intensity at the center of the line is less than in the wings. This process is also sometimes called self-absorption . Radiation emitted by a moving source is subject to Doppler shift due to

2660-643: 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. One astronomical unit (about 150 million kilometres; 93 million miles) is defined as the mean distance between the centres of the Sun and the Earth. The instantaneous distance varies by about ± 2.5 million km or 1.55 million miles as Earth moves from perihelion on ~ January 3rd to aphelion on ~ July 4th. At its average distance, light travels from

2800-436: Is defined to begin at the distance where the flow of the solar wind becomes superalfvénic —that is, where the flow becomes faster than the speed of Alfvén waves, at approximately 20 solar radii ( 0.1 AU ). Turbulence and dynamic forces in the heliosphere cannot affect the shape of the solar corona within, because the information can only travel at the speed of Alfvén waves. The solar wind travels outward continuously through

2940-533: Is due to effects which hold in a small region around the emitting element, usually small enough to assure local thermodynamic equilibrium . Broadening due to extended conditions may result from changes to the spectral distribution of the radiation as it traverses its path to the observer. It also may result from the combining of radiation from a number of regions which are far from each other. The lifetime of excited states results in natural broadening, also known as lifetime broadening. The uncertainty principle relates

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3080-482: Is emphasized by the analysis of the dynamics of the main structures of the corona, which evolve at differential times. Studying coronal variability in its complexity is not easy because the times of evolution of the different structures can vary considerably: from seconds to several months. The typical sizes of the regions where coronal events take place vary in the same way, as it is shown in the following table. Flares take place in active regions and are characterized by

3220-402: Is facilitated by the full ionization of helium in the transition region, which significantly reduces radiative cooling of the plasma. The transition region does not occur at a well-defined altitude, but forms a kind of nimbus around chromospheric features such as spicules and filaments , and is in constant, chaotic motion. The transition region is not easily visible from Earth's surface, but

3360-412: Is faint near the Sun itself, but drops in brightness only gradually far from the Sun, extending far across the sky and becoming the zodiacal light . The F-corona is recognized to arise from small dust grains orbiting the Sun; these form a tenuous cloud that extends through much of the solar system. The "K-corona" is named for the fact that its spectrum is a continuum, with no major spectral features. It

3500-486: Is much hotter (by a factor from 150 to 450) than the visible surface of the Sun: the corona's temperature is 1 to 3 million kelvin compared to the photosphere 's average temperature – around 5 800 kelvin . The corona is far less dense than the photosphere, and produces about one-millionth as much visible light. The corona is separated from the photosphere by the relatively shallow chromosphere . The exact mechanism by which

3640-463: Is not properly a gas, because it is made of charged particles, basically protons and electrons, moving at different velocities. Supposing that they have the same kinetic energy on average (for the equipartition theorem ), electrons have a mass roughly 1 800 times smaller than protons, therefore they acquire more velocity. Metal ions are always slower. This fact has relevant physical consequences either on radiative processes (that are very different from

3780-414: Is observed depends on the type of material and its temperature relative to another emission source. An absorption line is produced when photons from a hot, broad spectrum source pass through a cooler material. The intensity of light, over a narrow frequency range, is reduced due to absorption by the material and re-emission in random directions. By contrast, a bright emission line is produced when photons from

3920-409: Is only 84% of what it was in the protostellar phase (before nuclear fusion in the core started). In the future, helium will continue to accumulate in the core, and in about 5 billion years this gradual build-up will eventually cause the Sun to exit the main sequence and become a red giant . The chemical composition of the photosphere is normally considered representative of the composition of

4060-441: Is readily observable from space by instruments sensitive to extreme ultraviolet . The corona is the next layer of the Sun. The low corona, near the surface of the Sun, has a particle density around 10  m to 10  m . The average temperature of the corona and solar wind is about 1,000,000–2,000,000 K; however, in the hottest regions it is 8,000,000–20,000,000 K. Although no complete theory yet exists to account for

4200-410: Is strongly attenuated by Earth's ozone layer , so that the amount of UV varies greatly with latitude and has been partially responsible for many biological adaptations, including variations in human skin color . High-energy gamma ray photons initially released with fusion reactions in the core are almost immediately absorbed by the solar plasma of the radiative zone, usually after traveling only

4340-483: Is suggested by a high abundance of heavy elements in the Solar System, such as gold and uranium , relative to the abundances of these elements in so-called Population II , heavy-element-poor, stars. The heavy elements could most plausibly have been produced by endothermic nuclear reactions during a supernova, or by transmutation through neutron absorption within a massive second-generation star. The Sun

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4480-465: Is sunlight light that is Thomson-scattered by free electrons in the hot plasma of the Sun's outer atmosphere. The continuum nature of the spectrum arises from Doppler broadening of the Sun's Fraunhofer absorption lines in the reference frame of the (hot and therefore fast-moving) electrons. Although the K-corona is a phenomenon of the electrons in the plasma, the term is frequently used to describe

4620-470: Is tens to hundreds of kilometers thick, and is slightly less opaque than air on Earth. Because the upper part of the photosphere is cooler than the lower part, an image of the Sun appears brighter in the center than on the edge or limb of the solar disk, in a phenomenon known as limb darkening . The spectrum of sunlight has approximately the spectrum of a black-body radiating at 5,772 K (9,930 °F), interspersed with atomic absorption lines from

4760-506: Is the particle number density , k B {\displaystyle k_{B}} the Boltzmann constant and T {\displaystyle T} the plasma temperature. It is evident from the equation that the plasma pressure lowers when the plasma temperature decreases with respect to the surrounding regions or when the zone of intense magnetic field empties. The same physical effect renders sunspots apparently dark in

4900-509: Is the star at the center of the Solar System . It is a massive, nearly perfect sphere of hot plasma , heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light and infrared radiation with 10% at ultraviolet energies. It is by far the most important source of energy for life on Earth . The Sun has been an object of veneration in many cultures. It has been

5040-437: Is the layer below which the Sun becomes opaque to visible light. Photons produced in this layer escape the Sun through the transparent solar atmosphere above it and become solar radiation, sunlight. The change in opacity is due to the decreasing amount of H ions , which absorb visible light easily. Conversely, the visible light perceived is produced as electrons react with hydrogen atoms to produce H ions. The photosphere

5180-424: Is the most prominent variation in which the number and size of sunspots waxes and wanes. The solar magnetic field extends well beyond the Sun itself. The electrically conducting solar wind plasma carries the Sun's magnetic field into space, forming what is called the interplanetary magnetic field . In an approximation known as ideal magnetohydrodynamics , plasma particles only move along magnetic field lines. As

5320-531: Is the only region of the Sun that produces an appreciable amount of thermal energy through fusion; 99% of the Sun's power is generated in the innermost 24% of its radius, and almost no fusion occurs beyond 30% of the radius. The rest of the Sun is heated by this energy as it is transferred outward through many successive layers, finally to the solar photosphere where it escapes into space through radiation (photons) or advection (massive particles). The proton–proton chain occurs around 9.2 × 10 times each second in

5460-420: Is the thickest layer of the Sun, at 0.45 solar radii. From the core out to about 0.7 solar radii , thermal radiation is the primary means of energy transfer. The temperature drops from approximately 7 million to 2 million kelvins with increasing distance from the core. This temperature gradient is less than the value of the adiabatic lapse rate and hence cannot drive convection, which explains why

5600-538: Is ultimately related to the word for sun in other branches of the Indo-European language family, though in most cases a nominative stem with an l is found, rather than the genitive stem in n , as for example in Latin sōl , ancient Greek ἥλιος ( hēlios ), Welsh haul and Czech slunce , as well as (with *l > r ) Sanskrit स्वर् ( svár ) and Persian خور ( xvar ). Indeed,

5740-567: Is very complex. However, two kinds of basic structures can be distinguished: As for temporal dynamics, three different phases are generally distinguished, whose duration are not comparable. The durations of those periods depend on the range of wavelengths used to observe the event: Sometimes also a phase preceding the flare can be observed, usually called as "pre-flare" phase. Often accompanying large solar flares and prominences are coronal mass ejections (CME). These are enormous emissions of coronal material and magnetic field that travel outward from

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5880-402: Is wave heating, in which sound, gravitational or magnetohydrodynamic waves are produced by turbulence in the convection zone. These waves travel upward and dissipate in the corona, depositing their energy in the ambient matter in the form of heat. The other is magnetic heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in

6020-547: The Alfvén surface , the boundary separating the corona from the solar wind, defined as where the coronal plasma's Alfvén speed and the large-scale solar wind speed are equal. During the flyby, Parker Solar Probe passed into and out of the corona several times. This proved the predictions that the Alfvén critical surface is not shaped like a smooth ball, but has spikes and valleys that wrinkle its surface. The Sun emits light across

6160-557: The Lyman series or Balmer series . Originally all spectral lines were classified into series: the principal series , sharp series , and diffuse series . These series exist across atoms of all elements, and the patterns for all atoms are well-predicted by the Rydberg-Ritz formula . These series were later associated with suborbitals. There are a number of effects which control spectral line shape . A spectral line extends over

6300-524: The Parker spiral . Sunspots are visible as dark patches on the Sun's photosphere and correspond to concentrations of magnetic field where convective transport of heat is inhibited from the solar interior to the surface. As a result, sunspots are slightly cooler than the surrounding photosphere, so they appear dark. At a typical solar minimum , few sunspots are visible, and occasionally none can be seen at all. Those that do appear are at high solar latitudes. As

6440-410: The corona , and the heliosphere . The coolest layer of the Sun is a temperature minimum region extending to about 500 km above the photosphere, and has a temperature of about 4,100  K . This part of the Sun is cool enough to allow for the existence of simple molecules such as carbon monoxide and water. The chromosphere, transition region, and corona are much hotter than the surface of

6580-409: The electromagnetic radiation that it emits and to that coming from lower layers. The plasma is very rarefied and the photon mean free path overcomes by far all the other length-scales, including the typical sizes of common coronal features. Electromagnetic radiation from the corona has been identified coming from three main sources, located in the same volume of space: Sun The Sun

6720-531: The gravitational collapse of matter within a region of a large molecular cloud . Most of this matter 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 . Every second, the Sun's core fuses about 600 billion kilograms (kg) of hydrogen into helium and converts 4 billion kg of matter into energy . About 4 to 7 billion years from now, when hydrogen fusion in

6860-614: The l -stem survived in Proto-Germanic as well, as * sōwelan , which gave rise to Gothic sauil (alongside sunnō ) and Old Norse prosaic sól (alongside poetic sunna ), and through it the words for sun in the modern Scandinavian languages: Swedish and Danish sol , Icelandic sól , etc. The principal adjectives for the Sun in English are sunny for sunlight and, in technical contexts, solar ( / ˈ s oʊ l ər / ), from Latin sol . From

7000-428: 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, the apparent visible surface of the Sun. By this measure, the Sun is a near-perfect sphere with an oblateness estimated at 9 millionths, which means that its polar diameter differs from its equatorial diameter by only 10 kilometers (6.2 mi). The tidal effect of

7140-431: The temperature and density of the material, so they are widely used to determine the physical conditions of stars and other celestial bodies that cannot be analyzed by other means. Depending on the material and its physical conditions, the energy of the involved photons can vary widely, with the spectral lines observed across the electromagnetic spectrum , from radio waves to gamma rays . Strong spectral lines in

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7280-509: The visible band of the full electromagnetic spectrum . Many spectral lines occur at wavelengths outside this range. At shorter wavelengths, which correspond to higher energies, ultraviolet spectral lines include the Lyman series of hydrogen . At the much shorter wavelengths of X-rays , the lines are known as characteristic X-rays because they remain largely unchanged for a given chemical element, independent of their chemical environment. Longer wavelengths correspond to lower energies, where

7420-466: The visible part of the electromagnetic spectrum often have a unique Fraunhofer line designation, such as K for a line at 393.366 nm emerging from singly-ionized calcium atom, Ca , though some of the Fraunhofer "lines" are blends of multiple lines from several different species . In other cases, the lines are designated according to the level of ionization by adding a Roman numeral to

7560-444: The visible spectrum , so its color is white , with a CIE color-space index near (0.3, 0.3), when viewed from space or when the Sun is high in the sky. The Solar radiance per wavelength peaks in the green portion of the spectrum when viewed from space. When the Sun is very low in the sky, atmospheric scattering renders the Sun yellow, red, orange, or magenta, and in rare occasions even green or blue . Some cultures mentally picture

7700-611: The 11-year solar cycle and becomes particularly simple during the minimum period, when the magnetic field of the Sun is almost similar to a dipolar configuration (plus a quadrupolar component). The interconnections of active regions are arcs connecting zones of opposite magnetic field, of different active regions. Significant variations of these structures are often seen after a flare. Some other features of this kind are helmet streamers – large, cap-like coronal structures with long, pointed peaks that usually overlie sunspots and active regions. Coronal streamers are considered to be sources of

7840-426: The 1871 and 1878 eclipses, that the size and shape of the corona changes with the sunspot cycle . In 1930, Bernard Lyot invented the "coronograph" (now "coronagraph") , which allows viewing the corona without a total eclipse. In 1952, American astronomer Eugene Parker proposed that the solar corona might be heated by myriad tiny 'nanoflares', miniature brightenings resembling solar flares that would occur all over

7980-465: The Greek helios comes the rare adjective heliac ( / ˈ h iː l i æ k / ). In English, the Greek and Latin words occur in poetry as personifications of the Sun, Helios ( / ˈ h iː l i ə s / ) and Sol ( / ˈ s ɒ l / ), while in science fiction Sol may be used to distinguish the Sun from other stars. The term sol with a lowercase s is used by planetary astronomers for

8120-544: The NASA Parker Solar Probe will approach the Sun very closely, allowing more direct observations. Large-scale structures are very long arcs which can cover over a quarter of the solar disk but contain plasma less dense than in the coronal loops of the active regions. They were first detected in the June 8, 1968, flare observation during a rocket flight. The large-scale structure of the corona changes over

8260-446: The Solar System . Long-term secular change in sunspot number is thought, by some scientists, to be correlated with long-term change in solar irradiance, which, in turn, might influence Earth's long-term climate. The solar cycle influences space weather conditions, including those surrounding Earth. For example, in the 17th century, the solar cycle appeared to have stopped entirely for several decades; few sunspots were observed during

8400-443: The Sun as yellow and some even red; the cultural reasons for this are debated. The Sun is classed as a G2 star, meaning it is a G-type star , with 2 indicating its surface temperature is in the second range of the G class. The solar constant is the amount of power that the Sun deposits per unit area that is directly exposed to sunlight. The solar constant is equal to approximately 1,368 W/m (watts per square meter) at

8540-560: The Sun at up to 3000 km/s, containing roughly 10 times the energy of the solar flare or prominence that accompanies them. Some larger CMEs can propel hundreds of millions of tons of material into interplanetary space at roughly 1.5 million kilometers an hour. Coronal stars are ubiquitous among the stars in the cool half of the Hertzsprung–Russell diagram . These coronae can be detected using X-ray telescopes . Some stellar coronae, particularly in young stars, are much more luminous than

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8680-424: The Sun extends from the center to about 20–25% of the solar radius. It has a density of up to 150 g/cm (about 150 times the density of water) and a temperature of close to 15.7 million kelvin (K). By contrast, the Sun's surface temperature is about 5800 K . Recent analysis of SOHO mission data favors the idea that the core is rotating faster than the radiative zone outside it. Through most of

8820-688: The Sun will shed its outer layers and become a dense type of cooling star (a white dwarf ), and no longer produce energy by fusion, but will still glow and give off heat from its previous fusion for perhaps trillions of years. After that, it is theorized to become a super dense black dwarf , giving off negligible energy. The English word sun developed from Old English sunne . Cognates appear in other Germanic languages , including West Frisian sinne , Dutch zon , Low German Sünn , Standard German Sonne , Bavarian Sunna , Old Norse sunna , and Gothic sunnō . All these words stem from Proto-Germanic * sunnōn . This

8960-413: The Sun's magnetic field . The Sun's convection zone extends from 0.7 solar radii (500,000 km) to near the surface. In this layer, the solar plasma is not dense or hot enough to transfer the heat energy of the interior outward via radiation. Instead, the density of the plasma is low enough to allow convective currents to develop and move the Sun's energy outward towards its surface. Material heated at

9100-398: The Sun's core by radiation rather than by convection (see Radiative zone below), so the fusion products are not lifted outward by heat; they remain in the core, and gradually an inner core of helium has begun to form that cannot be fused because presently the Sun's core is not hot or dense enough to fuse helium. In the current photosphere, the helium fraction is reduced, and the metallicity

9240-426: The Sun's core diminishes to the point where the Sun is no longer in hydrostatic equilibrium , its core will undergo a marked increase in density and temperature which will cause its outer layers to expand, eventually transforming the Sun into a red giant . This process will make the Sun large enough to render Earth uninhabitable approximately five billion years from the present. After the red giant phase, models suggest

9380-403: The Sun's horizon to Earth's horizon in about 8 minutes and 20 seconds, while light from the closest points of the Sun and Earth takes about two seconds less. The energy of this sunlight supports almost all life on Earth by photosynthesis , and drives Earth's climate and weather. The Sun does not have a definite boundary, but its density decreases exponentially with increasing height above

9520-499: The Sun's life, energy has been produced by nuclear fusion in the core region through the proton–proton chain ; this process converts hydrogen into helium. Currently, 0.8% of the energy generated in the Sun comes from another sequence of fusion reactions called the CNO cycle ; the proportion coming from the CNO cycle is expected to increase as the Sun becomes older and more luminous. The core

9660-551: The Sun's life, they account for 74.9% and 23.8%, respectively, of the mass of the Sun in the photosphere. All heavier elements, called metals in astronomy, account for less than 2% of the mass, with oxygen (roughly 1% of the Sun's mass), carbon (0.3%), neon (0.2%), and iron (0.2%) being the most abundant. The Sun's original chemical composition was inherited from the interstellar medium out of which it formed. Originally it would have been about 71.1% hydrogen, 27.4% helium, and 1.5% heavier elements. The hydrogen and most of

9800-659: The Sun's. For example, FK Comae Berenices is the prototype for the FK Com class of variable star . These are giants of spectral types G and K with an unusually rapid rotation and signs of extreme activity. Their X-ray coronae are among the most luminous ( L x ≥ 10 erg·s or 10W) and the hottest known with dominant temperatures up to 40 MK. The astronomical observations planned with the Einstein Observatory by Giuseppe Vaiana and his group showed that F-, G-, K- and M-stars have chromospheres and often coronae much like

9940-438: The Sun, to reach the surface. Because energy transport in the Sun is a process that involves photons in thermodynamic equilibrium with matter , the time scale of energy transport in the Sun is longer, on the order of 30,000,000 years. This is the time it would take the Sun to return to a stable state if the rate of energy generation in its core were suddenly changed. Electron neutrinos are released by fusion reactions in

10080-402: The Sun. The reason is not well understood, but evidence suggests that Alfvén waves may have enough energy to heat the corona. Above the temperature minimum layer is a layer about 2,000 km thick, dominated by a spectrum of emission and absorption lines. It is called the chromosphere from the Greek root chroma , meaning color, because the chromosphere is visible as a colored flash at

10220-401: The Sun. During periods of quiet, the corona is more or less confined to the equatorial regions, with coronal holes covering the polar regions. However, during the Sun's active periods, the corona is evenly distributed over the equatorial and polar regions, though it is most prominent in areas with sunspot activity. The solar cycle spans approximately 11 years, from one solar minimum to

10360-484: The Sun. The O-B stars , which do not have surface convection zones, have a strong X-ray emission. However these stars do not have coronae, but the outer stellar envelopes emit this radiation during shocks due to thermal instabilities in rapidly moving gas blobs. Also A-stars do not have convection zones but they do not emit at the UV and X-ray wavelengths. Thus they appear to have neither chromospheres nor coronae. The matter in

10500-538: The X-ray emission. Coronal holes are unipolar regions which look dark in the X-rays since they do not emit much radiation. These are wide zones of the Sun where the magnetic field is unipolar and opens towards the interplanetary space. The high speed solar wind arises mainly from these regions. In the UV images of the coronal holes, some small structures, similar to elongated bubbles, are often seen as they were suspended in

10640-407: The atomic and molecular components of stars and planets , which would otherwise be impossible. Spectral lines are the result of interaction between a quantum system (usually atoms , but sometimes molecules or atomic nuclei ) and a single photon . When a photon has about the right amount of energy (which is connected to its frequency) to allow a change in the energy state of the system (in

10780-486: The beginning and end of total solar eclipses. The temperature of the chromosphere increases gradually with altitude, ranging up to around 20,000 K near the top. In the upper part of the chromosphere helium becomes partially ionized . Above the chromosphere, in a thin (about 200 km ) transition region, the temperature rises rapidly from around 20,000 K in the upper chromosphere to coronal temperatures closer to 1,000,000 K . The temperature increase

10920-486: The case of an atom this is usually an electron changing orbitals ), the photon is absorbed. Then the energy will be spontaneously re-emitted, either as one photon at the same frequency as the original one or in a cascade, where the sum of the energies of the photons emitted will be equal to the energy of the one absorbed (assuming the system returns to its original state). A spectral line may be observed either as an emission line or an absorption line . Which type of line

11060-528: The cooler plasma below, thus creating the relatively dark sun spots. High-resolution X-ray images of the Sun's corona photographed by Skylab in 1973, by Yohkoh in 1991–2001, and by subsequent space-based instruments revealed the structure of the corona to be quite varied and complex, leading astronomers to classify various zones on the coronal disc. Astronomers usually distinguish several regions, as described below. Active regions are ensembles of loop structures connecting points of opposite magnetic polarity in

11200-460: The core, but, unlike photons, they rarely interact with matter, so almost all are able to escape the Sun immediately. However, measurements of the number of these neutrinos produced in the Sun are lower than theories predict by a factor of 3. In 2001, the discovery of neutrino oscillation resolved the discrepancy: the Sun emits the number of electron neutrinos predicted by the theory, but neutrino detectors were missing 2 ⁄ 3 of them because

11340-501: The core, converting about 3.7 × 10 protons into alpha particles (helium nuclei) every second (out of a total of ~8.9 × 10 free protons in the Sun), or about 6.2 × 10  kg/s . However, each proton (on average) takes around 9 billion years to fuse with another using the PP chain. Fusing four free protons (hydrogen nuclei) into a single alpha particle (helium nucleus) releases around 0.7% of

11480-403: The corona is heated is still the subject of some debate, but likely possibilities include episodic energy releases from the pervasive magnetic field and magnetohydrodynamic waves from below. The outer edges of the Sun's corona are constantly being transported away, creating the "open" magnetic flux entrained in the solar wind . The corona is not always evenly distributed across the surface of

11620-401: The corona reaches 1,000,000–2,000,000 K . The high temperature of the corona shows that it is heated by something other than direct heat conduction from the photosphere. It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere, and two main mechanisms have been proposed to explain coronal heating. The first

11760-402: The corona, estimates had put it somewhere between 10 and 20 solar radii from the surface of the Sun. On April 28, 2021, during its eighth flyby of the Sun, NASA's Parker Solar Probe encountered the specific magnetic and particle conditions at 18.8 solar radii that indicated that it penetrated the Alfvén surface. A portrait, as diversified as the one already pointed out for the coronal features,

11900-401: The coronal heating problem remains as these structures are being observed remotely, where many ambiguities are present (i.e., radiation contributions along the line-of-sight propagation ). In-situ measurements are required before a definitive answer can be determined, but due to the high plasma temperatures in the corona, in-situ measurements are, at present, impossible. The next mission of

12040-577: The designation of the chemical element . Neutral atoms are denoted with the Roman numeral I, singly ionized atoms with II, and so on, so that, for example: Cu II — copper ion with +1 charge, Cu Fe III — iron ion with +2 charge, Fe More detailed designations usually include the line wavelength and may include a multiplet number (for atomic lines) or band designation (for molecular lines). Many spectral lines of atomic hydrogen also have designations within their respective series , such as

12180-400: The duration of a solar day on another planet such as Mars . The astronomical symbol for the Sun is a circle with a center dot, [REDACTED] . It is used for such units as M ☉ ( Solar mass ), R ☉ ( Solar radius ) and L ☉ ( Solar luminosity ). The scientific study of the Sun is called heliology . The Sun is a G-type main-sequence star that makes up about 99.86% of

12320-500: The effects of inhomogeneous broadening is sometimes reduced by a process called motional narrowing . Certain types of broadening are the result of conditions over a large region of space rather than simply upon conditions that are local to the emitting particle. Opacity broadening is an example of a non-local broadening mechanism. Electromagnetic radiation emitted at a particular point in space can be reabsorbed as it travels through space. This absorption depends on wavelength. The line

12460-407: The equator and their extension increases during the periods of maximum of the solar cycle, while they almost disappear during each minimum. Therefore, the quiet Sun always coincides with the equatorial zone and its surface is less active during the maximum of the solar cycle. Approaching the minimum of the solar cycle (also named butterfly cycle), the extension of the quiet Sun increases until it covers

12600-455: The extent that decay rates can be artificially suppressed or enhanced. The atoms in a gas which are emitting radiation will have a distribution of velocities. Each photon emitted will be "red"- or "blue"-shifted by the Doppler effect depending on the velocity of the atom relative to the observer. The higher the temperature of the gas, the wider the distribution of velocities in the gas. Since

12740-451: The external part of the solar atmosphere is in the state of plasma , at very high temperature (a few million kelvin) and at very low density (of the order of 10 particles/m). According to the definition of plasma, it is a quasi-neutral ensemble of particles which exhibits a collective behaviour. The composition is similar to that in the Sun's interior, mainly hydrogen, but with much greater ionization of its heavier elements than that found in

12880-563: The external poloidal dipolar magnetic field is near its dynamo-cycle minimum strength; but an internal toroidal quadrupolar field, generated through differential rotation within the tachocline, is near its maximum strength. At this point in the dynamo cycle, buoyant upwelling within the convective zone forces emergence of the toroidal magnetic field through the photosphere, giving rise to pairs of sunspots, roughly aligned east–west and having footprints with opposite magnetic polarities. The magnetic polarity of sunspot pairs alternates every solar cycle,

13020-430: The following minimum. Since the solar magnetic field is continually wound up due to the faster rotation of mass at the Sun's equator ( differential rotation ), sunspot activity is more pronounced at solar maximum where the magnetic field is more twisted. Associated with sunspots are coronal loops , loops of magnetic flux , upwelling from the solar interior. The magnetic flux pushes the hotter photosphere aside, exposing

13160-446: The form of large solar flares and myriad similar but smaller events— nanoflares . Currently, it is unclear whether waves are an efficient heating mechanism. All waves except Alfvén waves have been found to dissipate or refract before reaching the corona. In addition, Alfvén waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms. Emission-line A spectral line

13300-404: The fused mass as energy, so the Sun releases energy at the mass–energy conversion rate of 4.26 billion kg/s (which requires 600 billion kg of hydrogen ), for 384.6  yottawatts ( 3.846 × 10  W ), or 9.192 × 10   megatons of TNT per second. The large power output of the Sun is mainly due to the huge size and density of its core (compared to Earth and objects on Earth), with only

13440-594: The ground configuration of highly ionised metals (the green Fe-XIV line from Fe at 5 303 Å , but also the red Fe-X line from Fe at 6 374 Å ). The solar corona has three recognized, and distinct, sources of light that occupy the same volume: the "F-corona" (for "Fraunhofer"), the "K-corona" (for "Kontinuierlich"), and the "E-corona" (for "emission"). The "F-corona" is named for the Fraunhofer spectrum of absorption lines in ordinary sunlight, which are preserved by reflection off small material objects. The F-corona

13580-482: The heliosphere, forming the solar magnetic field into a spiral shape, until it impacts the heliopause more than 50 AU from the Sun. In December 2004, the Voyager 1 probe passed through a shock front that is thought to be part of the heliopause. In late 2012, Voyager 1 recorded a marked increase in cosmic ray collisions and a sharp drop in lower energy particles from the solar wind, which suggested that

13720-432: The helium in the Sun would have been produced by Big Bang nucleosynthesis in the first 20 minutes of the universe, and the heavier elements were produced by previous generations of stars before the Sun was formed, and spread into the interstellar medium during the final stages of stellar life and by events such as supernovae . Since the Sun formed, the main fusion process has involved fusing hydrogen into helium. Over

13860-426: The high temperature of the coronal plasma, revealing that the corona is much hotter than the internal layers of the chromosphere. The corona behaves like a gas which is very hot but very light at the same time: the pressure in the corona is usually only 0.1 to 0.6 Pa in active regions, while on the Earth the atmospheric pressure is about 100 kPa, approximately a million times higher than on the solar surface. However it

14000-537: The lifetime of an excited state (due to spontaneous radiative decay or the Auger process ) with the uncertainty of its energy. Some authors use the term "radiative broadening" to refer specifically to the part of natural broadening caused by the spontaneous radiative decay. A short lifetime will have a large energy uncertainty and a broad emission. This broadening effect results in an unshifted Lorentzian profile . The natural broadening can be experimentally altered only to

14140-505: The mass of the Solar System. It has an absolute magnitude of +4.83, estimated to be brighter than about 85% of the stars in the Milky Way , most of which are red dwarfs . It is more massive than 95% of the stars within 7 pc (23 ly). The Sun is a Population I , or heavy-element-rich, star. Its formation approximately 4.6 billion years ago may have been triggered by shockwaves from one or more nearby supernovae . This

14280-482: The most energetic events can last several hours. Flares produce a high and rapid increase of the density and temperature. An emission in white light is only seldom observed: usually, flares are only seen at extreme UV wavelengths and into the X-rays, typical of the chromospheric and coronal emission. In the corona, the morphology of flares is described by observations in the UV, soft and hard X-rays, and in Hα wavelengths, and

14420-424: The nature of the perturbing force as follows: Inhomogeneous broadening is a general term for broadening because some emitting particles are in a different local environment from others, and therefore emit at a different frequency. This term is used especially for solids, where surfaces, grain boundaries, and stoichiometry variations can create a variety of local environments for a given atom to occupy. In liquids,

14560-444: The neutrinos had changed flavor by the time they were detected. The Sun has a stellar magnetic field that varies across its surface. Its polar field is 1–2 gauss (0.0001–0.0002  T ), whereas the field is typically 3,000 gauss (0.3 T) in features on the Sun called sunspots and 10–100 gauss (0.001–0.01 T) in solar prominences . The magnetic field varies in time and location. The quasi-periodic 11-year solar cycle

14700-406: The open-magnetic flux that can be found in coronal holes and the solar wind. Loops of magnetic flux well up from the solar body and fill with hot solar plasma. Due to the heightened magnetic activity in these coronal loop regions, coronal loops can often be the precursor to solar flares and CMEs. The solar plasma that feeds these structures is heated from under 6 000 K to well over 10 K from

14840-497: The order of seconds (in the case of flare events), minutes, hours or days. Where there is a balance in loop energy sources and sinks, coronal loops can last for long periods of time and are known as steady state or quiescent coronal loops ( example ). Coronal loops are very important to our understanding of the current coronal heating problem . Coronal loops are highly radiating sources of plasma and are therefore easy to observe by instruments such as TRACE . An explanation of

14980-419: The past 4.6 billion years, the amount of helium and its location within the Sun has gradually changed. The proportion of helium within the core has increased from about 24% to about 60% due to fusion, and some of the helium and heavy elements have settled from the photosphere toward the center of the Sun because of gravity . The proportions of heavier elements are unchanged. Heat is transferred outward from

15120-443: The photosphere, the so-called coronal loops. They generally distribute in two zones of activity, which are parallel to the solar equator. The average temperature is between two and four million kelvin, while the density goes from 10 to 10 particles per cubic centimetre. Active regions involve all the phenomena directly linked to the magnetic field, which occur at different heights above the Sun's surface: sunspots and faculae occur in

15260-439: The photosphere, through the transition region, and into the corona. Often, the solar plasma will fill these loops from one point and drain to another, called foot points ( siphon flow due to a pressure difference, or asymmetric flow due to some other driver). When the plasma rises from the foot points towards the loop top, as always occurs during the initial phase of a compact flare, it is defined as chromospheric evaporation. When

15400-457: The photosphere. Bright points are small active regions found on the solar disk. X-ray bright points were first detected on April 8, 1969, during a rocket flight. The fraction of the solar surface covered by bright points varies with the solar cycle. They are associated with small bipolar regions of the magnetic field. Their average temperature ranges from 1.1 MK to 3.4 MK. The variations in temperature are often correlated with changes in

15540-487: The photosphere. Heavier metals, such as iron, are partially ionized and have lost most of the external electrons. The ionization state of a chemical element depends strictly on the temperature and is regulated by the Saha equation in the lowest atmosphere, but by collisional equilibrium in the optically thin corona. Historically, the presence of the spectral lines emitted from highly ionized states of iron allowed determination of

15680-473: The photosphere; spicules , Hα filaments and plages in the chromosphere; prominences in the chromosphere and transition region; and flares and coronal mass ejections (CME) happen in the corona and chromosphere. If flares are very violent, they can also perturb the photosphere and generate a Moreton wave . On the contrary, quiescent prominences are large, cool, dense structures which are observed as dark, "snake-like" Hα ribbons (appearing like filaments) on

15820-407: The photospheric radiative processes), or on thermal conduction. Furthermore, the presence of electric charges induces the generation of electric currents and high magnetic fields. Magnetohydrodynamic waves (MHD waves) can also propagate in this plasma, even though it is still not clear how they can be transmitted or generated in the corona. Coronal plasma is optically thin and therefore transparent to

15960-414: The photospheric surface. Both coronal mass ejections and high-speed streams of solar wind carry plasma and the interplanetary magnetic field outward into the Solar System. The effects of solar activity on Earth include auroras at moderate to high latitudes and the disruption of radio communications and electric power . Solar activity is thought to have played a large role in the formation and evolution of

16100-455: The planets is weak and does not significantly affect the shape of the Sun. The Sun rotates faster at its equator than at its poles . This differential rotation is caused by convective motion due to heat transport and the Coriolis force due to the Sun's rotation. In a frame of reference defined by the stars, the rotational period is approximately 25.6 days at the equator and 33.5 days at

16240-480: The plasma itself (as distinct from the dust that gives rise to the F-corona). The "E-corona" is the component of the corona with an emission-line spectrum, either inside or outside the wavelength band of visible light. It is a phenomenon of the ion component of the plasma, as individual ions are excited by collision with other ions or electrons, or by absorption of ultraviolet light from the Sun. The Sun's corona

16380-457: The plasma rapidly cools and falls toward the photosphere, it is called chromospheric condensation. There may also be symmetric flow from both loop foot points, causing a build-up of mass in the loop structure. The plasma may cool rapidly in this region (for a thermal instability), its dark filaments obvious against the solar disk or prominences off the Sun's limb . Coronal loops may have lifetimes in

16520-473: The poles. Viewed from Earth as it orbits the Sun, the apparent rotational period of the Sun at its equator is about 28 days. Viewed from a vantage point above its north pole, the Sun rotates counterclockwise around its axis of spin. A survey of solar analogs suggest the early Sun was rotating up to ten times faster than it does today. This would have made the surface much more active, with greater X-ray and UV emission. Sun spots would have covered 5–30% of

16660-557: The poloidal to the toroidal field, but with a polarity that is opposite to the previous cycle. The process carries on continuously, and in an idealized, simplified scenario, each 11-year sunspot cycle corresponds to a change, then, in the overall polarity of the Sun's large-scale magnetic field. The Sun's magnetic field leads to many effects that are collectively called solar activity . Solar flares and coronal mass ejections tend to occur at sunspot groups. Slowly changing high-speed streams of solar wind are emitted from coronal holes at

16800-448: The primordial Solar System. Typically, the solar heavy-element abundances described above are measured both by using spectroscopy of the Sun's photosphere and by measuring abundances in meteorites that have never been heated to melting temperatures. These meteorites are thought to retain the composition of the protostellar Sun and are thus not affected by the settling of heavy elements. The two methods generally agree well. The core of

16940-470: The probe had passed through the heliopause and entered the interstellar medium , and indeed did so on August 25, 2012, at approximately 122 astronomical units (18 Tm) from the Sun. The heliosphere has a heliotail which stretches out behind it due to the Sun's peculiar motion through the galaxy. On April 28, 2021, NASA's Parker Solar Probe encountered the specific magnetic and particle conditions at 18.8 solar radii that indicated that it penetrated

17080-437: The shorter wavelengths. Solar ultraviolet radiation ionizes Earth's dayside upper atmosphere, creating the electrically conducting ionosphere . Ultraviolet light from the Sun has antiseptic properties and can be used to sanitize tools and water. This radiation causes sunburn , and has other biological effects such as the production of vitamin D and sun tanning . It is the main cause of skin cancer . Ultraviolet light

17220-459: The slow solar wind. Filament cavities are zones which look dark in the X-rays and are above the regions where Hα filaments are observed in the chromosphere. They were first observed in the two 1970 rocket flights which also detected coronal holes . Filament cavities are cooler clouds of plasma suspended above the Sun's surface by magnetic forces. The regions of intense magnetic field look dark in images because they are empty of hot plasma. In fact,

17360-425: The solar cycle progresses toward its maximum , sunspots tend to form closer to the solar equator, a phenomenon known as Spörer's law . The largest sunspots can be tens of thousands of kilometers across. An 11-year sunspot cycle is half of a 22-year Babcock –Leighton dynamo cycle, which corresponds to an oscillatory exchange of energy between toroidal and poloidal solar magnetic fields. At solar-cycle maximum,

17500-497: The solar disc. Their temperature is about 5 000 – 8 000 K , and so they are usually considered as chromospheric features. In 2013, images from the High Resolution Coronal Imager revealed never-before-seen "magnetic braids" of plasma within the outer layers of these active regions. Coronal loops are the basic structures of the magnetic solar corona. These loops are the closed-magnetic flux cousins of

17640-451: The solar wind. These are the coronal plumes. More precisely, they are long thin streamers that project outward from the Sun's north and south poles. The solar regions which are not part of active regions and coronal holes are commonly identified as the quiet Sun. The equatorial region has a faster rotation speed than the polar zones. The result of the Sun's differential rotation is that the active regions always arise in two bands parallel to

17780-448: The spectral line is a combination of all of the emitted radiation, the higher the temperature of the gas, the broader the spectral line emitted from that gas. This broadening effect is described by a Gaussian profile and there is no associated shift. The presence of nearby particles will affect the radiation emitted by an individual particle. There are two limiting cases by which this occurs: Pressure broadening may also be classified by

17920-399: The star (this effect usually referred to as rotational broadening). The greater the rate of rotation, the broader the line. Another example is an imploding plasma shell in a Z-pinch . Each of these mechanisms can act in isolation or in combination with others. Assuming each effect is independent, the observed line profile is a convolution of the line profiles of each mechanism. For example,

18060-485: The sum of the magnetic pressure and plasma pressure must be constant everywhere on the heliosphere in order to have an equilibrium configuration: where the magnetic field is higher, the plasma must be cooler or less dense. The plasma pressure p {\displaystyle p} can be calculated by the state equation of a perfect gas: p = n k B T {\displaystyle p=nk_{B}T} , where n {\displaystyle n}

18200-463: The surface of the Sun, known as the photosphere . Corona ( Latin for 'crown') is, in turn, derived from Ancient Greek κορώνη ( korṓnē )  'garland, wreath'. In 1724, French-Italian astronomer Giacomo F. Maraldi recognized that the aura visible during a solar eclipse belongs to the Sun, not to the Moon . In 1809, Spanish astronomer José Joaquín de Ferrer coined

18340-507: The surface of the Sun. The high temperature of the Sun's corona gives it unusual spectral features, which led some in the 19th century to suggest that it contained a previously unknown element, " coronium ". Instead, these spectral features have since been explained by highly ionized iron (Fe-XIV, or Fe). Bengt Edlén , following the work of Walter Grotrian in 1939, first identified the coronal spectral lines in 1940 (observed since 1869) as transitions from low-lying metastable levels of

18480-417: The surface. The rotation rate was gradually slowed by magnetic braking , as the Sun's magnetic field interacted with the outflowing solar wind. A vestige of this rapid primordial rotation still survives at the Sun's core, which has been found to be rotating at a rate of once per week; four times the mean surface rotation rate. The Sun consists mainly of the elements hydrogen and helium . At this time in

18620-431: The tachocline picks up heat and expands, thereby reducing its density and allowing it to rise. As a result, an orderly motion of the mass develops into thermal cells that carry most of the heat outward to the Sun's photosphere above. Once the material diffusively and radiatively cools just beneath the photospheric surface, its density increases, and it sinks to the base of the convection zone, where it again picks up heat from

18760-424: The temperature of the corona, at least some of its heat is known to be from magnetic reconnection . The corona is the extended atmosphere of the Sun, which has a volume much larger than the volume enclosed by the Sun's photosphere. A flow of plasma outward from the Sun into interplanetary space is the solar wind . The heliosphere, the tenuous outermost atmosphere of the Sun, is filled with solar wind plasma and

18900-422: The tenuous layers above the photosphere. The photosphere has a particle density of ~10  m (about 0.37% of the particle number per volume of Earth's atmosphere at sea level). The photosphere is not fully ionized—the extent of ionization is about 3%, leaving almost all of the hydrogen in atomic form. The Sun's atmosphere is composed of five layers: the photosphere, the chromosphere , the transition region ,

19040-479: The term 'corona'. Based on his own observations of the 1806 solar eclipse at Kinderhook (New York), de Ferrer also proposed that the corona was part of the Sun and not of the Moon. English astronomer Norman Lockyer identified the first element unknown on Earth in the Sun's chromosphere, which was called helium (from Greek helios 'sun'). French astronomer Jules Jenssen noted, after comparing his readings between

19180-404: The top of the radiative zone and the convective cycle continues. At the photosphere, the temperature has dropped 350-fold to 5,700 K (9,800 °F) and the density to only 0.2 g/m (about 1/10,000 the density of air at sea level, and 1 millionth that of the inner layer of the convective zone). The thermal columns of the convection zone form an imprint on the surface of the Sun giving it

19320-424: The total mass of the Solar System. Roughly three-quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen , carbon , neon , and iron . The Sun is a G-type main-sequence star (G2V), informally called a yellow dwarf , though its light is actually white. It formed approximately 4.6 billion years ago from

19460-418: The transfer of energy through this zone is by radiation instead of thermal convection. Ions of hydrogen and helium emit photons, which travel only a brief distance before being reabsorbed by other ions. The density drops a hundredfold (from 20 000 kg/m to 200 kg/m ) between 0.25 solar radii and 0.7 radii, the top of the radiative zone. The radiative zone and the convective zone are separated by

19600-409: The whole disk surface excluding some bright points on the hemisphere and the poles, where there are coronal holes. The Alfvén surface is the boundary separating the corona from the solar wind defined as where the coronal plasma's Alfvén speed and the large-scale solar wind speed are equal. Researchers were unsure exactly where the Alfvén critical surface of the Sun lay. Based on remote images of

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