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{{otheruses|Sun (disambiguation)|The Sun (disambiguation)}}{{pp-semi-indef}}{{pp-move-indef}}{{Featured article}}{{Short description|Star at the center of the Solar System}}{{Use American English|date=August 2019}}{{Use dmy dates|date=August 2019}}

PITJEVA LAST2=STANDISH DATE=2009 JOURNAL=CELESTIAL MECHANICS AND DYNAMICAL ASTRONOMY VOLUME=103 PAGES=365–372 ISSN=1572-9478 speed of light>light speedApparent magnitude>Visual brightness (V)DATE=1 JULY 2013 URL=HTTP://NSSDC.GSFC.NASA.GOV/PLANETARY/FACTSHEET/SUNFACT.HTML NASA GODDARD SPACE FLIGHT CENTER >ACCESSDATE=12 AUGUST 2013 ARCHIVE-DATE=15 JULY 2010, dead, | label4 = Absolute magnitude| data4 = 4.83Stellar classification>Spectral classificationDATE=1990 PUBLISHER=CAMBRIDGE UNIVERSITY PRESS,weblink | label6 = MetallicityZ = 0.0122ASPLUND LAST2=GREVESSE LAST3=SAUVAL DATE=2006 JOURNAL=COMMUNICATIONS IN ASTEROSEISMOLOGY PAGES=76–79 DOI=10.1553/CIA147S76, | label7 = Angular sizeminutes of arcECLIPSE 99: FREQUENTLY ASKED QUESTIONS PUBLISHER=NASA URL-STATUS=DEAD ARCHIVEDATE=27 MAY 2010, | label8 = Adjectives| data8 = Solar| header10 = Orbital characteristics| label11 = Mean distancefrom Milky Way core2.7u=km}}{{nowrapfmt=commasul=light-years}}}}Galactic year>Galactic period8}} yr| label13 = Velocity22020370DISPLAY-AUTHORS=ETAL TITLE=FIVE-YEAR WILKINSON MICROWAVE ANISOTROPY PROBE OBSERVATIONS: DATA PROCESSING, SKY MAPS, AND BASIC RESULTS THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES >VOLUME=180 PAGES=225–245 BIBCODE=2009APJS..180..225H Cosmic microwave background radiation#CMBR dipole anisotropy>cosmic microwave background)| header20 = Physical characteristics| label21 = Equatorial radiusSolar radius>695,700 km,E.E. >LAST1=MAMAJEK LAST2=PRSA LAST3=TORRES LAST4=ET EPRINT=1510.07674 CLASS=ASTRO-PH.SR first1=Marcelo first2=Jeff R. first3=Rock I. first4=Isabelle F. title=Measuring the Solar Radius from Space during the 2003 and 2006 Mercury Transits bibcode=2012ApJ...750..135E volume=750 journal=The Astrophysical Journal year=2012}}{{valu=× Earth}}| label22 = Equatorial circumference4.379u=km}}109 × EarthSOLAR SYSTEM EXPLORATION: PLANETS: SUN: FACTS & FIGURES >URL=HTTP://SOLARSYSTEM.NASA.GOV/PLANETS/PROFILE.CFM?OBJECT=SUN&DISPLAY=FACTS&SYSTEM=METRIC ARCHIVEDATE=2 JANUARY 2008, NASA, | label23 = Flattening9|e=-6}}| label24 = Surface area6.09u=km2}}{{nowrapfmt=commas|12000}}}} × Earth| label25 = Volume1.41u=km3}}{{nowrapfmt=commas|1300000}}}} × Earth| label26 = Mass1.9885e=30{{val333000Mass of Earth>× Earth}}}}| label27 = Average density1.408DATE=1999 URL=HTTP://HYPERTEXTBOOK.COM/FACTS/1999/MAYKO.SHTML WEBSITE=THE PHYSICS FACTBOOK, {{val|0.255}} × Earth| label28 = Center density (modeled)162.212.4}} × Earth| label29 = Equatorial surface gravity274|u=m/s2}}28 × Earth| label30 = Moment of inertia factor0.070}} (estimate)| label31 = Escape velocity(from the surface)617.7|u=km/s}}55 × Earth| label32 = Temperature1.57u=K}}Photosphere (effective): {{nowrapfmt=commasul=K}}}} Corona: ≈ {{vale=6|u=K}}| label33 = Luminosity (Lsol)3.828ul=W}}≈ {{vale=28lumen (unit)>lm}}≈ {{valu=lm/W}} efficacyColor index>Color (B-V)| data34 = 0.63| label35 = Mean radiance (Isol)2.009u=W·m−2·sr−1}}| label36 = AgeLAST2=SCHLATTL LAST3=PATERNò YEAR=2002 JOURNAL=ASTRONOMY AND ASTROPHYSICS ISSUE=3 ARXIV=ASTRO-PH/0204331 DOI=10.1051/0004-6361:20020749 DOI=10.1126/SCIENCE.1226919 VOLUME=338 PAGES=651–655 PMID=23118187 FIRST1=JN FIRST2=M FIRST3=AN FIRST4=Ã… FIRST5=D FIRST6=MA, {{Registration required}}| header40 = Rotation characteristicsAxial tilt>Obliquity| data41 = 7.25°(to the ecliptic)67.23°(to the galactic plane)Right ascensionof North poleSEIDELMANN DISPLAY-AUTHORS=ETAL URL=HTTP://WWW.HNSKY.ORG/IAU-IAG.HTM ACCESSDATE=22 MARCH 2006, 19 h 4 min 30 s}}| label43 = Declinationof North pole| data43 = +63.87°63° 52' NorthSolar rotation>rotation period (at equator)| data44 = 25.05 d| label45 = (at 16° latitude)25 d 9 h 7 min 12 s}}| label46 = (at poles)| data46 = 34.4 d| label47 = Rotation velocity(at equator)7.189u=km/h}}photosphere>Photospheric composition (by mass)| label51 = HydrogenPUBLISHER=STANFORD SOLAR CENTER DATE=1979 URL=HTTPS://HISTORY.NASA.GOV/SP-402/CONTENTS.HTM PUBLISHER=NASA, NASA SP-402, | label52 = Helium| data52 = 24.85%| label53 = Oxygen| data53 = 0.77%| label54 = Carbon| data54 = 0.29%| label55 = Iron| data55 = 0.16%| label56 = Neon| data56 = 0.12%| label57 = Nitrogen| data57 = 0.09%| label58 = Silicon| data58 = 0.07%| label59 = Magnesium| data59 = 0.05%| label60 = Sulphur| data60 = 0.04%}}The Sun, or Solweblink is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma,NEWS,weblink How Round is the Sun?, NASA, 2 October 2008, 7 March 2011, NEWS,weblink First Ever STEREO Images of the Entire Sun, NASA, 6 February 2011, 7 March 2011, with internal convective motion that generates a magnetic field via a dynamo process.JOURNAL, 10.1146/annurev-astro-081913-040012, Solar Dynamo Theory, Annual Review of Astronomy and Astrophysics, 52, 251–290, 2014, Charbonneau, P., 2014ARA&A..52..251C, It is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometers (864,000 miles), or 109 times that of Earth, and its mass is about 330,000 times that of Earth. It accounts for about 99.86% of the total mass of the Solar System.JOURNAL, Woolfson, M., 2000, The origin and evolution of the solar system, Astronomy & Geophysics, 41, 1, 12, 2000A&G....41a..12W, 10.1046/j.1468-4004.2000.00012.x, harv, 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.JOURNAL, Basu, S., Antia, H.M., 2008, Helioseismology and Solar Abundances, Physics Reports, 457, 5–6, 217–283, 0711.4590, 2008PhR...457..217B, 10.1016/j.physrep.2007.12.002, harv, The Sun is a G-type main-sequence star (G2V) based on its spectral class. As such, it is informally and not completely accurately referred to as a yellow dwarf (its light is closer to white than yellow). It formed approximately 4.6 billionAll numbers in this article are short scale. One billion is 109, or 1,000,000,000.JOURNAL, The Absolute Chronology and Thermal Processing of Solids in the Solar Protoplanetary Disk, Science (journal), Science, James N., Connelly, Martin, Bizzarro, Alexander N., Krot, Ã…ke, Nordlund, Daniel, Wielandt, Marina A., Ivanova, 338, 6107, 651–655, 2 November 2012, 10.1126/science.1226919, 2012Sci...338..651C, 23118187, years ago from 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. It is thought that almost all stars form by this process.The Sun currently fuses about 600 million tons of hydrogen into helium every second, converting 4 million tons of matter into energy every second as a result. This energy, which can take between 10,000 and 170,000 years to escape from its core, is the source of the Sun's light and heat. When hydrogen fusion in its core has diminished to the point at which the Sun is no longer in hydrostatic equilibrium, its core will undergo a marked increase in density and temperature while its outer layers expand, eventually transforming the Sun into a red giant. It is calculated that the Sun will become sufficiently large to engulf the current orbits of Mercury and Venus, and render Earth uninhabitable – but not for about five billion years. After this, it will shed its outer layers and become a dense type of cooling star known as a white dwarf, and no longer produce energy by fusion, but still glow and give off heat from its previous fusion.The enormous effect of the Sun on Earth has been recognized since prehistoric times, and the Sun has been regarded by some cultures as a deity. The synodic rotation of Earth and its orbit around the Sun are the basis of solar calendars, one of which is the predominant calendar in use today.

Name and etymology

The English proper name Sun developed from Old English sunne and may be related to south. Cognates to English sun appear in other Germanic languages, including Old Frisian sunne, sonne, Old Saxon sunna, Middle Dutch sonne, modern Dutch zon, Old High German sunna, modern German Sonne, Old Norse sunna, and Gothic sunnō. All Germanic terms for the Sun stem from Proto-Germanic *sunnōn.BOOK, Barnhart, R.K., 1995, The Barnhart Concise Dictionary of Etymology, 776, HarperCollins, 978-0-06-270084-1, BOOK, Mallory, J.P., 1989, In Search of the Indo-Europeans: Language, Archaeology and Myth, 129, Thames & Hudson, 978-0-500-27616-7, The Latin name for the Sun, Sol, is not commonly used in everyday English. Sol is also used by planetary astronomers to refer to the duration of a solar day on another planet, such as Mars.WEB, 15 November 2006, Opportunity's View, Sol 959 (Vertical),weblink NASA, 1 August 2007, The related word solar is the usual adjectival term used,BOOK, Little, William, Fowler, H.W., Coulson, J., Sol, Oxford Universal Dictionary on Historical Principles, 3rd, B000QS3QVQ, 1955, DICTIONARY, Sol,weblink Merriam-Webster, 19 July 2009, in terms such as solar day, solar eclipse, and Solar System.The English weekday name Sunday stems from Old English (Sunnandæg; "Sun's day", from before 700) and is ultimately a result of a Germanic interpretation of Latin dies solis, itself a translation of the Greek ἡμέρα ἡλίου (hēméra hēlíou).BOOK, Barnhart, R.K., 1995, The Barnhart Concise Dictionary of Etymology, 778, HarperCollins, 978-0-06-270084-1,

General characteristics

The Sun is a G-type main-sequence star that comprises about 99.86% of the mass of the Solar System. The Sun 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.NEWS, Than, K., 2006, Astronomers Had it Wrong: Most Stars are Single,,weblink 1 August 2007, JOURNAL, Lada, C.J., 2006, Stellar multiplicity and the initial mass function: Most stars are single, Astrophysical Journal Letters, 640, 1, L63–L66, astro-ph/0601375, 2006ApJ...640L..63L, 10.1086/503158, harv, The Sun is a Population I, or heavy-element-rich,{{efn|name=heavy elements}} star.BOOK, Zeilik, M.A., Gregory, S.A., 1998, Introductory Astronomy & Astrophysics, 4th, 322, Saunders College Publishing, 978-0-03-006228-5, The formation of the Sun may have been triggered by shockwaves from one or more nearby supernovae.JOURNAL, Falk, S.W., Lattmer, J.M., Margolis, S.H., 1977, Are supernovae sources of presolar grains?, Nature (journal), Nature, 270, 5639, 700–701, 1977Natur.270..700F, 10.1038/270700a0, harv, This 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 is by far the brightest object in the Earth's sky, with an apparent magnitude of −26.74.JOURNAL, Burton, W.B., 1986, Stellar parameters, Space Science Reviews, 43, 3–4, 244–250, 10.1007/BF00190626, harv, JOURNAL, Bessell, M.S., Castelli, F., Plez, B., 1998, Model atmospheres broad-band colors, bolometric corrections and temperature calibrations for O–M stars, Astronomy and Astrophysics, 333, 231–250, 1998A&A...333..231B, harv, This is about 13 billion times brighter than the next brightest star, Sirius, which has an apparent magnitude of −1.46. The mean distance of the Sun's center to Earth's center is approximately {{convert|1|AU|km mi|lk=in|disp=x| (about |)}}, though the distance varies as Earth moves from perihelion in January to aphelion in July.WEB, 31 January 2008, Equinoxes, Solstices, Perihelion, and Aphelion, 2000–2020,weblink US Naval Observatory, 17 July 2009, At this average distance, light travels from the Sun's horizon to Earth's horizon in about 8 minutes and 19 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 lifeHydrothermal vent communities live so deep under the sea that they have no access to sunlight. Bacteria instead use sulfur compounds as an energy source, via chemosynthesis. on Earth by photosynthesis,BOOK, Simon, A., The Real Science Behind the X-Files : Microbes, meteorites, and mutants,weblink 25–27, Simon & Schuster, 2001, 978-0-684-85618-6, and drives Earth's climate and weather.The Sun does not have a definite boundary, but its density decreases exponentially with increasing height above the photosphere.BOOK, Beer, J., McCracken, K., von Steiger, R., 2012, Cosmogenic Radionuclides: Theory and Applications in the Terrestrial and Space Environments, 41, Springer Science+Business Media, 978-3-642-14651-0, For the purpose of measurement, however, 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.BOOK, Phillips, K.J.H., 1995, Guide to the Sun, 73, Cambridge University Press, 978-0-521-39788-9, By this measure, the Sun is a near-perfect sphere with an oblateness estimated at about 9 millionths,JOURNAL, Godier, S., Rozelot, J.-P., 2000, The solar oblateness and its relationship with the structure of the tachocline and of the Sun's subsurface,weblink Astronomy and Astrophysics, 355, 365–374, 2000A&A...355..365G, harv, which means that its polar diameter differs from its equatorial diameter by only {{convert|10|km|mi}}.WEB, Jones, G., 16 August 2012, Sun is the most perfect sphere ever observed in nature,weblink The Guardian, 19 August 2013, The tidal effect of the planets is weak and does not significantly affect the shape of the Sun.BOOK, Schutz, B.F., 2003, Gravity from the ground up, 98–99, Cambridge University Press, 978-0-521-45506-0, 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 the poles. Viewed from Earth as it orbits the Sun, the apparent rotational period of the Sun at its equator is about 28 days.BOOK, Phillips, K.J.H., 1995, Guide to the Sun, 78–79, Cambridge University Press, 978-0-521-39788-9,


File:Sun (Earth POV).jpg|thumb|The Sun as seen from the Earth's point of view.]]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 {{val|1368|u=W/m2|fmt=commas}} (watts per square meter) at a distance of one astronomical unit (AU) from the Sun (that is, on or near Earth).WEB, Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present,weblink 5 October 2005, dead,weblink" title="">weblink 1 August 2011, Sunlight on the surface of Earth is attenuated by Earth's atmosphere, so that less power arrives at the surface (closer to {{val|1000|u=W/m2|fmt=commas}}) in clear conditions when the Sun is near the zenith.BOOK, El-Sharkawi, Mohamed A., Electric energy, 2005, CRC Press, 978-0-8493-3078-0, 87–88, 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.WEB,weblink Solar radiation, The atmosphere in particular filters out over 70% of solar ultraviolet, especially at the shorter wavelengths.WEB,weblink Reference Solar Spectral Irradiance: Air Mass 1.5, 12 November 2009, Solar ultraviolet radiation ionizes Earth's dayside upper atmosphere, creating the electrically conducting ionosphere.BOOK, Phillips, K.J.H., 1995, Guide to the Sun, 14–15, 34–38, Cambridge University Press, 978-0-521-39788-9, The Sun's 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. When measuring all the photons emitted, the Sun is actually emitting more photons in the green portion of the spectrum than any other.NEWS,weblink What Color is the Sun?, Universe Today, 23 May 2016, WEB,weblink What Color is the Sun?, Stanford University, Stanford Solar Center, 23 May 2016, When the Sun is low in the sky, atmospheric scattering renders the Sun yellow, red, orange, or magenta. Despite its typical whiteness, most people mentally picture the Sun as yellow; the reasons for this are the subject of debate.JOURNAL, Wilk, S.R., 2009, The Yellow Sun Paradox,weblinkweblink" title="">weblink dead, 18 June 2012, Optics & Photonics News, 12–13, harv, The Sun is a G2V star, with G2 indicating its surface temperature of approximately 5,778 K (5,505 Â°C, 9,941 Â°F), and V that it, like most stars, is a main-sequence star.BOOK, Phillips, K.J.H., 1995, Guide to the Sun, 47–53, Cambridge University Press, 978-0-521-39788-9, NEWS, Dr Karl's Great Moments In Science: Lazy Sun is less energetic than compost,weblink 25 February 2014, Australian Broadcasting Corporation, 17 April 2012, Karl S. Kruszelnicki, Every second, the Sun burns 620 million tonnes of hydrogen..., The average luminance of the Sun is about 1.88 giga candela per square metre, but as viewed through Earth's atmosphere, this is lowered to about 1.44 Gcd/m2.{{efn|1=1.88 Gcd/m2 is calculated from the solar illuminance of {{val|128000|u=lux}} (see sunlight) times the square of the distance to the center of the Sun, divided by the cross sectional area of the Sun. 1.44 Gcd/m2 is calculated using {{val|98000|u=lux}}.}} However, the luminance is not constant across the disk of the Sun (limb darkening).


File:BLUE STEREO 3D Time for Space Wiggle.gif|thumb|False-color wiggle animation of the Sun by STEREOSTEREO(File:Gamma_sun.jpg|thumb|Normally the Sun does not make gamma rays, but a flare on June 15, 1991 caused this Gamma-ray observation by the COMPTEL instrument on the Compton Gamma Ray Observatory. Neutrons from the Sun collided with the intrastellar medium to produce gamma-rays.)(File:Skylab Solar flare.jpg|thumb|1973 Solar flare as recorded by Skylab){{see also|Molecules in stars}}The Sun is composed primarily of the chemical elements hydrogen and helium. At this time in the Sun's life, they account for 74.9% and 23.8% of the mass of the Sun in the photosphere, respectively.JOURNAL, 10.1086/375492, Lodders, Katharina, 10 July 2003, Solar System Abundances and Condensation Temperatures of the Elements, The Astrophysical Journal, 591, 2, 1220–1247,weblink 2003ApJ...591.1220L, harv, 1 September 2015,weblink" title="">weblink 7 November 2015, dead, JOURNAL, Lodders, K., Abundances and Condensation Temperatures of the Elements,weblink Meteoritics & Planetary Science, 38, suppl, 5272, 2003, 2003M&PSA..38.5272L, harv,
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.
BOOK, Hansen, C.J., Kawaler, S.A., Trimble, V., Stellar Interiors: Physical Principles, Structure, and Evolution, 19–20, 2nd, Springer Science+Business Media, Springer, 2004, 978-0-387-20089-7, The Sun's original chemical composition was inherited from the interstellar medium out of which it formed. Originally it would have contained about 71.1% hydrogen, 27.4% helium, and 1.5% heavier elements. The hydrogen and most of 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.BOOK, Hansen, C.J., Kawaler, S.A., Trimble, V., Stellar Interiors: Physical Principles, Structure, and Evolution, 77–78, 2nd, Springer Science+Business Media, Springer, 2004, 978-0-387-20089-7, 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. Within the core, the proportion of helium has increased from about 24% to about 60% due to fusion, and some of the helium and heavy elements have settled from the photosphere towards the center of the Sun because of gravity. The proportions of metals (heavier elements) is unchanged. Heat is transferred outward from 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 coreBOOK, Hansen, C.J., Kawaler, S.A., Trimble, V., Stellar Interiors: Physical Principles, Structure, and Evolution, § 9.2.3, yes, 2nd, Springer Science+Business Media, Springer, 2004, 978-0-387-20089-7, 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 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.Iben, I Jnr (1965) "Stellar Evolution. II. The Evolution of a 3 M_{sun} Star from the Main Sequence Through Core Helium Burning". (Astrophysical Journal, vol. 142, p. 1447)The chemical composition of the photosphere is normally considered representative of the composition of the primordial Solar System.JOURNAL, Aller, L.H., The chemical composition of the Sun and the solar system, Proceedings of the Astronomical Society of Australia, 1, 4, 133, 1968, 1968PASAu...1..133A, harv, 10.1017/S1323358000011048, The solar heavy-element abundances described above are typically measured both 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 settling of heavy elements. The two methods generally agree well.

Singly ionized iron-group elements

In the 1970s, much research focused on the abundances of iron-group elements in the Sun.JOURNAL, Biemont, E., 1978, Abundances of singly ionized elements of the iron group in the Sun, Monthly Notices of the Royal Astronomical Society, 184, 4, 683–694, 1978MNRAS.184..683B, harv, 10.1093/mnras/184.4.683, Ross and Aller 1976, Withbroe 1976, Hauge and Engvold 1977, cited in Biemont 1978. Although significant research was done, until 1978 it was difficult to determine the abundances of some iron-group elements (e.g. cobalt and manganese) via spectrography because of their hyperfine structures.The first largely complete set of oscillator strengths of singly ionized iron-group elements were made available in the 1960s,Corliss and Bozman (1962 cited in Biemont 1978) and Warner (1967 cited in Biemont 1978) and these were subsequently improved.Smith (1976 cited in Biemont 1978) In 1978, the abundances of singly ionized elements of the iron group were derived.

Isotopic composition

Various authors have considered the existence of a gradient in the isotopic compositions of solar and planetary noble gases,Signer and Suess 1963; Manuel 1967; Marti 1969; Kuroda and Manuel 1970; Srinivasan and Manuel 1971, all cited in Manuel and Hwaung 1983 e.g. correlations between isotopic compositions of neon and xenon in the Sun and on the planets.Kuroda and Manuel 1970 cited in Manuel and Hwaung 1983:7Prior to 1983, it was thought that the whole Sun has the same composition as the solar atmosphere.JOURNAL, Manuel, O.K., Hwaung, G., 1983, Solar abundances of the elements, Meteoritics (journal), Meteoritics, 18, 3, 209–222, 1983Metic..18..209M, 10.1111/j.1945-5100.1983.tb00822.x, harv, In 1983, it was claimed that it was fractionation in the Sun itself that caused the isotopic-composition relationship between the planetary and solar-wind-implanted noble gases.

Structure and fusion

(File:Sun poster.svg|thumb|upright=2.25|The structure of the Sun)The structure of the Sun contains the following layers:
*Core – the innermost 20–25% of the Sun's radius, where temperature (energies) and pressure are sufficient for nuclear fusion to occur. Hydrogen fuses into helium (which cannot currently be fused at this point in the Sun's life). The fusion process releases energy, and the helium gradually accumulates to form an inner core of helium within the core itself. * Radiative zone – Convection cannot occur until much nearer the surface of the Sun. Therefore, between about 20–25% of the radius, and 70% of the radius, there is a "radiative zone" in which energy transfer occurs by means of radiation (photons) rather than by convection. * Tachocline – the boundary region between the radiative and convective zones. * Convective zone – Between about 70% of the Sun's radius and a point close to the visible surface, the Sun is cool and diffuse enough for convection to occur, and this becomes the primary means of outward heat transfer, similar to weather cells which form in the earth's atmosphere. * Photosphere – the deepest part of the Sun which we can directly observe with visible light. Because the Sun is a gaseous object, it does not have a clearly defined surface; its visible parts are usually divided into a 'photosphere' and 'atmosphere'. * Atmosphere – a gaseous 'halo' surrounding the Sun, comprising the chromosphere, solar transition region, corona and heliosphere. These can be seen when the main part of the Sun is hidden, for example, during a solar eclipse.


The core of the Sun extends from the center to about 20–25% of the solar radius.JOURNAL, García, R., 2007, Tracking solar gravity modes: the dynamics of the solar core, Science (journal), Science, 316, 5831, 1591–1593, 2007Sci...316.1591G, 10.1126/science.1140598, 17478682, harv, etal, It has a density of up to {{val|150|u=g|up=cm3}}JOURNAL, Basu, S., etal, 2009, Fresh insights on the structure of the solar core, The Astrophysical Journal, 699, 2, 1403–1417, 0905.0651, 2009ApJ...699.1403B, 10.1088/0004-637X/699/2/1403, WEB, 18 January 2007, NASA/Marshall Solar Physics,weblink Marshall Space Flight Center, 11 July 2009, (about 150 times the density of water) and a temperature of close to 15.7 million kelvins (K). By contrast, the Sun's surface temperature is approximately 5,800 K. Recent analysis of SOHO mission data favors a faster rotation rate in the core than in the radiative zone above. Through most of the Sun's life, energy has been produced by nuclear fusion in the core region through a series of nuclear reactions called the p–p (proton–proton) chain; this process converts hydrogen into helium.CONFERENCE, XXIII Physics in Collisions Conference, Zeuthen, Germany, Broggini, C., 2003, Physics in Collision, Proceedings of the XXIII International Conference: Nuclear Processes at Solar Energy,weblink 21, astro-ph/0308537, 2003phco.conf...21B, harv, Only 0.8% of the energy generated in the Sun comes from another sequence of fusion reactions called the CNO cycle, though this proportion is expected to increase as the Sun becomes older.JOURNAL, Goupil, M.J., Lebreton, Y., Marques, J.P., Samadi, R., Baudin, F., 2011, Open issues in probing interiors of solar-like oscillating main sequence stars 1. From the Sun to nearly suns, (Journal of Physics: Conference Series), 271, 1, 012031, 1102.0247, 2011JPhCS.271a2031G, 10.1088/1742-6596/271/1/012031, The core is the only region in the Sun that produces an appreciable amount of thermal energy through fusion; 99% of the power is generated within 24% of the Sun's radius, and by 30% of the radius, fusion has stopped nearly entirely. The remainder of the Sun is heated by this energy as it is transferred outwards through many successive layers, finally to the solar photosphere where it escapes into space through radiation (photons) or advection (massive particles).BOOK, Zirker, J.B., 2002, Journey from the Center of the Sun, 15–34, Princeton University Press, 978-0-691-05781-1, The proton–proton chain occurs around {{val|9.2|e=37}} times each second in the core, converting about 3.7{{e|38}} protons into alpha particles (helium nuclei) every second (out of a total of ~8.9{{e|56}} free protons in the Sun), or about 6.2{{e|11}} kg/s. Fusing four free protons (hydrogen nuclei) into a single alpha particle (helium nucleus) releases around 0.7% of the fused mass as energy,BOOK, Shu, F.H., 1982, The Physical Universe: An Introduction to Astronomy, 102, University Science Books, 978-0-935702-05-7,weblink so the Sun releases energy at the mass–energy conversion rate of 4.26 million metric tons per second (which requires 600 metric megatons of hydrogen WEB, Ask Us: Sun,weblink Cosmicopia, NASA, 2012, 13 July 2017, ), for 384.6 yottawatts ({{val|3.846|e=26|u=W}}), or 9.192{{e|10}} megatons of TNT per second. However, 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 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,
WEB, Cohen, H., 9 November 1998, Table of temperatures, power densities, luminosities by radius in the Sun,weblink Contemporary Physics Education Project, 30 August 2011,weblinkfusedweb%2Ellnl%2Egov/cpep/chart_pages/5%2Eplasmas/sunlayers%2Ehtml, 29 November 2001, which is about the same rate of power production as takes place in reptile metabolism or a compost pile.WEB,, Lazy Sun is less energetic than compost, 17 April 2012, {{efn|name=power production density}}
The fusion rate in the core is in a self-correcting equilibrium: a slightly higher rate of fusion would cause the core to heat up more and expand slightly against the weight of the outer layers, reducing the density and hence the fusion rate and correcting the perturbation; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the density and increasing the fusion rate and again reverting it to its present rate.JOURNAL, Haubold, H.J., Mathai, A.M., 1994, Solar Nuclear Energy Generation & The Chlorine Solar Neutrino Experiment, 320, 1994, 102–116, AIP Conference Proceedings, astro-ph/9405040, 1995AIPC..320..102H, 10.1063/1.47009, harv,, WEB, Myers, S.T., 18 February 1999, Lecture 11 – Stellar Structure I: Hydrostatic Equilibrium, Introduction to Astrophysics II, 15 July 2009,weblink

Radiative zone

From the core out to about 0.7 solar radii, thermal radiation is the primary means of energy transfer.WEB,weblink NASA, Sun, World Book at NASA, 10 October 2012,weblink" title="">weblink 10 May 2013, 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 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 g/cm3 to 0.2 g/cm3) from 0.25 solar radii to the 0.7 radii, the top of the radiative zone.


The radiative zone and the convective zone are separated by 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.BOOK, Tobias, S.M., 2005, The solar tachocline: Formation, stability and its role in the solar dynamo,weblink 193–235, A.M. Soward, etal, Fluid Dynamics and Dynamos in Astrophysics and Geophysics, CRC Press, 978-0-8493-3355-2, Presently, it is hypothesized (see Solar dynamo) that a magnetic dynamo within this layer generates the Sun's magnetic field.

Convective zone

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 enough 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 the majority 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 to 5,700 K and the density to only 0.2 g/m3 (about 1/6,000 the density of air at sea level).The thermal columns of the convection zone form an imprint on the surface of the Sun giving it 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 hexagonal prisms.BOOK, Mullan, D.J, 2000, Solar Physics: From the Deep Interior to the Hot Corona,weblink 22, Page, D., Hirsch, J.G., From the Sun to the Great Attractor, Springer Science+Business Media, Springer, 978-3-540-41064-5,


File:EffectiveTemperature 300dpi e.png|thumb|The effective temperature, or black bodyblack bodyThe visible surface of the Sun, the photosphere, 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 we see is produced as electrons react with hydrogen atoms to produce H− ions.BOOK, Gibson, Edward G., 1973, The Quiet Sun (NASA SP-303), NASA, B0006C7RS0, BOOK, Shu, F.H., The Physics of Astrophysics, 1, University Science Books, 1991, 978-0-935702-64-4, 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 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 5,777 K, interspersed with atomic absorption 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 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.JOURNAL, Rast, M., Nordlund, Ã…., Stein, R., Toomre, J., 1993, Ionization Effects in Three-Dimensional Solar Granulation Simulations, The Astrophysical Journal Letters, 408, 1, L53–L56, 1993ApJ...408L..53R, 10.1086/186829, During early studies of the optical spectrum of the photosphere, some absorption lines were found that did not correspond to any chemical elements then known on Earth. In 1868, Norman Lockyer hypothesized that these absorption lines were caused by a new element that he dubbed helium, after the Greek Sun god Helios. Twenty-five years later, helium was isolated on Earth.WEB, Parnel, C., Discovery of Helium,weblink University of St Andrews, 22 March 2006,


{{See also|Corona|Coronal loop}}File:2017 Total Solar Eclipse (35909952653).jpg|thumb|right|During a total solar eclipse, the solar coronacoronaDuring a total solar eclipse, when the disk of the Sun is covered by that of the Moon, parts of the Sun's surrounding atmosphere can be seen. It is composed of four distinct parts: the chromosphere, the transition region, the corona and the heliosphere.The coolest layer of the Sun is a temperature minimum region extending to about {{val|500|u=km}} above the photosphere, and has a temperature of about {{val|4100|ul=K|fmt=commas}}.JOURNAL, Abhyankar, K.D., 1977, A Survey of the Solar Atmospheric Models,weblink Bulletin of the Astronomical Society of India, 5, 40–44, 1977BASI....5...40A, harv, This part of the Sun is cool enough to allow the existence of simple molecules such as carbon monoxide and water, which can be detected via their absorption spectra.JOURNAL, Solanki, S.K., Livingston, W., Ayres, T., 1994, New Light on the Heart of Darkness of the Solar Chromosphere, Science (journal), Science, 17748350, 263, 5143, 64–66, 1994Sci...263...64S, 10.1126/science.263.5143.64, harv, The chromosphere, transition region, and corona are much hotter than the surface of the Sun. The reason is not well understood, but evidence suggests that Alfvén waves may have enough energy to heat the corona.JOURNAL, De Pontieu, B., 2007, Chromospheric Alfvénic Waves Strong Enough to Power the Solar Wind, Science (journal), Science, 318, 5856, 1574–1577, 2007Sci...318.1574D, 10.1126/science.1151747, 18063784, harv, etal, Above the temperature minimum layer is a layer about {{val|2000|u=km|fmt=commas}} 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 the beginning and end of total solar eclipses. The temperature of the chromosphere increases gradually with altitude, ranging up to around {{val|20000|u=K|fmt=commas}} near the top. In the upper part of the chromosphere helium becomes partially ionized.JOURNAL, Hansteen, V.H., Leer, E., Holzer, T.E., 1997, The role of helium in the outer solar atmosphere, The Astrophysical Journal, 482, 1, 498–509, 1997ApJ...482..498H, 10.1086/304111, harv, File:171879main LimbFlareJan12 lg.jpg|thumb|left|upright=1.6|Taken by Hinode's Solar Optical Telescope on 12 January 2007, this image of the Sun reveals the filamentary nature of the plasma connecting regions of different magnetic polarity.]]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 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. Rather, it 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 is readily observable from space by instruments sensitive to the extreme ultraviolet portion of the spectrum.JOURNAL, Dwivedi, B.N., 2006, Our ultraviolet Sun,weblink Current Science, 91, 5, 587–595, harv, 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.{{efn|name=particle density}} 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 the temperature of the corona, at least some of its heat is known to be from magnetic reconnection.BOOK, Russell, C.T., 2001, Solar wind and interplanetary magnetic filed: A tutorial,weblink 73–88, Song, Paul, Singer, Howard J., George Siscoe, Siscoe, George L., Space Weather (Geophysical Monograph), American Geophysical Union, 978-0-87590-984-4, 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 the solar wind plasma. This outermost layer of the Sun 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,BOOK, Emslie, A.G, Miller, J.A., 2003, Particle Acceleration,weblink Dwivedi, B.N., Dynamic Sun, 275, Cambridge University Press, 978-0-521-81057-9, 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,WEB, 22 April 2003, A Star with two North Poles,weblink Science @ NASA, NASA, dead,weblink 18 July 2009, JOURNAL, Riley, P., Linker, J.A., Mikić, Z., 2002, Modeling the heliospheric current sheet: Solar cycle variations,weblink Journal of Geophysical Research, 107, A7, SSH 8–1, 2002JGRA..107.1136R, 10.1029/2001JA000299, CiteID 1136, dead,weblink" title="">weblink 14 August 2009, 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.PRESS, 2005, The Distortion of the Heliosphere: Our Interstellar Magnetic Compass,weblink European Space Agency, 22 March 2006, 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 the probe had passed through the heliopause and entered the interstellar medium.BOOK,weblink The Cosmic Compendium: Interstellar Travel, 163–164, Anderson, Rupert W., 2015, 978-1-329-02202-7,

Photons and neutrinos

{{see also|Solar irradiance}}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 a few millimeters. Re-emission happens in a random direction and usually at a 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.WEB, 2007, Ancient sunlight,weblink Technology Through Time, NASA, 50, 24 June 2009, harv, In contrast, it takes only 2.3 seconds for the neutrinos, 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 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.JOURNAL, Stix, M., 2003, On the time scale of energy transport in the sun, Solar Physics (journal), Solar Physics, 212, 1, 3–6, 2003SoPh..212....3S, 10.1023/A:1022952621810, Neutrinos are also released by the fusion reactions in the core, but, unlike photons, they rarely interact with matter, so almost all are able to escape the Sun immediately. For many years measurements of the number of neutrinos produced in the Sun were lower than theories predicted by a factor of 3. This discrepancy was resolved in 2001 through the discovery of the effects of neutrino oscillation: the Sun emits the number of neutrinos predicted by the theory, but neutrino detectors were missing {{frac|2|3}} of them because the neutrinos had changed flavor by the time they were detected.JOURNAL, Schlattl, H., 2001, Three-flavor oscillation solutions for the solar neutrino problem, Physical Review D, 64, 1, 013009, hep-ph/0102063, 2001PhRvD..64a3009S, 10.1103/PhysRevD.64.013009, harv,

Magnetic activity

Magnetic field

{{See also|Stellar magnetic field|Sunspots|List of solar cycles|Solar phenomena}}(File:172197main NASA Flare Gband lg-withouttext.jpg|thumb|Visible light photograph of sunspot, 13 December 2006){{Multiple image| direction = vertical| width = 350| image1 = Sunspot butterfly diagram.svg| image2 = Sunspot area variation.svgSolar cycle#Sunspots>Butterfly diagram showing paired sunspot pattern. Graph is of sunspot area.}}File:Sun - August 1, 2010.jpg|thumb|In this false-color ultraviolet image, the Sun shows a C3-class solar flare (white area on upper left), a solar tsunami (wave-like structure, upper right) and multiple filaments of plasma following a magnetic field, rising from the stellar surface.]]File:Heliospheric-current-sheet.gif|thumb|right|The heliospheric current sheet extends to the outer reaches of the Solar System, and results from the influence of the Sun's rotating magnetic field on the plasma in the interplanetary mediuminterplanetary mediumThe Sun has a magnetic field that varies across the surface of the Sun. Its polar field is {{convert|1|-|2|G|sigfig=1|lk=on}}, whereas the field is typically {{convert|3000|G|sigfig=1}} in features on the Sun called sunspots and {{convert|10|-|100|G|sigfig=1}} in solar prominences.The magnetic field also varies in time and location. The quasi-periodic 11-year solar cycle is the most prominent variation in which the number and size of sunspots waxes and wanes.BOOK, Zirker, J.B., 2002, Journey from the Center of the Sun, 119–120, Princeton University Press, 978-0-691-05781-1, BOOK, Lang, Kenneth R., 2008, The Sun from Space, 75, Springer-Verlag, 978-3-540-76952-1, Sunspots are visible as dark patches on the Sun's photosphere, and correspond to concentrations of magnetic field where the convective transport of heat is inhibited from the solar interior to the surface. As a result, sunspots are slightly cooler than the surrounding photosphere, and, 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 the solar cycle progresses towards its maximum, sunspots tend form closer to the solar equator, a phenomenon known as Spörer's law. The largest sunspots can be tens of thousands of kilometers across.WEB, 30 March 2001, The Largest Sunspot in Ten Years,weblink Goddard Space Flight Center, 10 July 2009,weblink" title="">weblink 23 August 2007, 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, 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 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 known as the Hale cycle.JOURNAL, Hale, G.E., Ellerman, F., Nicholson, S.B., Joy, A.H., The Magnetic Polarity of Sun-Spots, The Astrophysical Journal, 49, 153, 1919, 10.1086/142452, 1919ApJ....49..153H, WEB, 4 January 2008, NASA Satellites Capture Start of New Solar Cycle, PhysOrg,weblink 10 July 2009, 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.NEWS, 16 February 2001, Sun flips magnetic field,weblink CNN, 11 July 2009, WEB, Phillips, T., 15 February 2001, The Sun Does a Flip,weblink NASA, 11 July 2009, dead,weblink 12 May 2009, 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 the 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 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 the Parker spiral. The interplanetary magnetic field is much stronger than the dipole component of the solar magnetic field. The Sun's dipole magnetic field of 50–400 Î¼T (at the photosphere) reduces with the inverse-cube of the distance to about 0.1 nT at the distance of Earth. However, according to spacecraft observations the interplanetary field at Earth's location is around 5 nT, about a hundred times greater.JOURNAL, Wang, Y.-M., Sheeley, N.R., 2003, Modeling the Sun's Large-Scale Magnetic Field during the Maunder Minimum, The Astrophysical Journal, 591, 2, 1248–1256, 2003ApJ...591.1248W, 10.1086/375449, harv, The difference is due to magnetic fields generated by electrical currents in the plasma surrounding the Sun.

Variation in activity

(File:Solar-cycle-data.png|thumb|right|Measurements from 2005 of solar cycle variation during the last 30 years)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 the photospheric surface. Both coronal-mass ejections and high-speed streams of solar wind carry plasma and interplanetary magnetic field outward into the Solar System.BOOK, Zirker, J.B., 2002, Journey from the Center of the Sun, 120–127, Princeton University Press, 978-0-691-05781-1, 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 the Solar System.With solar-cycle modulation of sunspot number comes a corresponding modulation of space weather conditions, including those surrounding Earth where technological systems can be affected.

Long-term change

Long-term secular change in sunspot number is thought, by some scientists, to be correlated with long-term change in solar irradiance,JOURNAL, Willson, R.C., Hudson, H.S., 1991, The Sun's luminosity over a complete solar cycle, Nature (journal), Nature, 351, 6321, 42–44, 10.1038/351042a0, 1991Natur.351...42W, which, in turn, might influence Earth's long-term climate.JOURNAL, John A. Eddy, Eddy, John A., The Maunder Minimum, Science (journal), Science, 192, 4245, 1189–1202, June 1976, 17771739, 10.1126/science.192.4245.1189, 17425839, 1976Sci...192.1189E, 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 the Maunder minimum. This coincided in time with the era of the Little Ice Age, when Europe experienced unusually cold temperatures.JOURNAL, Lean, J., Skumanich, A., White, O., 1992, Estimating the Sun's radiative output during the Maunder Minimum, Geophysical Research Letters, 19, 15, 1591–1594, 10.1029/92GL01578, harv, 1992GeoRL..19.1591L, Earlier extended minima have been discovered through analysis of tree rings and appear to have coincided with lower-than-average global temperatures.BOOK, Mackay, R.M., Khalil, M.A.K, Greenhouse gases and global warming,weblink Singh, S.N., 2000, Trace Gas Emissions and Plants, 1–28, Springer (publisher), Springer, 978-0-7923-6545-7, A recent theory claims that there are magnetic instabilities in the core of the Sun that cause fluctuations with periods of either 41,000 or 100,000 years. These could provide a better explanation of the ice ages than the Milankovitch cycles.JOURNAL, Ehrlich, R., Solar Resonant Diffusion Waves as a Driver of Terrestrial Climate Change, Journal of Atmospheric and Solar–Terrestrial Physics, 69, 7, 759–766, 2007, 10.1016/j.jastp.2007.01.005, astro-ph/0701117, harv, 2007JASTP..69..759E,weblink JOURNAL, Clark, S., Sun's fickle heart may leave us cold,weblink New Scientist, 2588, 12, 2007, 10.1016/S0262-4079(07)60196-1, 193, harv,

Life phases

The Sun today is roughly halfway through the most stable part of its life. It has not changed dramatically for over four billion years, and will remain fairly stable for more than 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 giant molecular cloud that consisted mostly of hydrogen and helium and that probably gave birth to many other stars.BOOK, Zirker, Jack B., Journey from the Center of the Sun, 2002, Princeton University Press, 978-0-691-05781-1, 7–8,
This age is estimated using computer models of stellar evolution and through nucleocosmochronology. The result is consistent with the radiometric date of the oldest Solar System material, at 4.567 billion years ago.
JOURNAL, Amelin, Y., Krot, A., Hutcheon, I., Ulyanov, A., Lead isotopic ages of chondrules and calcium-aluminum-rich inclusions, Science (journal), Science, 297, 5587, 1678–1683, 2002, 10.1126/science.1073950, 12215641, harv, 2002Sci...297.1678A, JOURNAL, Baker, J., Bizzarro, M., Wittig, N., Connelly, J., Haack, H., Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites, Nature (journal), Nature, 436, 7054, 1127–1131, 2005, 16121173, 10.1038/nature03882, harv, 2005Natur.436.1127B, Studies of ancient meteorites reveal traces of stable daughter nuclei of short-lived isotopes, such as iron-60, that form only in exploding, short-lived stars. This indicates that one or more supernovae must have occurred near the location where the Sun formed. A shock 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.JOURNAL, Williams, J., The astrophysical environment of the solar birthplace, Contemporary Physics, 51, 5, 381–396, 2010, 10.1080/00107511003764725, 2010ConPh..51..381W, 1008.2973,, As one fragment of the cloud collapsed it also began to rotate because of conservation of angular momentum and heat up with the increasing pressure. 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. Gravity and pressure within the core of the cloud generated a lot of heat as it accreted more matter from the surrounding disk, eventually triggering nuclear fusion.HD 162826 and HD 186302 are hypothesized stellar siblings of the Sun, having formed in the same molecular cloud.

Main sequence

File:Solar evolution (English).svg|right|thumb|upright=1.45|Evolution of the Sun's luminosity, radius and (effective temperature]] compared to the present Sun. After Ribas (2010)JOURNAL, Ribas, Ignasi, Proceedings of the IAU Symposium 264 'Solar and Stellar Variability – Impact on Earth and Planets': The Sun and stars as the primary energy input in planetary atmospheres, 264, 3–18, February 2010, 10.1017/S1743921309992298, 2010IAUS..264....3R, Proceedings of the International Astronomical Union, 0911.4872, )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 million tonnes of matter are converted into energy within the Sun's core, producing neutrinos and solar 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 billion years as a main-sequence star.BOOK, Goldsmith, D., Owen, T., The search for life in the universe,weblink 96, University Science Books, 2001, 978-1-891389-16-0, The Sun is gradually becoming hotter during its time on the main sequence, because the helium atoms in the core occupy less volume than the hydrogen atoms that were fused. The core is therefore shrinking, allowing the outer layers of the Sun to move closer to the center and experience a stronger gravitational force, according to the inverse-square law. This stronger force increases the pressure on the core, which is resisted by a gradual increase in the rate at which fusion occurs. This process speeds up as the core gradually becomes denser. It is estimated that the Sun has become 30% brighter in the last 4.5 billion years.WEB,weblink The Sun's Evolution, At present, it is increasing in brightness by about 1% every 100 million years.WEB,weblink Earth Won't Die as Soon as Thought, 22 January 2014,

After core hydrogen exhaustion

File:Sun red giant.svg|thumb|upright=1.35|left|The size of the current Sun (now in the main sequencemain sequenceThe Sun does not have enough mass to explode as a supernova. Instead it will exit the main sequence in approximately 5 billion years and start to turn into a red giant.WEB, Nola Taylor Redd, Red Giant Stars: Facts, Definition & the Future of the Sun,weblink, 20 February 2016, JOURNAL, Schröder, K.-P., Connon Smith, R., 10.1111/j.1365-2966.2008.13022.x, Distant future of the Sun and Earth revisited, Monthly Notices of the Royal Astronomical Society, 386, 1, 155–163, 2008, 0801.4031, 2008MNRAS.386..155S, As a red giant, the Sun will grow so large that it will engulf Mercury, Venus, and probably Earth.JOURNAL, Boothroyd, A.I., Sackmann, I.‐J., 10.1086/306546, The CNO Isotopes: Deep Circulation in Red Giants and First and Second Dredge‐up, The Astrophysical Journal, 510, 1, 232–250, 1999, 1999ApJ...510..232B, astro-ph/9512121, Even before it becomes a red giant, the luminosity of the Sun will have nearly doubled, and Earth will receive as much sunlight as Venus receives today. Once the core hydrogen is exhausted in 5.4 billion years, the Sun will expand into a subgiant phase and slowly double in size over about half a billion years. It will then expand more rapidly over about half a billion years until it is over two hundred times larger than today and a couple of thousand times more luminous. This then starts the red-giant-branch phase where the Sun will spend around a billion years and lose around a third of its mass.File:Evolution of a Sun-like star.svg|upright=1.35|right|thumb|Evolution of a Sun-like star. The track of a one solar mass star on the Hertzsprung–Russell diagramHertzsprung–Russell diagramAfter the red-giant branch the Sun has approximately 120 million years of active life left, but much happens. First, the core, full of degenerate helium ignites violently in the helium flash, where 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 the triple-alpha process.WEB,weblink The End Of The Sun, 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 the red clump or horizontal branch, but a star of the Sun's mass 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.When the helium is exhausted, the Sun will repeat the expansion it followed when the hydrogen in the core was exhausted, except that this time it all happens faster, and the Sun becomes larger and more luminous. This is the asymptotic-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 and thermal 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 and the radius to over 1 AU.JOURNAL, Vassiliadis, E., Wood, P.R., 10.1086/173033, Evolution of low- and intermediate-mass stars to the end of the asymptotic giant branch with mass loss, The Astrophysical Journal, 413, 641, 1993, 1993ApJ...413..641V, According to a 2008 model, Earth's orbit is shrinking due to tidal forces (and, eventually, drag from the lower chromosphere), so that it will be engulfed by the Sun near the tip of the red giant branch phase, 3.8 and 1 million years after Mercury and Venus have respectively suffered the same fate. 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. For the Sun, four thermal pulses are predicted before it completely loses its outer envelope and starts to make a planetary nebula. By the end of that phase—lasting approximately 500,000 years—the Sun will only have about half of its current mass.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 a planetary nebula as the exposed core reaches 30,000 K. The final naked core, a white dwarf, will have a temperature of over 100,000 K, and contain an estimated 54.05% of the Sun's present day mass. 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 black dwarf.{{bibcode|1995A&A...297..727B}}{{bibcode|1995A&A...299..755B}}

Orbit and location

(File:Artist%27s impression of the Milky Way (updated - annotated).jpg|thumb|right|Illustration of the Milky Way, showing the location of the Sun)The Sun lies close to the inner rim of the Milky Way's Orion Arm, in the Local Interstellar Cloud or the Gould Belt, at a distance of 7.5–8.5 kpc (25,000–28,000 light-years) from the Galactic Center.Our Local Galactic Neighborhood, NASA {{webarchive |url= |date=7 November 2015 }}WEB,weblink Into the Interstellar Void, Centauri Dreams, JOURNAL, Reid, M.J., The distance to the center of the Galaxy, Annual Review of Astronomy and Astrophysics, 1993, 31, 1, 345–372, 10.1146/annurev.aa.31.090193.002021, 1993ARA&A..31..345R, harv, JOURNAL, Eisenhauer, F., A Geometric Determination of the Distance to the Galactic Center, Astrophysical Journal, 597, 2, L121–L124, 2003, 10.1086/380188, 2003ApJ...597L.121E, harv, astro-ph/0306220, etal, JOURNAL, Horrobin, M., First results from SPIFFI. I: The Galactic Center,weblink Astronomische Nachrichten, 325, 2, 120–123, 2004, 10.1002/asna.200310181, harv, 2004AN....325...88H, etal, JOURNAL, Eisenhauer, F., SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month, Astrophysical Journal, 628, 1, 246–259, 2005, 10.1086/430667, 2005ApJ...628..246E, harv, astro-ph/0502129, etal, The Sun is contained within the Local Bubble, a space of rarefied hot gas, possibly produced by the supernova remnant Geminga,JOURNAL, Gehrels, Neil, Chen, Wan, 25 February 1993, The Geminga supernova as a possible cause of the local interstellar bubble, Nature, 361, 6414, 706–707, 10.1038/361704a0, Mereghetti, S., harv, 1993Natur.361..704B, or multiple supernovae in subgroup B1 of the Pleiades moving group.JOURNAL, The origin of the young stellar population in the solar neighborhood – a link to the formation of the Local Bubble?, T.W., Berghoefer, D., Breitschwerdt, 1 July 2002, Astronomy & Astrophysics, 390, 1, 299–306, 10.1051/0004-6361:20020627, astro-ph/0205128, 2002A&A...390..299B, The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years.PRESS, English, J., Exposing the Stuff Between the Stars,weblink Hubble News Desk, 2000, 10 May 2007, The Sun, and thus the Solar System, is found in what scientists call the galactic habitable zone.The Apex of the Sun's Way, or the solar apex, is the direction that the Sun travels relative to other nearby stars. This motion is towards a point in the constellation Hercules, near the star Vega.Within {{Convert|10|pc|ly|1|abbr=on|disp=out}} of the Sun there are 315 known stars in 227 systems, as of 2000, including 163 single stars. It is estimated that a further 130 systems within this range have not yet been identified. Out to {{Convert|25|pc|ly|1|abbr=on|disp=out}}, there may be up to 7,500 stars, of which around 2,600 are known. The number of substellar objects in that volume are expected to be comparable to the number of stars.{{citation |title=New nearby stars among bright APM high proper motion stars |last1=Scholz |first1=R.-D. |last2=Ibata |first2=R. |last3=Irwin |first3=M. |last4=Lehmann |first4=I. |last5=Salvato |first5=M. |last6=Schweitzer |first6=A. |journal=Monthly Notices of the Royal Astronomical Society |volume=329 |issue=1 |pages=109–114 |date=January 2002 |doi=10.1046/j.1365-8711.2002.04945.x |bibcode=2002MNRAS.329..109S}} Of the 50 nearest stellar systems within 17 light-years from Earth (the closest being the red dwarf Proxima Centauri at approximately 4.2 light-years), the Sun ranks fourth in mass.JOURNAL, Adams, F.C., Graves, G., Laughlin, G.J.M., 2004, Red Dwarfs and the End of the Main Sequence,weblink Revista Mexicana de Astronomía y Astrofísica, 22, 46–49, 2004RMxAC..22...46A, harv, dead,weblink" title="">weblink 26 July 2011,

Orbit in Milky Way

The Sun orbits the center of the Milky Way, and it is presently moving in the direction of the constellation of Cygnus. A simple model of the motion of a star in the galaxy gives the galactic coordinates {{mvar|X}}, {{mvar|Y}}, and {{mvar|Z}} as:
X(t)=X(0)+frac{U(0)}kappasin(kappa t)+frac{V(0)}{2B}(1-cos(kappa t)) Y(t)=Y(0)+2Aleft(X(0)+frac{V(0)}{2B}right)t-frac{Omega_0}{Bkappa}V(0)sin(kappa t)+frac{2Omega_0}{kappa^2}U(0)(1-cos(kappa t)) Z(t)=frac{W(0)}nusin(nu t)+Z(0)cos(nu t)
where {{mvar|U}}, {{mvar|V}}, and {{mvar|W}} are the respective velocities with respect to the local standard of rest, {{mvar|A}} and {{mvar|B}} are the Oort constants, Omega_0=A-B is the angular velocity of galactic rotation for the local standard of rest, kappa=sqrt{-4Omega_0B} is the "epicyclic frequency", and ν is the vertical oscillation frequency.JOURNAL, B. Fuchs, etal, The search for the origin of the Local Bubble redivivus, MNRAS, 373, 3, 993–1003, 2006, 10.1111/j.1365-2966.2006.11044.x, astro-ph/0609227, 2006MNRAS.373..993F, For the sun, the present values of {{mvar|U}}, {{mvar|V}}, and {{mvar|W}} are estimated as (U(0),V(0),W(0))=(10.00, 5.25, 7.17) km/s, and estimates for the other constants are {{mvar|A}} = 15.5 km/s/kpc, {{mvar|B}} = âˆ’12.2 km/s/kpc, κ = 37 km/s/kpc, and ν=74 km/s/kpc. We take {{math|X(0)}} and {{math|Y(0)}} to be zero and {{math|Z(0)}} is estimated to be 17 parsecs.JOURNAL, Bobylev, Vadim V., 2010, Searching for Stars Closely Encountering with the Solar System, Astronomy Letters, 36, 3, 220–226, 10.1134/S1063773710030060, 1003.2160, 2010AstL...36..220B, This model implies that the Sun circulates around a point that is itself going around the galaxy. The period of the Sun's circulation around the point is 2pi/kappa. which, using the equivalence that a parsec equals 1 km/s times 0.978 million years, comes to 166 million years, shorter than the time it takes for the point to go around the galaxy. In the ({{mvar|X, Y}}) coordinates, the Sun describes an ellipse around the point, whose length in the {{mvar|Y}} direction is
2timessqrt{left(frac{2Omega_0}{kappa^2}U(0)right)^2+left(frac{Omega_0}{Bkappa}V(0)right)^2}=1035text{ parsec}.
and whose width in the {{mvar|X}} direction is
2timessqrt{left(frac{U(0)}kapparight)^2+left(frac{V(0)}{2B}right)^2}=691text{ parsec}
The ratio of length to width of this ellipse, the same for all stars in our neighborhood, is 2Omega/kappaapprox 1.50.The moving point is presently at
X=frac{V(0)}{2B}=-215text{ parsec} Y=frac{2Omega_0}{kappa^2}U(0)=405text{ parsec}.
The oscillation in the {{mvar|Z}} direction takes the Sun
sqrt{left(frac{W(0)}nuright)^2+Z(0)^2}=98text{ parsec}
above the galactic plane and the same distance below it, with a period of 2pi/nu or 83 million years, approximately 2.7 times per orbit.BOOK, Moore, Patrick, Rees, Robin, Patrick Moore's Data Book of Astronomy, 2014, Cambridge University Press, Cambridge, 978-1-139-49522-6, harv, Although 2pi/Omega_0 is 222 million years, the value of Omega at the point around which the Sun circulates is
OmegaapproxOmega_0-frac{2A}{R_0}Delta Xapprox 26.1text{ km/s/kpc}
(see Oort constants), corresponding to 235 million years, and this is the time that the point takes to go once around the galaxy. Other stars with the same value of X+V/(2B) have take the same amount of time to go around the galaxy as the sun and thus remain in the same general vicinity as the Sun.The Sun's orbit around the Milky Way is perturbed due to the non-uniform mass distribution in Milky Way, such as that in and between the galactic spiral arms. It has been argued that the Sun's passage through the higher density spiral arms often coincides with mass extinctions on Earth, perhaps due to increased impact events.JOURNAL, Gillman, M., Erenler, H., The galactic cycle of extinction, International Journal of Astrobiology, 7, 1, 17–26, 2008, 10.1017/S1473550408004047, harv, 2008IJAsB...7...17G,weblink, It takes the Solar System about 225–250 million years to complete one orbit through the Milky Way (a galactic year),WEB, Leong, S., Period of the Sun's Orbit around the Galaxy (Cosmic Year),weblink The Physics Factbook, 2002, 10 May 2007, so it is thought to have completed 20–25 orbits during the lifetime of the Sun. The orbital speed of the Solar System about the center of the Milky Way is approximately 251 km/s (156 mi/s).JOURNAL, Croswell, K., 2008, Milky Way keeps tight grip on its neighbor,weblink New Scientist, 199, 2669, 8, 10.1016/S0262-4079(08)62026-6, harv, At this speed, it takes around 1,190 years for the Solar System to travel a distance of 1 light-year, or 7 days to travel 1 AU.BOOK, Garlick, M.A., The Story of the Solar System, 46, Cambridge University Press, 2002, 978-0-521-80336-6,weblink The Milky Way is moving with respect to the cosmic microwave background radiation (CMB) in the direction of the constellation Hydra with a speed of 550 km/s, and the Sun's resultant velocity with respect to the CMB is about 370 km/s in the direction of Crater or Leo.JOURNAL, Kogut, A., 1993, Dipole Anisotropy in the COBE Differential Microwave Radiometers First-Year Sky Maps, Astrophysical Journal, 419, 1993, 1, astro-ph/9312056, 10.1086/173453, 1993ApJ...419....1K, etal,

Theoretical problems

File:Map of the full sun.jpg|thumb|Map of the full Sun by STEREO and SDO spacecraft]]

Coronal heating problem

The temperature of the photosphere is approximately 6,000 K, whereas the temperature of 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 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.JOURNAL, Alfvén, H., Magneto-hydrodynamic waves, and the heating of the solar corona, Monthly Notices of the Royal Astronomical Society, 107, 2, 211–219, 1947, 1947MNRAS.107..211A, harv, 10.1093/mnras/107.2.211, The other is magnetic heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in the form of large solar flares and myriad similar but smaller events—nanoflares.JOURNAL, Parker, E.N., Nanoflares and the solar X-ray corona, Astrophysical Journal, 330, 1, 474, 1988, 10.1086/166485, 1988ApJ...330..474P, harv, 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.JOURNAL, Sturrock, P.A., Uchida, Y., Coronal heating by stochastic magnetic pumping, Astrophysical Journal, 246, 1, 331, 1981, 10.1086/158926, 1981ApJ...246..331S, harv, 2060/19800019786, In addition, Alfvén waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms.JOURNAL, Erdèlyi, R., Ballai, I., Heating of the solar and stellar coronae: a review, 2007, Astron. Nachr., 328, 8, 726–733, 10.1002/asna.200710803, harv, 2007AN....328..726E,

Faint young Sun problem

Theoretical models of the Sun's development suggest that 3.8 to 2.5 billion years ago, during the Archean eon, the Sun was only about 75% as bright as it is today. Such a weak star would not have been able to sustain liquid water on Earth's surface, and thus life should not have been able to develop. However, the geological record demonstrates that Earth has remained at a fairly constant temperature throughout its history, and that the young Earth was somewhat warmer than it is today. One theory among scientists is that the atmosphere of the young Earth contained much larger quantities of greenhouse gases (such as carbon dioxide, methane) than are present today, which trapped enough heat to compensate for the smaller amount of solar energy reaching it.JOURNAL, Kasting, J.F., Ackerman, T.P., Climatic Consequences of Very High Carbon Dioxide Levels in the Earth's Early Atmosphere, Science (journal), Science, 234, 4782, 1383–1385, 1986, 10.1126/science.11539665, 11539665, harv,weblink However, examination of Archaean sediments appears inconsistent with the hypothesis of high greenhouse concentrations. Instead, the moderate temperature range may be explained by a lower surface albedo brought about by less continental area and the "lack of biologically induced cloud condensation nuclei". This would have led to increased absorption of solar energy, thereby compensating for the lower solar output.JOURNAL, Rosing, Minik T., Bird, Dennis K., Sleep, Norman H., Bjerrum, Christian J., No climate paradox under the faint early Sun, Nature, 464, 7289, 744–747, 1 April 2010, 20360739, 10.1038/nature08955, 2010Natur.464..744R,

Observational history

The enormous effect of the Sun on Earth has been recognized since prehistoric times, and the Sun has been regarded by some cultures as a deity.

Early understanding

File:Solvognen DO-6865 2000.jpg|thumb|right|The Trundholm sun chariot pulled by a horse is a sculpture believed to be illustrating an important part of Nordic Bronze Age mythology. The sculpture is probably from around 1350 BC. It is displayed at the National Museum of DenmarkNational Museum of Denmark{{See also|The Sun in culture}}The Sun has been an object of veneration in many cultures throughout human history. Humanity's most fundamental understanding of the Sun is as the luminous disk in the sky, whose presence above the horizon creates day and whose absence causes night. In many prehistoric and ancient cultures, the Sun was thought to be a solar deity or other supernatural entity. Worship of the Sun was central to civilizations such as the ancient Egyptians, the Inca of South America and the Aztecs of what is now Mexico. In religions such as Hinduism, the Sun is still considered a god. Many ancient monuments were constructed with solar phenomena in mind; for example, stone megaliths accurately mark the summer or winter solstice (some of the most prominent megaliths are located in Nabta Playa, Egypt; Mnajdra, Malta and at Stonehenge, England); Newgrange, a prehistoric human-built mount in Ireland, was designed to detect the winter solstice; the pyramid of El Castillo at Chichén Itzá in Mexico is designed to cast shadows in the shape of serpents climbing the pyramid at the vernal and autumnal equinoxes.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 of Elagabalus in the late Roman Empire the Sun's birthday was a holiday celebrated as Sol Invictus (literally "Unconquered Sun") soon after the winter solstice, which may have been an antecedent to Christmas. Regarding the fixed stars, the Sun appears from Earth to revolve once a year along the ecliptic through the zodiac, and so Greek astronomers categorized it as one of the seven planets (Greek planetes, "wanderer"); the naming of the days of the weeks after the seven planets dates to the Roman era.WEB,weblink Oxford Dictionaries, Planet, 22 March 2015, December 2007, JOURNAL, Bernard R., Goldstein, Saving the phenomena : the background to Ptolemy's planetary theory, Journal for the History of Astronomy, 28, 1, 1997, 1–12, 1997JHA....28....1G, harv, 10.1177/002182869702800101, BOOK, Ptolemy's Almagest, Ptolemy, Toomer, G.J., Princeton University Press, 1998, 978-0-691-00260-6,

Development of scientific understanding

In the early first millennium BC, Babylonian astronomers observed that the Sun's motion along the ecliptic is not uniform, though they did not know why; it is today known that this is due to the movement of Earth in an elliptic orbit around the Sun, with Earth moving faster when it is nearer to the Sun at perihelion and moving slower when it is farther away at aphelion.BOOK, Babylon to Voyager and beyond: a history of planetary astronomy, David, Leverington, Cambridge University Press, 2003, 978-0-521-80840-8, 6–7, harv, One of the first people to offer a scientific or philosophical explanation for the Sun was the Greek philosopher Anaxagoras. He reasoned that it was not the chariot of Helios, but instead a giant flaming ball of metal even larger than the land of the Peloponnesus and that the Moon reflected the light of the Sun.JOURNAL, Sider, D., Anaxagoras on the Size of the Sun, 269068, Classical Philology (journal), Classical Philology, 68, 2, 128–129, 1973, 10.1086/365951, harv, For teaching this heresy, he was imprisoned by the authorities and sentenced to death, though he was later released through the intervention of Pericles. Eratosthenes estimated the distance between Earth and the Sun in the 3rd century BC as "of stadia myriads 400 and 80000", the translation of which is ambiguous, implying either 4,080,000 stadia (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 1st century AD, Ptolemy estimated the distance as 1,210 times the radius of Earth, approximately {{convert|{{#expr:1.210*6.371round2}}|e6km|AU|sp=us}}.JOURNAL, Goldstein, B.R., The Arabic Version of Ptolemy's Planetary Hypotheses, Transactions of the American Philosophical Society, 57, 4, 9–12, 1967, 10.2307/1006040, harv, 1006040, The theory that the Sun is the center around which the planets orbit was first proposed by the ancient Greek Aristarchus of Samos in the 3rd century BC, and later adopted by Seleucus of Seleucia (see Heliocentrism). This view was developed in a more detailed mathematical model of a heliocentric system in the 16th century by Nicolaus Copernicus.Observations of sunspots were recorded during the Han Dynasty (206 BC–AD 220) by Chinese astronomers, who maintained records of these observations for centuries. Averroes also provided a description of sunspots in the 12th century.BOOK, Ead, Hamed A., Averroes As A Physician, University of Cairo, The invention of the telescope in the early 17th century permitted detailed observations of sunspots by Thomas 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.WEB, Galileo Galilei (1564–1642),weblink BBC, 22 March 2006, Arabic astronomical contributions include Albatenius' discovery that the direction of the Sun's apogee (the place in the Sun's orbit against the fixed stars where it seems to be moving slowest) is changing.A short History of scientific ideas to 1900, C. Singer, Oxford University Press, 1959, p. 151. (In modern heliocentric terms, this is caused by a gradual motion of the aphelion of the Earth's orbit). Ibn Yunus observed more than 10,000 entries for the Sun's position for many years using a large astrolabe.The Arabian Science, C. Ronan, pp. 201–244 in The Cambridge Illustrated History of the World's Science, Cambridge University Press, 1983; at pp. 213–214.File:Sun-bonatti.png|thumb|Sol, the Sun, from a 1550 edition of Guido BonattiGuido BonattiFrom an observation of a transit of Venus in 1032, the Persian astronomer and polymath Avicenna concluded that Venus is closer to Earth than the Sun.JOURNAL, Theory and Observation in Medieval Astronomy, Bernard R., Goldstein, Isis (journal), Isis, 63, 1, March 1972, 39–47 [44], 10.1086/350839, harv, In 1672 Giovanni Cassini and Jean Richer determined the distance to Mars and were thereby able to calculate the distance to the Sun.In 1666, Isaac Newton observed the Sun's light using a prism, and showed that it is made up of light of many colors.WEB, Sir Isaac Newton (1643–1727),weblink BBC, 22 March 2006, In 1800, William Herschel discovered infrared radiation beyond the red part of the solar spectrum.WEB, Herschel Discovers Infrared Light,weblink Cool Cosmos, 22 March 2006, dead,weblink" title="">weblink 25 February 2012, The 19th century saw advancement in spectroscopic studies of the Sun; Joseph von Fraunhofer recorded more than 600 absorption lines in the spectrum, the strongest of which are still often referred to as Fraunhofer lines. 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.JOURNAL, Thomson, W., On the Age of the Sun's Heat,weblink Macmillan's Magazine, 1862, 5, 388–393, harv, Kelvin and Hermann von Helmholtz then proposed a gravitational 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.JOURNAL, 2000, Kelvin's age of the Earth paradox revisited, Journal of Geophysical Research, 105, B6, 13155–13158, 2000JGR...10513155S, 10.1029/2000JB900028, Stacey, Frank D., In 1890 Joseph Lockyer, who discovered helium in the solar spectrum, proposed a meteoritic hypothesis for the formation and evolution of the Sun.JOURNAL, Lockyer, J.N., The meteoritic hypothesis; a statement of the results of a spectroscopic inquiry into the origin of cosmical systems, London and New York, 1890,, 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 suggested radioactive decay as the source.WEB, Darden, L., The Nature of Scientific Inquiry,weblink 1998, However, it would be Albert Einstein who would provide the essential clue to the source of the Sun's energy output with his mass–energy equivalence relation {{nowrap|E {{=}} mc2}}.BOOK, Hawking, S.W., Stephen Hawking, 2001, The Universe in a Nutshell, Bantam Books, 978-0-553-80202-3, In 1920, Sir Arthur 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.WEB, Studying the stars, testing relativity: Sir Arthur Eddington,weblink Space Science, European Space Agency, 2005, 1 August 2007, The preponderance of hydrogen in the Sun was confirmed in 1925 by Cecilia Payne using the ionization theory developed by Meghnad Saha, an Indian physicist. The theoretical concept of fusion was developed in the 1930s by the astrophysicists Subrahmanyan Chandrasekhar and Hans Bethe. Hans Bethe calculated the details of the two main energy-producing nuclear reactions that power the Sun.JOURNAL, Bethe, H., On the Formation of Deuterons by Proton Combination, Physical Review, 54, 10, 862, 1938, 10.1103/PhysRev.54.862.2, Critchfield, C., harv, 1938PhRv...54Q.862B, JOURNAL, Bethe, H., Energy Production in Stars, Physical Review, 55, 1, 434–456, 1939, 10.1103/PhysRev.55.434, harv, 1939PhRv...55..434B, In 1957, Margaret Burbidge, Geoffrey Burbidge, William Fowler and Fred Hoyle showed that most of the elements in the universe have been synthesized by nuclear reactions inside stars, some like the Sun.JOURNAL, E.M., Burbidge, G.R., Burbidge, W.A., Fowler, F., Hoyle, Synthesis of the Elements in Stars, Reviews of Modern Physics, 29, 4, 547–650, 1957, 10.1103/RevModPhys.29.547, 1957RvMP...29..547B, harv,

Solar space missions

{{see also|Solar observatory}}File:Sunspots and Solar Flares.jpg|thumb|The Sun giving out a large (geomagnetic storm]] on 1:29 pm, EST, 13 March 2012)(File:Moon transit of sun large.ogv|thumb|left|A lunar transit of the Sun captured during calibration of STEREO B's ultraviolet imaging camerasWEB, Phillips, T., Stereo Eclipse,weblink Science@NASA, NASA, 2007, 19 June 2008, dead,weblink 10 June 2008, )The first satellites designed for long term observation of the Sun from interplanetary space were NASA's Pioneers 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.WEB, Wade, M., Pioneer 6-7-8-9-E,weblink 2008, Encyclopedia Astronautica, 22 March 2006, dead,weblink" title="">weblink 22 April 2006, dmy-all, WEB, Solar System Exploration: Missions: By Target: Our Solar System: Past: Pioneer 9,weblink NASA, 30 October 2010, NASA maintained contact with Pioneer 9 until May 1983, dead,weblink" title="">weblink 2 April 2012, dmy-all, In the 1970s, two Helios spacecraft and the Skylab Apollo 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. 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. Skylab made the first time-resolved observations of the solar transition region and of ultraviolet emissions from the solar corona. Discoveries included the first observations of coronal mass ejections, then called "coronal transients", and of coronal holes, now known to be intimately associated with the solar wind.JOURNAL, Burlaga, L.F., Magnetic Fields and plasmas in the inner heliosphere: Helios results, 2001, Planetary and Space Science, 49, 14–15, 1619–1627, 10.1016/S0032-0633(01)00098-8, harv, 2001P&SS...49.1619B,weblink File:PIA22197-SunFormsQuestionMark-20171222.jpg|thumb|left|Coronal hole on the Sun forms a question markquestion markIn 1980, the Solar Maximum Mission was launched by NASA. This spacecraft was designed to observe gamma rays, X-rays and UV 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 Shuttle Challenger mission STS-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 before re-entering Earth's atmosphere in June 1989.WEB, Burkepile, C.J., Solar Maximum Mission Overview,weblink 1998, 22 March 2006,weblink" title="">weblink 5 April 2006, Launched in 1991, Japan's Yohkoh (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.PRESS, Result of Re-entry of the Solar X-ray Observatory "Yohkoh" (SOLAR-A) to the Earth's Atmosphere,weblink Japan Aerospace Exploration Agency, 2005, 22 March 2006, One of the most important solar missions to date has been the Solar and Heliospheric Observatory, jointly built by the European Space Agency and NASA and launched on 2 December 1995. Originally intended to serve a two-year mission, a mission extension through 2012 was approved in October 2009.WEB, 7 October 2009,weblink Mission extensions approved for science missions, ESA Science and Technology, 16 February 2010, It has proven so useful that a follow-on mission, the Solar Dynamics Observatory (SDO), was launched in February 2010.WEB, 11 February 2010,weblink NASA Successfully Launches a New Eye on the Sun, NASA Press Release Archives, 16 February 2010, Situated at the Lagrangian 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. Besides its direct solar observation, SOHO has enabled the discovery of a large number of comets, mostly tiny sungrazing comets that incinerate as they pass the Sun.WEB, Sungrazing Comets,weblink Large Angle and Spectrometric Coronagraph, LASCO (US Naval Research Laboratory), 19 March 2009, (File:Giant prominence on the sun erupted.jpg|thumb|upright=1.35|A solar prominence erupts in August 2012, as captured by SDO)All these satellites have observed the Sun from the plane of the ecliptic, and so have only observed its equatorial regions in detail. The Ulysses 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. Once Ulysses 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.WEB, Jet Propulsion Laboratory, JPL/California Institute of Technology, CALTECH, Ulysses: Primary Mission Results,weblink NASA, 2005, 22 March 2006, dead,weblink" title="">weblink 6 January 2006, Elemental abundances in the photosphere are well known from spectroscopic 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.JOURNAL, Calaway, M.J., Genesis capturing the Sun: Solar wind irradiation at Lagrange 1, Nuclear Instruments and Methods in Physics Research B, 267, 7, 1101–1108, 2009, 10.1016/j.nimb.2009.01.132, Stansbery, Eileen K., Keller, Lindsay P., harv, 2009NIMPB.267.1101C,weblink The Solar Terrestrial Relations Observatory (STEREO) mission was launched in October 2006. Two identical spacecraft were launched into orbits that cause them to (respectively) pull further ahead of and fall gradually behind Earth. This enables stereoscopic imaging of the Sun and solar phenomena, such as coronal mass ejections.WEB, 8 March 2006,weblink STEREO Spacecraft & Instruments, NASA Missions, 30 May 2006, JOURNAL, Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI), Howard, R.A., Moses, J.D., Socker, D.G., Dere, K.P., Cook, J.W., Advances in Space Research, 29, 12, 2017–2026, 2002, harv, 2008SSRv..136...67H, 10.1007/s11214-008-9341-4,weblink The Indian Space Research Organisation has scheduled the launch of a 100 kg satellite named Aditya for mid 2020. Its main instrument will be a coronagraph for studying the dynamics of the Solar corona.WEB,weblink Aditya L-1: After Chandrayaan 2, ISRO to pursue India's first mission to the Sun in 2020,

Observation and effects

File:Anatomy of a Sunset-2.jpg|thumb|right|upright=1.35|During certain atmospheric conditions, the Sun becomes clearly visible to the naked eye, and can be observed without stress to the eyes. Click on this photo to see the full cycle of a sunset, as observed from the high plains of the Mojave DesertMojave DesertFile:STS-134 EVA4 view to the Russian Orbital Segment.jpg|thumb|right|The Sun, as seen from low Earth orbit overlooking the International Space StationInternational Space StationThe brightness of the Sun can cause pain from looking at it with the naked eye; however, doing so for brief periods is not hazardous for normal non-dilated eyes.JOURNAL, T.J., White, M.A., Mainster, P.W., Wilson, J.H., Tips, Chorioretinal temperature increases from solar observation, Bulletin of Mathematical Biophysics, 33, 1, 1–17, 1971, 10.1007/BF02476660, harv, JOURNAL, M.O.M., Tso, F.G., La Piana, The Human Fovea After Sungazing, Transactions of the American Academy of Ophthalmology and Otolaryngology, 1975, 79, OP788–95, 1209815, 6, harv, Looking directly at the Sun causes phosphene 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.JOURNAL, Hope-Ross, M.W., Ultrastructural findings in solar retinopathy, Eye (journal), Eye, 7, 4, 1993, 10.1038/eye.1993.7, 8325420, Mahon, GJ, Gardiner, TA, Archer, DB, harv, 29–33, JOURNAL, Solar Retinopathy from Sun-Gazing Under Influence of LSD, Schatz, H., Mendelblatt, F., British Journal of Ophthalmology, 57, 4, 1973, 10.1136/bjo.57.4.270, 4707624, harv, 1214879, 270–273, UV exposure gradually yellows the lens of the eye over a period of years, and is thought to contribute to the formation of cataracts, but this depends on general exposure to solar UV, and not whether one looks directly at the Sun.WEB, Chou, B.R., Eye Safety During Solar Eclipses,weblink 2005, "While environmental exposure to UV radiation is known to contribute to the accelerated aging of the outer layers of the eye and the development of cataracts, the concern over improper viewing of the Sun during an eclipse is for the development of "eclipse blindness" or retinal burns." Long-duration viewing of the direct Sun with the naked eye can begin to cause UV-induced, sunburn-like lesions on the retina after about 100 seconds, particularly under conditions where the UV light from the Sun is intense and well focused;JOURNAL, W.T. Jr., Ham, H.A., Mueller, D.H., Sliney, Nature (journal), Nature, Retinal sensitivity to damage from short wavelength light, 260, 5547, 153–155, 1976, 10.1038/260153a0, 815821, harv, 1976Natur.260..153H, BOOK, W.T. Jr., Ham, H.A., Mueller, J.J. Jr., Ruffolo, D. III, Guerry, Solar Retinopathy as a function of Wavelength: its Significance for Protective Eyewear, The Effects of Constant Light on Visual Processes, Williams, T.P., Baker, B.N., Plenum Press, 319–346, 1980, 978-0-306-40328-6, conditions are worsened by young eyes or new lens implants (which admit more UV than aging natural eyes), Sun angles near the zenith, and observing locations at high altitude.Viewing the Sun through light-concentrating optics such as binoculars 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 or IR rays, can actually harm the eye at high brightness levels.BOOK, T., Kardos, Earth science,weblink 87, J.W. Walch, 2003, 978-0-8251-4500-1, Herschel wedges, also called Solar Diagonals, are effective and inexpensive for small telescopes. The sunlight that is destined for the eyepiece is reflected from an unsilvered surface of a piece of glass. Only a very small fraction of the incident light is reflected. The rest passes through the glass and leaves the instrument. If the glass breaks because of the heat, no light at all is reflected, making the device fail-safe. Simple filters made of darkened glass allow the full intensity of sunlight to pass through if they break, endangering the observer's eyesight. Unfiltered binoculars can deliver hundreds of times as much energy as using the naked eye, possibly causing immediate damage. It is claimed that even brief glances at the midday Sun through an unfiltered telescope can cause permanent damage.BOOK, Macdonald, Lee, 2012, How to Observe the Sun Safely, Springer Science + Business Media, New York, 2. Equipment for Observing the Sun, 17, 10.1007/978-1-4614-3825-0_2, NEVER LOOK DIRECTLY AT THE SUN THROUGH ANY FORM OF OPTICAL EQUIPMENT, EVEN FOR AN INSTANT. A brief glimpse of the Sun through a telescope is enough to cause permanent eye damage, or even blindness. Even looking at the Sun with the naked eye for more than a second or two is not safe. Do not assume that it is safe to look at the Sun through a filter, no matter how dark the filter appears to be., Patrick Moore's Practical Astronomy Series, 978-1-4614-3824-3, File:Doppelsonne Halo Echzell Hessen 12-08-2012.jpg|thumb|left|Halo with sun dogsun dogPartial solar eclipses are hazardous to view because the eye's pupil is not adapted to the unusually high visual contrast: the pupil dilates according to the total amount of light in the field of view, not by the brightest object in the field. During partial eclipses most sunlight is blocked by the Moon passing in front of the Sun, but the uncovered parts of the photosphere have the same surface brightness as during a normal day. In the overall gloom, the pupil expands from ~2 mm to ~6 mm, and each retinal cell exposed to the solar image receives up to ten times more light than it would looking at the non-eclipsed Sun. This can damage or kill those cells, resulting in small permanent blind spots for the viewer.WEB, Espenak, Fred, Eye Safety During Solar Eclipses,weblink NASA, 26 April 1996, The hazard is insidious for inexperienced observers and for children, because there is no perception of pain: it is not immediately obvious that one's vision is being destroyed.(File:Actual Sunrise.jpeg|upright=1.35|thumb|right|A sunrise)(File:D85 1116 Sunset in Thailand photographed by Trisorn Triboon.jpg|thumb|A sunset in Thailand in summer)During sunrise and sunset, sunlight is attenuated because of Rayleigh scattering and Mie scattering from a particularly long passage through Earth's atmosphere,JOURNAL, Haber, Jorg, Magnor, Marcus, Seidel, Hans-Peter, Physically based Simulation of Twilight Phenomena, 2005, ACM Transactions on Graphics, 24, 4, 1353–1373, 10.1145/1095878.1095884,, harv, 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.JOURNAL, Diurnal asymmetries in global radiation, I.G., Piggin, Springer, 1972, 20, 1, 10.1007/BF02243313, 41–48, harv, 1972AMGBB..20...41P, An optical phenomenon, known as a green flash, can sometimes be seen shortly after sunset or before sunrise. The flash is caused by light from the Sun just below the horizon being bent (usually through a temperature 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 is scattered more, leaving light that is perceived as green.WEB, The Green Flash,weblink BBC, 10 August 2008,weblink" title="">weblink 16 December 2008, Ultraviolet light from the Sun has antiseptic properties and can be used to sanitize tools and water. It also causes sunburn, and has other biological effects such as the production of vitamin D and sun tanning. Ultraviolet light 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 in different regions of the Earth.JOURNAL, Barsh, G.S., What Controls Variation in Human Skin Color?, PLoS Biology, 1, 1, e7, 2003, 14551921, 212702, 10.1371/journal.pbio.0000027, harv,

Planetary system

The Sun has eight known planets. This includes four terrestrial planets (Mercury, Venus, Earth, and Mars), two gas giants (Jupiter and Saturn), and two ice giants (Uranus and Neptune). The Solar System also has at least five dwarf planets, an asteroid belt, numerous comets, and a large number of icy bodies which lie beyond the orbit of Neptune.

Religious aspects

Solar deities play a major role in many world religions and mythologies.BOOK, Coleman, J.A., Davidson, George, The Dictionary of Mythology: An A–Z of Themes, Legends, and Heroes, 2015, Arcturus Publishing Limited, London, 978-1-78404-478-7, 316, The ancient Sumerians believed that the sun was Utu,BOOK, Black, Jeremy, Anthony, Green, Gods, Demons and Symbols of Ancient Mesopotamia: An Illustrated Dictionary,weblink The British Museum Press, 1992, 978-0-7141-1705-8, 182–184, harv, {{citation |last=Nemet-Nejat |first=Karen Rhea |authorlink=Karen Rhea Nemet-Nejat |date=1998 |title=Daily Life in Ancient Mesopotamia |publisher=Greenwood |series=Daily Life |isbn=978-0-313-29497-6 |page=203 |url=}} the god of justice and twin brother of Inanna, the Queen of Heaven, who was identified as the planet Venus. Later, Utu was identified with the East Semitic god Shamash. Utu was regarded as a helper-deity, who aided those in distress, and, in iconography, he is usually portrayed with a long beard and clutching a saw, which represented his role as the dispenser of justice.From at least the 4th Dynasty of Ancient Egypt, the Sun was worshipped as the god Ra, portrayed as a falcon-headed divinity surmounted by the solar disk, and surrounded by a serpent. In the New Empire period, the Sun became identified with the dung beetle, whose spherical ball of dung was identified with the Sun. In the form of the sun disc Aten, the Sun had a brief resurgence during the Amarna Period when it again became the preeminent, if not only, divinity for the Pharaoh Akhenaton.BOOK, Teeter, Emily, Religion and Ritual in Ancient Egypt, 2011, Cambridge University Press, New York, 978-0-521-84855-8, BOOK, Frankfort, Henri, Ancient Egyptian Religion: an Interpretation, 2011, Dover Publications, 978-0-486-41138-5, In Proto-Indo-European religion, the Sun was personified as the goddess *Seh2ul.JOURNAL, Dexter, Miriam Robbins, Proto-Indo-European Sun Maidens and Gods of the Moon, Mankind Quarterly, 25, 1 & 2, 1984, 137–144, ENCYCLOPEDIA, 1997, Encyclopedia of Indo-European Culture, Mallory, James P., J. P. Mallory, Adams, Douglas Q., Douglas Q. Adams, London, Routledge, (EIEC),weblink 978-1-884964-98-5, harv, Derivatives of this goddess in Indo-European languages include the Old Norse Sól, Sanskrit Surya, Gaulish Sulis, Lithuanian Saulė, and Slavic Solntse. In ancient Greek religion, the sun deity was the male god Helios, but traces of an earlier female solar deity are preserved in Helen of Troy. In later times, Helios was syncretized with Apollo.BOOK, Burkert, Walter, Walter Burkert, 1985, Greek Religion, Harvard University Press, Cambridge, 978-0-674-36281-9, 120, In the Bible, {{Bibleverse||Malachi|4:2|9}} mentions the "Sun of Righteousness" (sometimes translated as the "Sun of Justice"),{{citation |title=Bible, Book of Malachi |publisher=King James Version |url=}} which some Christians have interpreted as a reference to the Messiah (Christ).BOOK, Spargo, Emma Jane Marie, The Category of the Aesthetic in the Philosophy of Saint Bonaventure,weblink 86, 1953, The Franciscan Institute, St. Bonaventure, New York; E. Nauwelaerts, Louvain, Belgium; F. Schöningh, Paderborn, Germany, In ancient Roman culture, Sunday was the day of the sun god. It was adopted as the Sabbath day by Christians who did not have a Jewish background. The symbol of light was a pagan device adopted by Christians, and perhaps the most important one that did not come from Jewish traditions. In paganism, the Sun was a source of life, giving warmth and illumination to mankind. 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 the winter solstice (which influenced Christmas) was part of the Roman cult of the unconquered Sun (Sol Invictus). Christian churches were built with an orientation so that the congregation faced toward the sunrise in the East.BOOK, Owen Chadwick, A History of Christianity,weblink 1998, St. Martin's Press, 22, 978-0-312-18723-1, Tonatiuh, the Aztec god of the sun, was usually depicted holding arrows and a shieldBOOK, State and Cosmos in the Art of Tenochtitlan, Townsend, Richard, Dumbarton Oaks, 1979, Washington, DC, 66, and was closely associated with the practice of human sacrifice. The sun goddess Amaterasu is the most important deity in the Shinto religion,BOOK, Roberts, Jeremy, Japanese Mythology A To Z, New York, Chelsea House Publishers, 2010, 2nd, 978-1-60413-435-3, 4–5, BOOK, Wheeler, Post, The Sacred Scriptures of the Japanese, New York, Henry Schuman, 393–395, 1952, 978-1-4254-8787-4, and she is believed to be the direct ancestor of all Japanese emperors.

See also

{{Wikipedia books |1=The Sun}}{hide}cmn|colwidth=25em| {edih}


{{notes| notes ={{efn| name = heavy elementsastronomy>astronomical sciences, the term heavy elements (or metals) refers to all elements except hydrogen and helium.}}{{efn| name = power production density| A 50 kg adult human has a volume of about 0.05 m3, which corresponds to 13.8 watts, at the volumetric power of the solar center. This is 285 kcal/day, about 10% of the actual average caloric intake and output for humans in non-stressful conditions.}}{{efn| name = particle density25}} m−3.}}}}



Further reading

  • BOOK, Cohen, Richard, 2010, Chasing the Sun: the Epic Story of the Star that Gives us Life, Simon & Schuster, 978-1-4000-6875-3,
  • JOURNAL, Thompson, M.J., 2004, Solar interior: Helioseismology and the Sun's interior, Astronomy & Geophysics, 45, 4, 21–25,
  • Solar Activity Scholarpedia Hugh Hudson 3(3):3967. {{doi|10.4249/scholarpedia.3967}}

External links

{{Sister project links|Sun}} {{The Sun|state=uncollapsed}}{{Sun spacecraft}}{{Solar System}}{{Star}}{{Nearest star systems|1}}{{Astronomy navbar}}{{Authority control}}

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