The Sun orbits theGalactic Center at a distance of 24,000 to 28,000light-years. From Earth, it is1 astronomical unit (1.496×108 km) or about 8light-minutes away.Its diameter is about1,391,400 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 the total mass of the Solar System. Roughly three-quarters of the Sun'smass consists ofhydrogen (~73%); the rest is mostlyhelium (~25%), with much smaller quantities of heavier elements, includingoxygen,carbon,neon, andiron.
The Sun is aG-type main-sequence star (G2V), informally called a yellow dwarf, though its light is actually white. It formed approximately 4.6 billion[a] years ago from thegravitational collapse of matter within a region of a largemolecular cloud. Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk thatbecame the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in itscore. Every second, the Sun's core fuses about 600 billionkilograms (kg) of hydrogen into helium and converts 4 billion kg ofmatter into energy.
About 4 to 7 billion years from now, whenhydrogen fusion in the Sun's core diminishes to the point where the Sun is no longer inhydrostatic 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 ared giant. After the red giant phase, models suggest the Sun will shed its outer layers and become a dense type of cooling star (awhite 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 denseblack dwarf, giving off negligible energy.
Etymology
The English wordsun developed fromOld Englishsunne. Cognates appear in otherGermanic languages, includingWest Frisiansinne,Dutchzon,Low GermanSünn,Standard GermanSonne,BavarianSunna,Old Norsesunna, andGothicsunnō. All these words stem fromProto-Germanic*sunnōn.[17][18] This is ultimately related to the word forsun in other branches of theIndo-European language family, though in most cases anominative stem with anl is found, rather than thegenitive stem inn, as for example inLatinsōl,ancient Greekἥλιος (hēlios),Welshhaul andCzechslunce, as well as (with *l >r) Sanskritस्वर् (svár) andPersianخور (xvar). Indeed, thel-stem survived in Proto-Germanic as well, as*sōwelan, which gave rise to Gothicsauil (alongsidesunnō) and Old Norse prosaicsól (alongside poeticsunna), and through it the words forsun in the modern Scandinavian languages:Swedish andDanishsol,Icelandicsól, etc.[18]
The principal adjectives for the Sun in English aresunny for sunlight and, in technical contexts,solar (/ˈsoʊlər/),[3] from Latinsol.[19] From the Greekhelios comes the rare adjectiveheliac (/ˈhiːliæk/).[20] In English, the Greek and Latin words occur in poetry as personifications of the Sun,Helios (/ˈhiːliəs/) andSol (/ˈsɒl/),[2][1] while in science fictionSol may be used to distinguish the Sun from other stars. The termsol with a lowercases is used by planetary astronomers for the duration of asolar day on another planet such asMars.[21]
Size comparison of the Sun, all theplanets of theSolar System and some larger stars. The Sun is 1.4 million kilometers (4.643light-seconds) wide, about 109 timeswider than Earth, or four times theLunar distance, and contains 99.86% of all Solar Systemmass.
The Sun is aG-type main-sequence star that makes up about 99.86% of the mass of the Solar System.[26] It has anabsolute magnitude of +4.83, estimated to be brighter than about 85% of the stars in theMilky Way, most of which arered dwarfs.[27][28] It is more massive than 95% of the stars within 7 pc (23 ly).[29]The Sun is aPopulation I, or heavy-element-rich,[b] star.[30] Its formation approximately 4.6 billion years ago may have been triggered by shockwaves from one or more nearbysupernovae.[31][32] This is suggested by a highabundance of heavy elements in the Solar System, such asgold anduranium, relative to the abundances of these elements in so-calledPopulation II, heavy-element-poor, stars. The heavy elements could most plausibly have been produced byendothermic nuclear reactions during a supernova, or bytransmutation throughneutron absorption within a massive second-generation star.[30]
Oneastronomical unit (about 150 million kilometres; 93 million miles) is defined as the mean distance between the centers of the Sun and the Earth. The instantaneous distance varies by about±2.5 million kilometres (1.6 million miles) as Earth moves fromperihelion around 3 January toaphelion around 4 July.[36] At its average distance, light travels from the Sun's horizon to Earth's horizon in about 8 minutes and 20 seconds,[37] while light from the closest points of the Sun and Earth takes about two seconds less. The energy of thissunlight supports almost all life[c] on Earth byphotosynthesis,[38] and drivesEarth's climate and weather.[39]
The Sun does not have a definite boundary, but its density decreases exponentially with increasing height above thephotosphere.[40] 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.[41] The roundness of the Sun is relative difference between its radius at its equator,, and at its pole,, called theoblateness,[42]The value is difficult to measure. Atmospheric distortion means the measurement must be done on satellites; the value is very small meaning very precise technique is needed.[43]
The oblateness was once proposed to be sufficient to explain theperihelion precession of Mercury but Einstein proposed thatgeneral relativity could explain the precession using a spherical Sun.[43] When high precision measurements of the oblateness became available via theSolar Dynamics Observatory[44] and thePicard satellite[42] the measured value was even smaller than expected,[43] 8.2 x 10−6, or 8 parts per million.This makes the Sun the natural object closest to a perfect sphere.[45] The oblateness value remains constant independent of solar irradiation changes.[42] The tidal effect of the planets is weak and does not significantly affect the shape of the Sun.[46]
The Sun rotates faster at its equator than at itspoles. Thisdifferential rotation is caused byconvective motion due to heat transport and theCoriolis 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 the poles. Viewed from Earth as it orbits the Sun, theapparent rotational period of the Sun at its equator is about 28 days.[47] Viewed from a vantage point above its north pole, the Sun rotatescounterclockwise around its axis of spin.[d][48]
A survey ofsolar 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 the surface.[49] The rotation rate was gradually slowed bymagnetic braking, as the Sun's magnetic field interacted with the outflowingsolar wind.[50] 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.[51][52]
The Sun consists mainly of the elementshydrogen andhelium. At this time in the Sun's life, they account for 74.9% and 23.8%, respectively, of the mass of the Sun in the photosphere.[53] All heavier elements, calledmetals in astronomy, account for less than 2% of the mass, withoxygen (roughly 1% of the Sun's mass),carbon (0.3%),neon (0.2%), andiron (0.2%) being the most abundant.[54]
The Sun's original chemical composition was inherited from theinterstellar medium out of which it formed. Originally it would have been about 71.1% hydrogen, 27.4% helium, and 1.5% heavier elements.[53] The hydrogen and most of the helium in the Sun would have been produced byBig Bang nucleosynthesis in the first 20 minutes of the universe, and the heavier elements wereproduced by previous generations of stars before the Sun was formed, and spread into the interstellar medium during thefinal stages of stellar life and by events such assupernovae.[55]
Since the Sun formed, the main fusion process has involved fusing hydrogen into helium. Over 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 ofgravity. The proportions of heavier elements are unchanged.Heat is transferred outward from the Sun's core by radiation rather than by convection (seeRadiative zone below), so the fusion products are not lifted outward by heat; they remain in the core,[56] 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 themetallicity is only 84% of what it was in theprotostellar 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 themain sequence and become ared giant.[57]
The chemical composition of the photosphere is normally considered representative of the composition of the primordial Solar System.[58] Typically, the solar heavy-element abundances described above are measured both by usingspectroscopy of the Sun's photosphere and by measuring abundances inmeteorites 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.[59]
The core of the Sun extends from the center to about 20–25% of the solar radius.[60] It has a density of up to150 g/cm3[61][62] (about 150 times the density of water) and a temperature of close to 15.7 millionkelvin (K).[62] By contrast, the Sun's surface temperature is about5800 K. Recent analysis ofSOHO mission data favors the idea that the core is rotating faster than the radiative zone outside it.[60] Through most of the Sun's life, energy has been produced by nuclear fusion in the core region through theproton–proton chain; this process converts hydrogen into helium.[63] Currently, 0.8% of the energy generated in the Sun comes from another sequence of fusion reactions called theCNO cycle; the proportion coming from the CNO cycle is expected to increase as the Sun becomes older and more luminous.[64][65]
The core is the only region of the Sun that produces an appreciable amount ofthermal 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).[66][67]
Illustration of a proton-proton reaction chain, from hydrogen formingdeuterium,helium-3, and regularhelium-4
The proton–proton chain occurs around9.2×1037 times each second in the core, converting about 3.7×1038 protons intoalpha particles (helium nuclei) every second (out of a total of ~8.9×1056 free protons in the Sun), or about6.2×1011 kg/s. However, each proton (on average) takes around 9 billion years to fuse with another using the PP chain.[66] Fusing four freeprotons (hydrogen nuclei) into a single alpha particle (helium nucleus) releases around 0.7% of the fused mass as energy,[68] so the Sun releases energy at the mass–energy conversion rate of 4.26 billion kg/s (which requires 600 billion kg of hydrogen[69]), for 384.6 yottawatts (3.846×1026 W),[5] or 9.192×1010megatons 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 a fairly small amount of power being generated percubic metre. Theoretical models of the Sun's interior indicate a maximum power density, or energy production, of approximately 276.5watts per cubic metre at the center of the core,[70] which, according toKarl Kruszelnicki, is about the same power density inside acompost pile.[71]
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 andexpand slightly against the weight of the outer layers, reducing the density and hence the fusion rate and correcting theperturbation; 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.[72][73]
Illustration of different stars' internal structure based on mass. The Sun in the middle has an inner radiating zone and an outer convective zone.
The radiative zone is the thickest layer of the Sun, at 0.45 solar radii. From the core out to about 0.7solar radii,thermal radiation is the primary means of energy transfer.[74] The temperature drops from approximately 7 million to 2 million kelvins with increasing distance from the core.[62] Thistemperature gradient is less than the value of theadiabatic lapse rate and hence cannot drive convection, which explains why the transfer of energy through this zone is byradiation instead of thermal convection.[62]Ions of hydrogen and helium emit photons, which travel only a brief distance before being reabsorbed by other ions.[74] The density drops a hundredfold (from 20,000 kg/m3 to 200 kg/m3) between 0.25 solar radii and 0.7 radii, the top of the radiative zone.[74]
The radiative zone and the convective zone are separated by a transition layer, thetachocline. This is a region where the sharp regime change between the uniform rotation of the radiative zone and the differential rotation of theconvection zone results in a largeshear between the two—a condition where successive horizontal layers slide past one another.[75] Presently, it is hypothesized that a magnetic dynamo, orsolar dynamo, within this layer generates the Sun'smagnetic field.[62]
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 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 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/m3 (about 1/10,000 the density of air at sea level, and 1 millionth that of the inner layer of the convective zone).[62]
The thermal columns of the convection zone form an imprint on the surface of the Sun giving it a granular appearance called thesolar granulation at the smallest scale andsupergranulation 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.[62] The Sun's thermal columns areBénard cells and take the shape of roughly hexagonal prisms.[76]
The visible surface of the Sun, the photosphere, is the layer below which the Sun becomesopaque to visible light.[77] 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 ofH− ions, which absorb visible light easily.[77] Conversely, the visible light perceived is produced as electrons react with hydrogen atoms to produce H− ions.[78][79]
The photosphere 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 orlimb of the solar disk, in a phenomenon known aslimb darkening.[77] The spectrum of sunlight has approximately the spectrum of ablack-body radiating at 5,772 K (9,930 °F),[12] interspersed with atomicabsorption lines from the tenuous layers above the photosphere. The photosphere has a particle density of ~1023 m−3 (about 0.37% of the particle number per volume ofEarth'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.[80]
The coolest layer of the Sun is a temperature minimum region extending to about500 km above the photosphere, and has a temperature of about4,100 K.[77] This part of the Sun is cool enough to allow for the existence of simple molecules such ascarbon monoxide and water.[81] The chromosphere, transition region, and corona are much hotter than the surface of the Sun.[77] The reason is not well understood, but evidence suggests thatAlfvén waves may have enough energy to heat the corona.[82]
The Sun's transition region taken byHinode's Solar Optical Telescope
Above the temperature minimum layer is a layer about2,000 km thick, dominated by a spectrum of emission and absorption lines.[77] It is called thechromosphere from the Greek rootchroma, meaning color, because the chromosphere is visible as a colored flash at the beginning and end of total solar eclipses.[74] The temperature of the chromosphere increases gradually with altitude, ranging up to around20,000 K near the top.[77] In the upper part of the chromosphere helium becomes partiallyionized.[83]
Above the chromosphere, in a thin (about200 km) transition region, the temperature rises rapidly from around20,000 K in the upper chromosphere to coronal temperatures closer to1,000,000 K.[84] The temperature increase is facilitated by the full ionization of helium in the transition region, which significantly reduces radiative cooling of the plasma.[83] The transition region does not occur at a well-defined altitude, but forms a kind ofnimbus around chromospheric features such asspicules andfilaments, and is in constant, chaotic motion.[74] The transition region is not easily visible from Earth's surface, but is readily observable fromspace by instruments sensitive toextreme ultraviolet.[85]
During asolar eclipse the solar corona can be seen with the naked eye during totality.
The corona is the next layer of the Sun. The low corona, near the surface of the Sun, has a particle density around 1015 m−3 to 1016 m−3.[83][e] 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.[84] Although no complete theory yet exists to account for the temperature of the corona, at least some of its heat is known to be frommagnetic reconnection.[84][86]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 intointerplanetary space is thesolar wind.[86]
The heliosphere, the tenuous outermost atmosphere of the Sun, is filled with solar wind plasma and is defined to begin at the distance where the flow of the solar wind becomessuperalfvénic—that is, where the flow becomes faster than the speed of Alfvén waves,[87] 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 the heliosphere,[88][89] forming the solar magnetic field into aspiral shape,[86] until it impacts theheliopause more than50 AU from the Sun. In December 2004, theVoyager 1 probe passed through a shock front that is thought to be part of the heliopause.[90] In late 2012,Voyager 1 recorded a marked increase incosmic ray collisions and a sharp drop in lower energy particles from the solar wind, which suggested that the probe had passed through the heliopause and entered theinterstellar medium,[91] and indeed did so on August 25, 2012, at approximately 122 astronomical units (18 Tm) from the Sun.[92] The heliosphere has aheliotail which stretches out behind it due to the Sun'speculiar motion through the galaxy.[93]
On April 28, 2021, NASA'sParker Solar Probe encountered the specific magnetic and particle conditions at 18.8 solar radii that indicated that it penetrated theAlfvén 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.[94][95] 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.[94]
The Sun emits light across thevisible spectrum, so its color iswhite, with aCIE 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.[96][97] When the Sun is very low in the sky,atmospheric scattering renders the Sun yellow, red, orange, or magenta, and in rare occasions evengreen or blue. Some cultures mentally picture the Sun as yellow and some even red; the cultural reasons for this are debated.[98] The Sun is classed as aG2 star,[66] meaning it is aG-type star, with2 indicating itssurface temperature is in the second range of the G class.
Thesolar 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 approximately1,368 W/m2 (watts per square meter) at a distance of oneastronomical unit (AU) from the Sun (that is, at or near Earth's orbit).[99] Sunlight on the surface of Earth isattenuated byEarth's atmosphere, so that less power arrives at the surface (closer to1,000 W/m2) in clear conditions when the Sun is near thezenith.[100] 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.[101] The atmosphere filters out over 70% of solar ultraviolet, especially at the shorter wavelengths.[102] Solarultraviolet radiation ionizes Earth's dayside upper atmosphere, creating the electrically conductingionosphere.[103]
Ultraviolet light from the Sun hasantiseptic properties and can be used to sanitize tools and water. This radiation causessunburn, and has other biological effects such as the production ofvitamin D andsun tanning. It is the main cause ofskin cancer. Ultraviolet light is strongly attenuated by Earth'sozone layer, so that the amount of UV varies greatly withlatitude and has been partially responsible for many biological adaptations, including variations inhuman skin color.[104]
High-energygamma rayphotons initially released with fusion reactions in the core are almost immediately absorbed by the solar plasma of the radiative zone, usually after traveling only 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.[105] In contrast, it takes only 2.3 seconds forneutrinos, which account for about 2% of the total energy production of the Sun, to reach the surface. Because energy transport in the Sun is a process that involves photons inthermodynamic equilibrium withmatter, 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.[106]
Electron neutrinos are released by fusion reactions in 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 arelower than theories predict by a factor of 3. In 2001, the discovery ofneutrino oscillation resolved the discrepancy: the Sun emits the number of electron neutrinos predicted by the theory, but neutrino detectors were missing2⁄3 of them because the neutrinos had changedflavor by the time they were detected.[107]
Magnetic activity
The Sun has astellar magnetic field that varies across its surface. Its polar field is 1–2gauss (0.0001–0.0002 T), whereas the field is typically 3,000 gauss (0.3 T) in features on the Sun calledsunspots and 10–100 gauss (0.001–0.01 T) insolar prominences.[5] The magnetic field varies in time and location. The quasi-periodic 11-yearsolar cycle is the most prominent variation in which the number and size of sunspots waxes and wanes.[108][109][110]
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 theinterplanetary magnetic field.[86] In an approximation known as idealmagnetohydrodynamics, plasma particles only move along magnetic field lines. As 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 thincurrent 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 anArchimedean spiral structure called theParker spiral.[86]
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 typicalsolar minimum, few sunspots are visible, and occasionally none can be seen at all. Those that do appear are at high solar latitudes. As the solar cycle progresses toward itsmaximum, sunspots tend to form closer to the solar equator, a phenomenon known asSpörer's law. The largest sunspots can be tens of thousands of kilometers across.[111]
An 11-year sunspot cycle is half of a 22-yearBabcock–Leightondynamo cycle, which corresponds to an oscillatory exchange of energy betweentoroidal and poloidal solar magnetic fields. At solar-cycle maximum, 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, a phenomenon described byHale's law.[112][113]
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 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.[114][115]
Measurements from 2005 of solar cycle variation during the previous 30 years
The Sun's magnetic field leads to many effects that are collectively calledsolar activity.Solar flares andcoronal mass ejections tend to occur at sunspot groups. Slowly changing high-speed streams of solar wind are emitted fromcoronal holes at 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.[116] The effects of solar activity on Earth includeauroras at moderate to high latitudes and the disruption of radio communications andelectric power. Solar activity is thought to have played a large role in theformation and evolution of the Solar System.[117]
Change in solar irradiance over the 11 year solar cycle has been correlated with change in sunspot number.[118] The solar cycle influencesspace 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 a period known as theMaunder minimum. This coincided in time with the era of theLittle Ice Age, when Europe experienced unusually cold temperatures.[119][120] Earlier extended minima have been discovered through analysis oftree rings and appear to have coincided with lower-than-average global temperatures.[121]
The temperature of the photosphere is approximately 6,000 K, whereas the temperature of the corona reaches1,000,000–2,000,000 K.[84] The high temperature of the corona shows that it is heated by something other than directheat conduction from the photosphere.[86]
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.[84] The first is wave heating, in which sound, gravitational or magnetohydrodynamic waves are produced by turbulence in the convection zone.[84] These waves travel upward and dissipate in the corona, depositing their energy in the ambient matter in the form of heat.[122] The other is magnetic heating, in which magnetic energy is continuously built up by photospheric motion and released throughmagnetic reconnection in the form of large solar flares and myriad similar but smaller events—nanoflares.[123]
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.[124] In addition, Alfvén waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms.[84]
Overview of the evolution of a star like the Sun, from collapsingprotostar at left tored giant stage at right
The Sun today is roughly halfway through the main-sequence portion of its life. It has not changed dramatically in over four billion[a] years and will remain fairly stable for about five billion more. However, after hydrogen fusion in its core has stopped, the Sun will undergo dramatic changes, both internally and externally.
The Sun formed about 4.6 billion years ago from the collapse of part of a giantmolecular cloud that consisted mostly of hydrogen and helium and that probably gave birth to many other stars.[125] This age is estimated usingcomputer models ofstellar evolution and throughnucleocosmochronology.[13] The result is consistent with theradiometric date of the oldest Solar System material, at 4.567 billion years ago.[126][127] Studies of ancientmeteorites reveal traces of stable daughter nuclei of short-lived isotopes, such asiron-60, that form only in exploding, short-lived stars. This indicates that one or moresupernovae must have occurred near the location where the Sun formed. Ashock wave from a nearby supernova would have triggered the formation of the Sun by compressing the matter within the molecular cloud and causing certain regions to collapse under their own gravity.[128] As one fragment of the cloud collapsed it also began to rotate due toconservation of angular momentum and heat up with the increasing pressure.[129] Much of the mass became concentrated in the center, whereas the rest flattened out into a disk that would become the planets and other Solar System bodies.[130][131] Gravity and pressure within the core of the cloud generated a lot of heat as it accumulated more matter from the surrounding disk, eventually triggeringnuclear fusion.[132]
The starsHD 162826 andHD 186302 share similarities with the Sun and are thus hypothesized to be its stellar siblings, formed in the same molecular cloud.[133][134]
Evolution of a Sun-like star. The track of a one solar mass star on theHertzsprung–Russell diagram is shown from the main sequence to the white dwarf stage.
The Sun is about halfway through its main-sequence stage, during which nuclear fusion reactions in its core fuse hydrogen into helium. Each second, more than four billion kilograms of matter are converted into energy within the Sun's core, producing neutrinos andsolar radiation. At this rate, the Sun has so far converted around 100 times the mass of Earth into energy, about 0.03% of the total mass of the Sun. The Sun will spend a total of approximately 10 to 11 billion years as a main-sequence star before thered giant phase of the Sun.[135] At the 8 billion year mark, the Sun will be at its hottest point according to the ESA'sGaia space observatory mission in 2022.[136]
The Sun is gradually becoming hotter in its core, hotter at the surface, larger in radius, and more luminous during its time on the main sequence: since the beginning of its main sequence life, it has expanded in radius by 15% and the surface has increased in temperature from 5,620 K (9,660 °F) to 5,772 K (9,930 °F), resulting in a 48% increase in luminosity from 0.677solar luminosities to its present-day 1.0 solar luminosity. This occurs because the helium atoms in the core have a higher meanmolecular weight than thehydrogen atoms that were fused, resulting in less thermal pressure. The core is therefore shrinking, allowing the outer layers of the Sun to move closer to the center, releasinggravitational potential energy. According to thevirial theorem, half of this released gravitational energy goes into heating, which leads to a gradual increase in the rate at which fusion occurs and thus an increase in the luminosity. This process speeds up as the core gradually becomes denser.[137] At present, it is increasing in brightness by about 1% every 100 million years. It will take at least 1 billion years from now to deplete liquid water from the Earth from such increase.[138] After that, the Earth will cease to be able to support complex, multicellular life and the last remaining multicellular organisms on the planet will suffer a final, completemass extinction.[139]
After core hydrogen exhaustion
The size of the current Sun (now in themain sequence) compared to its estimated size during its red-giant phase in the future
The Sun does not have enough mass to explode as asupernova. Instead, when it runs out of hydrogen in the core in approximately 5 billion years, core hydrogen fusion will stop, and there will be nothing to prevent the core from contracting. The release of gravitational potential energy will cause the luminosity of the Sun to increase, ending the main sequence phase and leading the Sun to expand over the next billion years: first into asubgiant, and then into ared giant.[137][140][141] The heating due to gravitational contraction will also lead to expansion of the Sun and hydrogen fusion in a shell just outside the core, where unfused hydrogen remains, contributing to the increased luminosity, which will eventually reach more than 1,000 times its present luminosity.[137] When the Sun enters itsred-giant branch (RGB) phase, it will engulf (and very likely destroy)Mercury andVenus. According to a 2008 article, Earth's orbit will have initially expanded to at most 1.5 AU (220 million km; 140 million mi) due to the Sun's loss of mass. However, Earth's orbit will then start shrinking due totidal forces (and, eventually, drag from the lower chromosphere) so that it is engulfed by the Sun during thetip of the red-giant branch phase 7.59 billion years from now, 3.8 and 1 million years after Mercury and Venus have respectively suffered the same fate.[141]
By the time the Sun reaches the tip of the red-giant branch, it will be about 256 times larger than it is today, with a radius of 1.19 AU (178 million km; 111 million mi).[141][142] The Sun will spend around a billion years in the RGB and lose around a third of its mass.[141]
After the red-giant branch, the Sun has approximately 120 million years of active life left, but much happens. First, the core (full ofdegenerate helium) ignites violently in thehelium flash; it is estimated that 6% of the core—itself 40% of the Sun's mass—will be converted into carbon within a matter of minutes through thetriple-alpha process.[143] The Sun then shrinks to around 10 times its current size and 50 times the luminosity, with a temperature a little lower than today. It will then have reached thered clump orhorizontal branch, but a star of the Sun's metallicity does not evolve blueward along the horizontal branch. Instead, it just becomes moderately larger and more luminous over about 100 million years as it continues to react helium in the core.[141]
When the helium is exhausted, the Sun will repeat the expansion it followed when the hydrogen in the core was exhausted. This time, however, it all happens faster, and the Sun becomes larger and more luminous. This is theasymptotic-giant-branch phase, and the Sun is alternately reacting hydrogen in a shell or helium in a deeper shell. After about 20 million years on the early asymptotic giant branch, the Sun becomes increasingly unstable, with rapid mass loss andthermal pulses that increase the size and luminosity for a few hundred years every 100,000 years or so. The thermal pulses become larger each time, with the later pulses pushing the luminosity to as much as 5,000 times the current level. Despite this, the Sun's maximum AGB radius will not be as large as its tip-RGB maximum: 179R☉, or about 0.832 AU (124.5 million km; 77.3 million mi).[141][144]
Models vary depending on the rate and timing of mass loss. Models that have higher mass loss on the red-giant branch produce smaller, less luminous stars at the tip of the asymptotic giant branch, perhaps only 2,000 times the luminosity and less than 200 times the radius.[141] For the Sun, four thermal pulses are predicted before it completely loses its outer envelope and starts to make aplanetary nebula.[145]
The post-asymptotic-giant-branch evolution is even faster. The luminosity stays approximately constant as the temperature increases, with the ejected half of the Sun's mass becoming ionized into aplanetary nebula as the exposed core reaches 30,000 K (53,500 °F), as if it is in a sort ofblue loop. The final naked core, awhite dwarf, will have a temperature of over 100,000 K (180,000 °F) and contain an estimated 54.05% of the Sun's present-day mass.[141] Simulations indicate that the Sun may be among the least massive stars capable of forming a planetary nebula.[146] The planetary nebula will disperse in about 10,000 years, but the white dwarf will survive for trillions of years before fading to a hypothetical super-denseblack dwarf.[147][148][149] As such, it would give off no more energy.[150]
The Sun has eight known planets orbiting it. This includes fourterrestrial planets (Mercury,Venus,Earth, andMars), twogas giants (Jupiter andSaturn), and twoice giants (Uranus andNeptune). The Solar System also has nine bodies generally considered asdwarf planets and some morecandidates, anasteroid belt, numerouscomets, and a large number of icy bodies which lie beyond the orbit of Neptune. Six of the planets and many smaller bodies also have their ownnatural satellites: in particular, the satellite systems of Jupiter, Saturn, and Uranus are in some ways like miniature versions of the Sun's system.[151]
The Sun is moved by the gravitational pull of the planets. The center of the Sun moves around the Solar Systembarycenter, within a range from 0.1 to 2.2 solar radii. The Sun's motion around the barycenter approximately repeats every 179 years, rotated by about 30° due primarily to thesynodic period of Jupiter and Saturn.[152]
The Sun's gravitational field is estimated todominate the gravitational forces of surrounding stars out to about two light-years (125,000 AU). Lower estimates for the radius of theOort cloud, by contrast, do not place it farther than50,000 AU.[153] Most of the mass is orbiting in the region between 3,000 and100,000 AU.[154] The furthest known objects, such asComet West, have aphelia around70,000 AU from the Sun.[155] The Sun'sHill sphere with respect to the galactic nucleus, the effective range of its gravitational influence, was calculated byG. A. Chebotarev to be 230,000 AU.[156]
Diagram of theLocal Interstellar Cloud, theG-Cloud and surrounding stars. As of 2022, the precise location of the Solar System in the clouds is an open question in astronomy.[157]
Within 10 light-years of the Sun there are relatively few stars, the closest being the triple star systemAlpha Centauri, which is about 4.4 light-years away and may be in the Local Bubble'sG-Cloud.[158] Alpha Centauri A and B are a closely tied pair ofSun-like stars, whereas the closest star to the Sun, the smallred dwarfProxima Centauri, orbits the pair at a distance of 0.2 light-years. In 2016, a potentially habitableexoplanet was found to be orbiting Proxima Centauri, calledProxima Centauri b, the closest confirmed exoplanet to the Sun.[159]
The Solar System is surrounded by theLocal Interstellar Cloud, although it is not clear if it is embedded in the Local Interstellar Cloud or if it lies just outside the cloud's edge.[160] Multiple otherinterstellar clouds exist in the region within 300 light-years of the Sun, known as theLocal Bubble.[160] The latter feature is an hourglass-shaped cavity orsuperbubble in the interstellar medium roughly 300 light-years across. The bubble is suffused with high-temperature plasma, suggesting that it may be the product of several recent supernovae.[161]
The Local Bubble is a small superbubble compared to the neighboring widerRadcliffe Wave andSplit linear structures (formerlyGould Belt), each of which are some thousands of light-years in length.[162] All these structures are part of theOrion Arm, which contains most of the stars in the Milky Way that are visible to the unaided eye.[163]
Groups of stars form together instar clusters, before dissolving into co-moving associations. A prominent grouping that is visible to the naked eye is theUrsa Major moving group, which is around 80 light-years away within the Local Bubble. The nearest star cluster isHyades, which lies at the edge of the Local Bubble. The closest star-forming regions are theCorona Australis Molecular Cloud, theRho Ophiuchi cloud complex and theTaurus molecular cloud; the latter lies just beyond the Local Bubble and is part of the Radcliffe wave.[164]
Stellar flybys that pass within 0.8 light-years of the Sun occur roughly once every 100,000 years. Theclosest well-measured approach wasScholz's Star, which approached to ~50,000 AU of the Sun some ~70 thousands years ago, likely passing through the outer Oort cloud.[165] There is a 1% chance every billion years that a star will pass within100 AU of the Sun, potentially disrupting the Solar System.[166]
The general motion and orientation of the Sun, with Earth and the moon as its Solar System satellites.
The Sun, taking along the whole Solar System, orbitsthe galaxy's center of mass at an average speed of 230 km/s (828,000 km/h) or 143 mi/s (514,000 mph),[167] taking about 220–250 millionEarth years to complete a revolution (aGalactic year),[168] having done so about 20 times since the Sun's formation.[169][170] The direction of the Sun's motion, theSolar apex, is roughly in the direction of the starVega.[171]
The Sun's idealized orbit around the Galactic Center in an artist's top-down depiction of the current layout of the Milky Way.
The Milky Way is moving with respect to thecosmic microwave background radiation (CMB) in the direction of the constellationHydra with a speed of 550 km/s. Since the Sun is moving with respect to the galactic center in the direction of Cygnus (galactic longitude 90°; latitude 0°) at more than 200km/sec, the resultant velocity with respect to the CMB is about 370 km/s in the direction ofCrater orLeo (galactic latitude 264°, latitude 48°).[172]
In many prehistoric and ancient cultures, the Sun was thought to be asolar deity or othersupernatural entity.[173][174] In the early first millennium BC,Babylonian astronomers observed that the Sun's motion along theecliptic is not uniform, though they did not know why; it is today known that this is due to the movement of Earth in anelliptic orbit, moving faster when it is nearer to the Sun at perihelion and moving slower when it is farther away at aphelion.[175]
One of the first people to offer a scientific or philosophical explanation for the Sun was theGreek philosopherAnaxagoras. He reasoned that it was a giant flaming ball of metal even larger than the land of thePeloponnesus and that the Moon reflected the light of the Sun.[176]Eratosthenes estimated the distance between Earth and the Sun in the third century BC as "of stadiamyriads 400 and 80000", the translation of which is ambiguous, implying either 4,080,000stadia (755,000 km) or 804,000,000 stadia (148 to 153 million kilometers or 0.99 to 1.02 AU); the latter value is correct to within a few percent. In the first century AD,Ptolemy estimated the distance as 1,210 timesthe radius of Earth, approximately 7.71 million kilometers (0.0515 AU).[177]
The theory that the Sun is the center around which the planets orbit was first proposed by the ancient GreekAristarchus of Samos in the third century BC,[178] and later adopted bySeleucus of Seleucia (seeHeliocentrism).[179] This view was developed in a more detailed mathematical model of a heliocentric system in the 16th century byNicolaus Copernicus.[180]
Development of scientific understanding
Sol, the Sun, from a 1550 edition ofGuido Bonatti'sLiber astronomiae
Observations of sunspots were recorded during theHan dynasty (206 BC–AD 220) byChinese astronomers, who maintained records of these observations for centuries.Averroes also provided a description of sunspots in the 12th century.[181] The invention of the telescope in the early 17th century permitted detailed observations of sunspots byThomas Harriot,Galileo Galilei and other astronomers. Galileo posited that sunspots were on the surface of the Sun rather than small objects passing between Earth and the Sun.[182]
Arabic astronomical contributions includeAl-Battani's discovery that the direction of the Sun'sapogee (the place in the Sun's orbit against the fixed stars where it seems to be moving slowest) is changing.[183] In modern heliocentric terms, this is caused by a gradual motion of the aphelion of theEarth's orbit.Ibn Yunus observed more than 10,000 entries for the Sun's position for many years using a largeastrolabe.[184]
The first reasonably accurate distance to the Sun was determined in 1684 byGiovanni Domenico Cassini. Knowing that direct measurements of the solar parallax were difficult, he chose to measure the Martian parallax. Having sentJean Richer toCayenne, part ofFrench Guiana, for simultaneous measurements, Cassini in Paris determined the parallax ofMars when Mars was at its closest to Earth in 1672. Using the circumference distance between the two observations, Cassini calculated the Earth-Mars distance, then usedKepler's laws to determine the Earth-Sun distance. His value, about 10% smaller than modern values, was much larger than all previous estimates.[185]
From an observation of atransit of Venus in 1032, the Persian astronomer and polymathIbn Sina concluded that Venus was closer to Earth than the Sun.[186] In 1677,Edmond Halley observed a transit of Mercury across the Sun, leading him to realize that observations of thesolar parallax of a planet (more ideally using the transit of Venus) could be used totrigonometrically determine the distances between Earth,Venus, and the Sun.[187] Careful observations of the1769 transit of Venus allowed astronomers to calculate the average Earth–Sun distance as 93,726,900 miles (150,838,800 km), only 0.8% greater than the modern value.[188]
Sun as seen in Hydrogen-alpha light
In 1666,Isaac Newton observed the Sun's light using aprism, and showed that it is made up of light of many colors.[189] In 1800,William Herschel discoveredinfrared radiation beyond the red part of the solar spectrum.[190] The 19th century saw advancement in spectroscopic studies of the Sun;Joseph von Fraunhofer recorded more than 600absorption lines in the spectrum, the strongest of which are still often referred to asFraunhofer lines. The 20th century brought about several specialized systems for observing the Sun, especially at different narrowband wavelengths, such as those using Calcium H (396.9 nm), K (393.37 nm) andHydrogen-alpha (656.46 nm)filtering.[191]
During early studies of theoptical spectrum of the photosphere, some absorption lines were found that did not correspond to anychemical elements then known on Earth. In 1868,Norman Lockyer hypothesized that these absorption lines were caused by a new element that he dubbedhelium, after the Greek Sun godHelios. Twenty-five years later, helium was isolated on Earth.[192]
In the early years of the modern scientific era, the source of the Sun's energy was a significant puzzle.Lord Kelvin suggested that the Sun is a gradually cooling liquid body that is radiating an internal store of heat.[193] Kelvin andHermann von Helmholtz then proposed agravitational contraction mechanism to explain the energy output, but the resulting age estimate was only 20 million years, well short of the time span of at least 300 million years suggested by some geological discoveries of that time.[193][194] In 1890, Lockyer proposed a meteoritic hypothesis for the formation and evolution of the Sun.[195]
Not until 1904 was a documented solution offered.Ernest Rutherford suggested that the Sun's output could be maintained by an internal source of heat, and suggestedradioactive decay as the source.[196] However, it would beAlbert Einstein who would provide the essential clue to the source of the Sun's energy output with hismass–energy equivalence relationE =mc2.[197] In 1920, SirArthur Eddington proposed that the pressures and temperatures at the core of the Sun could produce a nuclear fusion reaction that merged hydrogen (protons) into helium nuclei, resulting in a production of energy from the net change in mass.[198] The preponderance of hydrogen in the Sun was confirmed in 1925 byCecilia Payne using the ionization theory developed byMeghnad Saha. The theoretical concept of fusion was developed in the 1930s by the astrophysicistsSubrahmanyan Chandrasekhar andHans Bethe. Hans Bethe calculated the details of the two main energy-producing nuclear reactions that power the Sun.[199][200] In 1957,Margaret Burbidge,Geoffrey Burbidge,William Fowler andFred Hoyle showed that most of the elements in the universe have beensynthesized by nuclear reactions inside stars, some like the Sun.[201]
The first satellites designed for long term observation of the Sun from interplanetary space were NASA'sPioneers 6, 7, 8 and 9, which were launched between 1959 and 1968. These probes orbited the Sun at a distance similar to that of Earth, and made the first detailed measurements of the solar wind and the solar magnetic field.Pioneer 9 operated for a particularly long time, transmitting data until May 1983.[202][203]
In the 1970s, twoHelios spacecraft and the SkylabApollo Telescope Mount provided scientists with significant new data on solar wind and the solar corona. The Helios 1 and 2 probes were U.S.–German collaborations that studied the solar wind from an orbit carrying the spacecraft inside Mercury's orbit at perihelion.[204] The Skylab space station, launched by NASA in 1973, included a solar observatory module called the Apollo Telescope Mount that was operated by astronauts resident on the station.[85] Skylab made the first time-resolved observations of the solar transition region and of ultraviolet emissions from the solar corona.[85] Discoveries included the first observations of coronal mass ejections, then called "coronal transients", and ofcoronal holes, now known to be intimately associated with the solar wind.[204]
In 1980, theSolar Maximum Mission probes were launched by NASA. This spacecraft was designed to observe gamma rays,X-rays andUV radiation from solar flares during a time of high solar activity and solar luminosity. Just a few months after launch, however, an electronics failure caused the probe to go into standby mode, and it spent the next three years in this inactive state. In 1984,Space ShuttleChallenger missionSTS-41C retrieved the satellite and repaired its electronics before re-releasing it into orbit. The Solar Maximum Mission subsequently acquired thousands of images of the solar corona beforere-entering Earth's atmosphere in June 1989.[205]
Launched in 1991, Japan'sYohkoh (Sunbeam) satellite observed solar flares at X-ray wavelengths. Mission data allowed scientists to identify several different types of flares and demonstrated that the corona away from regions of peak activity was much more dynamic and active than had previously been supposed. Yohkoh observed an entire solar cycle but went into standby mode when an annular eclipse in 2001 caused it to lose its lock on the Sun. It was destroyed by atmospheric re-entry in 2005.[206]
TheSolar and Heliospheric Observatory, jointly built by theEuropean Space Agency and NASA, was launched on 2 December 1995.[85] Originally intended to serve a two-year mission,[207] SOHO remains in operation as of 2024.[208] Situated at theLagrangian point between Earth and the Sun (at which the gravitational pull from both is equal), SOHO has provided a constant view of the Sun at many wavelengths since its launch.[85] Besides its direct solar observation, SOHO has enabled the discovery of a large number ofcomets, mostly tinysungrazing comets that incinerate as they pass the Sun.[209]
All these satellites have observed the Sun from the plane of the ecliptic, and so have only observed its equatorial regions in detail. TheUlysses probe was launched in 1990 to study the Sun's polar regions. It first traveled to Jupiter, to "slingshot" into an orbit that would take it far above the plane of the ecliptic. OnceUlysses was in its scheduled orbit, it began observing the solar wind and magnetic field strength at high solar latitudes, finding that the solar wind from high latitudes was moving at about 750 km/s, which was slower than expected, and that there were large magnetic waves emerging from high latitudes that scattered galactic cosmic rays.[210]
Elemental abundances in the photosphere are well known fromspectroscopic studies, but the composition of the interior of the Sun is more poorly understood. A solar wind sample return mission,Genesis, was designed to allow astronomers to directly measure the composition of solar material.[211]
Observation by eyes
Exposure to the eye
The Sun seen from Earth, withglare from the lenses. The eye also sees glare when looked towards the Sun directly.
The brightness of the Sun can cause pain from looking at it with thenaked eye; however, doing so for brief periods is not hazardous for normal non-dilated eyes.[212][213] Looking directly at the Sun (sungazing) causesphosphene visual artifacts and temporary partial blindness. It also delivers about 4 milliwatts of sunlight to the retina, slightly heating it and potentially causing damage in eyes that cannot respond properly to the brightness.[214][215] Viewing of the direct Sun with the naked eye can cause UV-induced, sunburn-like lesions on the retina beginning after about 100 seconds, particularly under conditions where the UV light from the Sun is intense and well focused.[216][217]
Viewing the Sun through light-concentratingoptics such asbinoculars may result in permanent damage to the retina without an appropriate filter that blocks UV and substantially dims the sunlight. When using an attenuating filter to view the Sun, the viewer is cautioned to use a filter specifically designed for that use. Some improvised filters that pass UV orIR rays, can actually harm the eye at high brightness levels.[218] Brief glances at the midday Sun through an unfiltered telescope can cause permanent damage.[219]
During sunrise and sunset, sunlight is attenuated because ofRayleigh scattering andMie scattering from a particularly long passage through Earth's atmosphere,[220] and the Sun is sometimes faint enough to be viewed comfortably with the naked eye or safely with optics (provided there is no risk of bright sunlight suddenly appearing through a break between clouds). Hazy conditions, atmospheric dust, and high humidity contribute to this atmospheric attenuation.[221]
Phenomena
Anoptical phenomenon, known as agreen flash, can sometimes be seen shortly after sunset or before sunrise. The flash is caused by light from the Sun just below the horizon beingbent (usually through atemperature inversion) towards the observer. Light of shorter wavelengths (violet, blue, green) is bent more than that of longer wavelengths (yellow, orange, red) but the violet and blue light isscattered more, leaving light that is perceived as green.[222]
Solar deities play a major role in many world religions and mythologies.[223]Worship of the Sun was central to civilizations such as theancient Egyptians, theInca of South America and theAztecs of what is now Mexico. In religions such asHinduism, the Sun is still considered a god, known asSurya. Many ancient monuments were constructed with solar phenomena in mind; for example, stonemegaliths accurately mark the summer or wintersolstice (for example inNabta Playa, Egypt;Mnajdra, Malta; andStonehenge, England);Newgrange, a prehistoric human-built mount in Ireland, was designed to detect the winter solstice; the pyramid ofEl Castillo atChichén Itzá in Mexico is designed to cast shadows in the shape of serpents climbing thepyramid at the vernal and autumnalequinoxes.[224]
From at least theFourth Dynasty of Ancient Egypt, the Sun was worshipped as thegod Ra, portrayed as a falcon-headed divinity surmounted by the solar disk. In theNew Empire period, the Sun became identified with thedung beetle. In the form of the sun discAten, the Sun had a brief resurgence during theAmarna Period when it again became the preeminent, if not only, divinity for the PharaohAkhenaton.[227][228] The Egyptians portrayed the god Ra as being carried across the sky in a solar barque, accompanied by lesser gods, and to the Greeks, he was Helios, carried by a chariot drawn by fiery horses. From the reign ofElagabalus in thelate Roman Empire the Sun's birthday was a holiday celebrated asSol Invictus (literally "Unconquered Sun") soon after the winter solstice. The Sun appears from Earth to revolve once a year along theecliptic through thezodiac, and so Greek astronomers categorized it as one of the sevenplanets (Greekplanetes, "wanderer"); the naming of thedays of the weeks after the seven planets dates to theRoman era.[229][230][231]
In ancient Roman culture,Sunday was the day of the sun god. In paganism, the Sun was a source of life. It was the center of a popular cult among Romans, who would stand at dawn to catch the first rays of sunshine as they prayed. The celebration of thewinter solstice (which influenced Christmas) was part of the Roman cult of the unconquered Sun (Sol Invictus). It was adopted as theSabbath day by Christians. The symbol of light was a pagan device adopted by Christians, and perhaps the most important one that did not come from Jewish traditions. Christian churches were built so that the congregation faced toward the sunrise.[236] In theBible, theBook of Malachi mentions the "Sun of Righteousness", which someChristians have interpreted as a reference to theMessiah (Christ).[237]
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