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The Sun (in Greek: Helios, in Latin: Sol [a]) is the star  at the center of the Solar System and is by far

the most important source of energy for life on arth! "t is a nearly perfect spherical ball of

hot plasma,[#$][#%] &ith internal con'ecti'e motion that generates a magnetic field 'ia a dynamo

process![#] "ts diameter is about #* times that of arth, and it has a mass about %%, times that

of arth, accounting for about **!+- of the total mass of the Solar System! [#.] /bout three 0uarters

of the Sun1s mass consists of  hydrogen2 the rest is mostly helium, &ith much smaller 0uantities of

hea'ier elements, including o3ygen, carbon, neon andiron![#]

The Sun is a G4type main4se0uence star  (G$5) based on spectral class and it is informally referred

to as a yello& d&arf! "t formed appro3imately !.6 billion[b][#6] years ago from the gra'itational

collapse of matter &ithin a region of a large molecular cloud! 7ost of this matter gathered in the

center, &hereas the rest flattened into an orbiting disk that became the Solar System! The central

mass became increasingly hot and dense, e'entually initiating nuclear fusion in its core! "t is thought

that almost all stars form by this process!

The Sun is roughly middle aged and has not changed dramatically for four billion [b] years, and &ill

remain fairly stable for another four billion yearrs! 8o&e'er, after hydrogen fusion in its core has

stopped, the Sun &ill undergo se'ere changes and become a red giant! "t is calculated that the Sun

&ill become sufficiently large to engulf the current orbits of  7ercury, 5enus, and possibly arth!

The enormous effect of the Sun on the arth has been recogni9ed since prehistoric times, and the

Sun has been regarded by some cultures as a deity! arth1s mo'ement around the Sun is the basis

of the solar calendar , &hich is the predominant calendar in use today!

Contents

  [hide] 

• # ame and etymology

o #!# ;eligious aspects

• $ <haracteristics

• % Sunlight

• <omposition

o !# Singly ioni9ed iron4group elements

o !$ "sotopic composition

• . Structure

o .!# <ore

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o .!$ ;adiati'e 9one

o .!% Tachocline

o .! <on'ecti'e 9one

o .!. =hotosphere

o .! /tmosphere

o .!6 =hotons and neutrinos

• 7agnetism and acti'ity

o !# 7agnetic field

o !$ 5ariation in acti'ity

o !% Long4term change

• 6 Life phases

o 6!# >ormation

o 6!$ 7ain se0uence

o 6!% /fter core hydrogen e3haustion

• + 7otion and location

o +!# ?rbit in 7ilky @ay

• * Theoretical problems

o *!# <oronal heating problem

o *!$ >aint young Sun problem

• # 8istory of obser'ation

o #!# arly understanding

o #!$ Ae'elopment of scientific understanding

o #!% Solar space missions

• ## ?bser'ation and effects

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• #$ See also

• #% otes

• # ;eferences

• #. >urther reading

• # 3ternal links

Name and etymology 

The nglish proper noun Sun de'eloped from ?ld nglish sunne and may be related to south!

<ognates to nglish sun appear in other Germanic languages, including ?ld

>risian sunne, sonne, ?ld Sa3on sunna,7iddle Autch sonne, modern Autch zon, ?ld 8igh

German sunna, modern German Sonne, ?ld orse sunna, and Gothic sunnō! /ll Germanic terms for 

the Sun stem from =roto4Germanic Bsunnōn![#+][#*]

The nglish &eekday name Sunday  stems from ?ld nglish (Sunnandæg 2 CSun1s dayC, from before

6) and is ultimately a result of a Germanic interpretation of Latin dies solis, itself a translation of

the Greek DEF HIJK (ἡ ἡ   hēméra hēlíou)![$] The Latin name for the Sun, Sol , is &idely kno&n but is

not common in general nglish language use2 the adecti'al form is the related &ord solar ![$#][$$] The

term sol  is also used by planetary astronomers to refer to the duration of a solar day on another

planet, such as 7ars![$%] / mean arth solar day is appro3imately $ hours, &hereas a mean 7artian

1sol1 is $ hours, %* minutes, and %.!$ seconds![$]

Religious aspectsThe Sun is 'ie&ed as a goddess in Germanic paganism, SMlNSunna![#*] Scholars theori9e that the

Sun, as a Germanic goddess, may represent an e3tension of an earlier =roto4"ndo4uropean Sun

deity because of "ndo4uropean linguistic connections bet&een ?ld

orse Sól , Sanskrit Surya, Gaulish Sulis, Lithuanian Saulė, and Sla'ic Solntse![#*]

"n ancient ;oman culture, Sunday &as the day of the Sun god! "t &as adopted as the Sabbath day

by <hristians &ho did not ha'e a Oe&ish background! The symbol of light &as a pagan de'ice

adopted by <hristians P perhaps the most important one that did not come from Oe&ish traditions! "n

paganism, the sun &as a source of life, gi'ing &armth and illumination to mankind! "t &as the center

of a popular cult among ;omans, &ho &ould stand at da&n to catch the first rays of sunshine as

they prayed! The celebration of the &inter solstice (&hich influenced <hristmas) &as part of the

;oman cult of the sun! <hristian churches &ere built &ith an orientation so that the congregation

faced to&ard the sunrise in the ast![$.]

Characteristics

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The Sun is a G4type main4se0uence star  that comprises about **!+- of the mass of the Solar

System! The Sun has an absolute magnitude of Q!+%, estimated to be brighter than about +.- of

the stars in the 7ilky @ay, most of &hich are red d&arfs![$][$6] The Sun is a =opulation ", or hea'y4

element4rich,[c] star ![$+] The formation of the Sun may ha'e been triggered by shock&a'es from one or 

more nearby superno'ae![$*]

 This is suggested by a high abundance of hea'y elements in the SolarSystem, such as gold and uranium, relati'e to the abundances of these elements in so4

called =opulation "", hea'y4element4poor, stars! These elements could most plausibly ha'e been

produced by endothermic nuclear reactions during a superno'a, or by transmutation through neutron

absorption &ithin a massi'e second4generation star![$+]

The Sun is by far the brightest obect in the sky, &ith an apparent magnitude of R$!6![%][%#] This is

about #% billion times brighter than the ne3t brightest star, Sirius, &hich has an apparent

magnitude of R#!! The mean distance of the Sun to arth is appro3imately # astronomical

unit (about #.,, km2 *%,, mi), though the distance 'aries as arth mo'es

from perihelion in Oanuary to aphelion in Ouly![%$] /t this a'erage distance, light tra'els from the Sun to

arth in about + minutes and #* seconds! The energy of this sunlight supports almost all life [d] on

arth by photosynthesis,[%%] and dri'es arth1s climate and &eather!

The Sun does not ha'e a definite boundary, and in its outer parts its density decreases e3ponentially

&ith increasing distance from its center ![%] >or the purpose of measurement, ho&e'er, the Sun1s

radius is considered to be the distance from its center to the edge of the photosphere, the apparent

'isible surface of the Sun! [%.] y this measure, the Sun is a near4perfect sphere &ith

an oblateness estimated at about * millionths, [%] &hich means that its polar diameter differs from its

e0uatorial diameter by only # kilometres (!$ mi)![%6] The tidal effect of the planets is &eak and does

not significantly affect the shape of the Sun! [%+]The Sun rotates faster at its e0uator  than at its poles!

This differential rotation is caused by con'ecti'e motion due to heat transport and the <oriolis

force due to the Sun1s rotation! "n a frame of reference defined by the stars, the rotational period is

appro3imately $.! days at the e0uator and %%!. days at the poles! 5ie&ed from arth as it orbits

the Sun, the apparent rotational period  of the Sun at its e0uator is about $+ days![%*]

Sunlight

Main article: Sunlight 

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This spectrum, created at the 7c7ath4=ierce Solar ?bser'atory, sho&s that the Sun emits light of nearly e'ery

color, but is brightest in yello&4green light! The dark patches in the spectrum arise from gas at or abo'e the

Sun1s surface absorbing light! ecause different types of gas absorb different colors of light, it is possible to

determine &hich gases are present in the Sun1s photosphere! 8elium, for e3ample, &as first disco'ered in #+6

by analy9ing the solar spectrum and &as only later found on arth!

Sunlight is arth1s primary source of energy! The other significant source of arth1s energy is the

store of fissionable materials generated by the cataclysmic death of other stars! These fissionable

materials trapped in arth1s crust gi'e rise to geothermal energy, &hich dri'es the 'olcanism on

arth and also makes it possible for humans to fuel nuclear reactors! The solar constant is the

amount of po&er that the Sun deposits per unit area that is directly e3posed to sunlight! The solar

constant is e0ual to appro3imately #,%+ @Nm$ (&atts per s0uare meter) at a distance of

one astronomical unit (/) from the Sun (that is, on or near arth)![]Sunlight on the surface of arth

is attenuated by arth1s atmosphere so that less po&er arri'es at the surfaceUcloser

to #, @Nm$ in clear conditions &hen the Sun is near the 9enith![#] Sunlight at the top of arth1s

atmosphere is composed (by total energy) of about .- infrared light, - 'isible light, and #-

ultra'iolet light![$] The atmosphere in particular filters out o'er 6- of solar ultra'iolet, especially at

the shorter &a'elengths![%] Solar ultra'iolet radiation ioni9es the arth1s dayside upper atmosphere,

creating the electrically conducting ionosphere![]

The Sun1s color is &hite, &ith a <" color4space inde3 near (!%, !%), &hen 'ie&ed from space or

&hen high in the sky2 &hen lo& in the sky, atmospheric scattering renders the Sun yello&, red,

orange, or magenta! Aespite its typical &hiteness, most people mentally picture the Sun as yello&2

the reasons for this are the subect of debate![.] The Sun is a G$5 star, &ith G  indicating its surface

temperature of appro3imately .,66+ V (.,.. W<, *,*# W>), and !  that it, like most stars, is a main4

se0uence star ![][6] The luminance of the Sun is about #!++ giga candela per s0uare metre, but as

'ie&ed through arth1s atmosphere, this is lo&ered to about #! GcdNm$![+]

Composition

See also: Molecules in stars

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The Sun is composed primarily of the chemical elements hydrogen and helium2 they account for

6!*- and $%!+- of the mass of the Sun in the photosphere, respecti'ely![*]  /ll hea'ier elements,

called metals in astronomy, account for less than $- of the mass, &ith o3ygen (roughly #- of the

Sun1s mass), carbon (!%-), neon (!$-), and iron (!$-) being the most abundant![.]

The Sun inherited its chemical composition from the interstellar medium out of &hich it formed! The

hydrogen and helium in the Sun &ere produced by ig ang nucleosynthesis, and the hea'ier

elements &ere produced by stellar nucleosynthesis in generations of stars that completed

their  stellar e'olution and returned their material to the interstellar medium before the formation of

the Sun![.#] The chemical composition of the photosphere is normally considered representati'e of

the composition of the primordial Solar System! [.$] 8o&e'er, since the Sun formed, some of the

helium and hea'y elements ha'e gra'itationally settled from the photosphere! Therefore, in today1s

photosphere the helium fraction is reduced and the metallicity is only +- of that in

the protostellar  phase (before nuclear fusion in the core started)! The protostellar Sun1s composition

&as reconstructed as 6#!#- hydrogen, $6!- helium, and #!.- hea'ier elements![*]

"n the inner portions of the Sun, nuclear fusion has modified the composition by con'erting hydrogen

into helium, so the innermost portion of the Sun is no& roughly - helium, &ith the abundance of

hea'ier elements unchanged! ecause the interior of the Sun is radiati'e, not con'ecti'e

(see ;adiati'e 9one belo&), none of the fusion products from the core ha'e risen to the

photosphere![.%]

The reacti'e core 9one of Chydrogen burningC, &here hydrogen is con'erted into helium, is starting to

surround the core of Chelium ashC! This de'elopment &ill continue and &ill e'entually cause the Sun

to lea'e themain se0uence, to become a red giant![.]

The solar hea'y4element abundances described abo'e are typically measured both

using spectroscopy of the Sun1s photosphere and by measuring abundances in meteorites that ha'e

ne'er 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 hea'y elements! The t&o methods

generally agree &ell![#]

Singly ionized iron-group elements

"n the #*6s, much research focused on the abundances of iron4group elements in the Sun![..]

[.]

 /lthough significant research &as done, until #*6+ it &as difficult to determine the abundances ofsome iron4group elements (e!g! cobalt and manganese) 'ia spectrography because of

their  hyperfine structures![..]

The first largely complete set of oscillator strengths of singly ioni9ed iron4group elements &ere made

a'ailable in the #*s,[.6] and these &ere subse0uentl