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| − | {{otheruses6|Sun (disambiguation)|Sol (disambiguation)}}
| + | :''For other usages, see [[sun]].'' |
| − | {{Solar_System_Infobox/Sun}}
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| − | The '''Sun''' ({{lang-la|Sol}}) is the [[star]] at the center of the [[Solar System]]<!-- Please don't change "the" to "our" - there is only one "Solar System", and thus "the" is correct. See Talk page for this article and Solar System. -->. The [[Earth]] and other matter (including other [[planet]]s, [[asteroid]]s, [[meteoroid]]s, [[comet]]s, and [[Cosmic dust|dust]]) [[orbit]] the Sun,<ref>[http://www.nasa.gov/lb/audience/forkids/home/F_Ames_What_is_the_Solar_System_Text.html What is the solar system?]</ref> which by itself accounts for about 99.8% of the [[Solar System]]'s [[mass]]. [[Energy]] from the Sun, in the form of sunlight and heat, supports almost all life on Earth via [[photosynthesis]], and drives the Earth's [[climate]] and weather.
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| − | The surface composition of the Sun consists of [[hydrogen]] (about 74% of its mass, or 92% of its volume), [[helium]] (about 24-25% of mass,<ref>{{cite journal | + | '''The Sun''' (sometimes referred to as '''Sol''') is the name of the [[star]] at the center of the [[Solar system]]. The Sun is the best known star, mainly due to [[Earth]]'s relative proximity to it. |
| − | | last=Basu | first=Sarbani | coauthors=Antia, H. M.
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| − | | title=Helioseismology and Solar Abundances
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| − | | journal=Physics Reports | year=2007
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| − | | url=http://front.math.ucdavis.edu/0711.4590
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| − | | accessdate=2007-12-09 }}</ref> 7% of volume), and trace quantities of other elements, including [[Iron]], [[Nickel]], [[Oxygen]], [[Silicon]], [[Sulfur]], [[Magnesium]], [[Carbon]], [[Neon]], [[Calcium]], and [[Chromium]].<ref name="manuel1983">Manuel O. K. and Hwaung Golden (1983), Meteoritics, Volume 18, Number 3, 30 September 1983, pp 209-222. Online: http://web.umr.edu/~om/archive/SolarAbundances.pdf (retrieved 7 December 2007 20:21 UTC).</ref> <span id="why_the_sun_is_yellow"></span>The Sun has a [[stellar classification|spectral class]] of G2V. ''G2'' means that it has a surface temperature of approximately 5,780 [[Kelvin|K]], giving it a [[color temperature|white]] color which, because of atmospheric [[scattering]], appears yellow as seen from the surface of the Earth. This is a subtractive effect, as the [[Rayleigh scattering|preferential scattering]] of blue photons (causing the sky color) removes enough blue light to leave a residual reddishness that is perceived as yellow. (When low enough in the sky, the Sun appears orange or red, due to this scattering.)
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| − | Its spectrum contains [[spectral line|line]]s of ionized and neutral metals as well as very weak hydrogen lines. The ''V'' ([[Roman numerals|Roman five]]) in the spectral class indicates that the Sun, like most stars, is a [[main sequence]] star. This means that it generates its energy by [[nuclear fusion]] of [[hydrogen]] nuclei into [[helium]]. There are more than 100 million G2 class stars in our galaxy. Once regarded as a small and relatively insignificant star, the Sun is now known to be brighter than 85% of the stars in the [[Milky Way|galaxy]], most of which are [[red dwarf]]s.<ref>{{cite news |first=Ker |last=Than |title=Astronomers Had it Wrong: Most Stars are Single |publisher=SPACE.com |date=January 30, 2006 |url=http://www.space.com/scienceastronomy/060130_mm_single_stars.html |accessdate=2007-08-01 }}</ref>
| + | [[Category:Stellar objects]] |
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| − | The Sun orbits the center of the [[Milky Way]] [[galaxy]] at a distance of approximately 26,000 [[light-year]]s from the [[galactic center]], completing one revolution in about 225–250 million years. Its approximate [[orbital speed]] is 220 kilometers per second, plus or minus 20 km/s. This is equivalent to about one light-year every 1,400 years, and about one [[Astronomical unit|AU]] every 8 days. These measurements of galactic distance and speed are as accurate as we can get given our current knowledge, but will change as we learn more.<ref name="Kerr">{{cite journal
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| − | |last=Kerr
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| − | |first=F. J.
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| − | |coauthors=Lynden-Bell D.
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| − | |year=1986
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| − | |url=http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1986MNRAS.221.1023K&data_type=PDF_HIGH&type=PRINTER&filetype=.pdf
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| − | |format=PDF
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| − | |title=Review of galactic constants
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| − | |journal=Monthly Notices of the Royal Astronomical Society
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| − | |volume=221
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| − | |pages=1023–1038}}</ref>
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| − | The Sun is currently traveling through the [[Local Interstellar Cloud]] in the low-density [[Local Bubble]] zone of diffuse high-temperature gas, in the inner rim of the [[Orion Arm]] of the [[Milky Way Galaxy]], between the larger [[Perseus arm|Perseus]] and [[Sagittarius arm]]s of the galaxy. Of the 50 [[Nearest stars|nearest stellar systems]] within 17 light years from the Earth, the Sun ranks 4th in [[absolute magnitude]] as a fourth magnitude star (M=4.83).
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| − | ==Overview==
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| − | The Sun is a [[metallicity#Population I stars|Population I]], or heavy element-rich<ref>In [[astronomy|astronomical]] [[jargon]], the term heavy elements ("[[metallicity|metals]]") refers to [[element]]s with [[atomic number]] greater than 2, i. e. all elements except [[hydrogen]] and [[helium]].</ref>, star.<ref name=zeilik /> The formation of the Sun may have been triggered by shockwaves from one or more nearby [[supernova]]e.<ref name="Falk">{{cite journal
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| − | |last=Falk
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| − | |first=S. W.
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| − | |coauthors=Lattmer, J. M., Margolis, S. H.
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| − | |year=1977
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| − | |url=http://www.nature.com/nature/journal/v270/n5639/abs/270700a0.html
| + | |
| − | |title=Are supernovae sources of presolar grains?
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| − | |journal=Nature
| + | |
| − | |volume=270
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| − | |pages=700-701}}</ref>
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| − | This is suggested by a high [[Abundance of the chemical elements|abundance]] of [[heavy metals|heavy elements]] such as [[gold]] and [[uranium]] in the solar system relative to the abundances of these elements in so-called [[metallicity#Population II stars|Population II]] (heavy element-poor) stars. These elements could most plausibly have been produced by [[endergonic]] nuclear reactions during a supernova, or by [[Nuclear transmutation|transmutation]] via [[neutron]] absorption inside a massive second-generation star.
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| − | Sunlight is Earth's primary source of energy. The [[solar constant]] is the amount of power that the Sun deposits per unit area that is directly exposed to sunlight. The solar constant is equal to approximately 1,370 [[watt]]s per square meter at a distance of one [[astronomical unit|AU]] from the Sun (that is, on or near Earth). Sunlight on the surface of Earth is [[attenuation (electromagnetic radiation)|attenuated]] by the Earth's atmosphere so that less power arrives at the surface—closer to 1,000 watts per directly exposed square meter in clear conditions when the Sun is near the [[zenith]]. This energy can be harnessed via a variety of natural and synthetic processes—[[photosynthesis]] by [[plant]]s captures the energy of sunlight and converts it to chemical form (oxygen and reduced carbon compounds), while direct heating or electrical conversion by [[solar cells]] are used by [[solar energy|solar power]] equipment to generate electricity or to do other useful work. The energy stored in [[petroleum]] and other [[fossil fuel]]s was originally converted from sunlight by [[photosynthesis]] in the distant past.
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| − | [[Ultraviolet]] light from the Sun has [[antiseptic]] properties and can be used to sanitize tools and water. It also causes [[sunburn]], and has other medical effects such as the production of [[Vitamin D]]. Ultraviolet light is strongly attenuated by Earth's [[ozone layer]], so that the amount of UV varies greatly with [[latitude]] and has been responsible for many biological adaptations, including variations in human [[skin color]] in different regions of the globe.<ref>Barsh G.S., 2003, [http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=212702 ''What Controls Variation in Human Skin Color?''], PLoS Biology, v. 1, p. 19 </ref>
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| − | Observed from Earth, the Sun's path across the sky varies throughout the year. The shape described by the Sun's position, considered at the same time each day for a complete year, is called the [[analemma]] and resembles a figure 8 aligned along a north/south axis. While the most obvious variation in the Sun's apparent position through the year is a north/south swing over 47 degrees of angle (because of the 23.5-degree tilt of the Earth with respect to the Sun), there is an east/west component as well, caused by the acceleration of the Earth as it approaches its [[perihelion]] with the Sun, and the reduction in the Earth's speed as it moves away to approach its [[aphelion]]. The north/south swing in apparent angle is the main source of [[seasons]] on Earth.
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| − | The Sun is a magnetically active star. It supports a strong, changing [[magnetic field]] that varies year-to-year and reverses direction about every eleven years around solar maximum. The Sun's magnetic field gives rise to many effects that are collectively called [[solar variation|solar activity]], including [[sunspot]]s on the surface of the Sun, [[solar flares]], and variations in [[solar wind]] that carry material through the Solar System. Effects of solar activity on Earth include [[aurora (astronomy)|aurora]]s 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 [[solar nebula|formation]] and evolution of the [[Solar System]]. Solar activity changes the structure of Earth's [[ionosphere|outer atmosphere]]. <!-- POV is in "strongly affects". Get a measure. -->
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| − | Although it is the nearest star to Earth and has been intensively studied by scientists, many questions about the Sun remain unanswered. Current topics of scientific inquiry include the Sun's regular cycle of [[sunspot]] activity, the physics and origin of [[solar flare|flares]] and [[solar prominence|prominences]], the magnetic interaction between the [[chromosphere]] and the [[corona]], and the origin (propulsion source) of [[solar wind]].
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| − | ==Location within the galaxy==
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| − | The Sun may be found close to the inner rim of the [[Milky Way Galaxy|Milky Way Galaxy's]] [[Orion Arm]], in the [[Local Fluff]] or the [[Gould Belt]], at a hypothesized distance of 7.62±0.32 [[Kiloparsec|kpc]] from the [[Galactic Center]].<ref name="distance1">{{cite journal
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| − | | last = Reid
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| − | | first = Mark J.
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| − | | title=The distance to the center of the Galaxy
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| − | | journal=Annual review of astronomy and astrophysics
| + | |
| − | | year=1993
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| − | | volume=31
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| − | | pages=345–372
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| − | | url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1993ARA%26A..31..345R&
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| − | | accessdate=2007-05-10 }}</ref><ref name="distance2">{{cite journal
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| − | | author=Eisenhauer, F.; Schödel, R.; Genzel, R.; Ott, T.; Tecza, M.; Abuter, R.; Eckart, A.; Alexander, T.
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| − | | title=A Geometric Determination of the Distance to the Galactic Center
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| − | | journal=The Astrophysical Journal
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| − | | year=2003
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| − | | volume=597
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| − | | pages=L121–L124
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| − | | url=http://adsabs.harvard.edu/abs/2003astro.ph..6220E
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| − | | accessdate=2007-05-10
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| − | | doi = 10.1086/380188 <!--Retrieved from CrossRef by DOI bot-->
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| − | }}</ref><ref name="distance3">{{cite journal
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| − | | author=Horrobin, M.; Eisenhauer, F.; Tecza, M.; Thatte, N.; Genzel, R.; Abuter, R.; Iserlohe, C.; Schreiber, J.; Schegerer, A.; Lutz, D.; Ott, T.; Schödel, R.
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| − | | title=First results from SPIFFI. I: The Galactic Center
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| − | | journal=Astronomische Nachrichten
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| − | | year=2004
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| − | | volume=325
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| − | | pages=120–123
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| − | | url=http://www.mpe.mpg.de/SPIFFI/preprints/first_result_an1.pdf
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| − | | format=PDF
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| − | | accessdate=2007-05-10
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| − | | doi = 10.1002/asna.200310181 <!--Retrieved from CrossRef by DOI bot-->
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| − | }}</ref><ref name="eisenhaueretal2005">{{cite journal
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| − | | title = SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month
| + | |
| − | | author = Eisenhauer, F. et al.
| + | |
| − | | journal = The Astrophysical Journal
| + | |
| − | | volume = 628
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| − | | issue = 1
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| − | | pages = 246–259
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| − | | year = 2005
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| − | | url = http://adsabs.harvard.edu/abs/2005ApJ...628..246E
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| − | | accessdate = 2007-08-12 | doi = 10.1086/430667 <!--Retrieved from CrossRef by DOI bot-->
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| − | }}</ref>
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| − | The distance between the local arm and the next arm out, the [[Perseus Arm]], is about 6,500 light-years.<ref name="fn9">{{cite news
| + | |
| − | | last = English
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| − | | first = Jayanne
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| − | | date = [[2000-01-14]]
| + | |
| − | | title = Exposing the Stuff Between the Stars
| + | |
| − | | publisher = Hubble News Desk
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| − | | date = [[1991-07-24]]
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| − | | url = http://www.ras.ucalgary.ca/CGPS/press/aas00/pr/pr_14012000/pr_14012000map1.html
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| − | | accessdate = 2007-05-10
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| − | }}</ref> The Sun, and thus the Solar System, is found in what scientists call the [[Habitable zone#Galactic habitable zone|galactic habitable zone]].
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| − | The Apex of the Sun's Way, or the [[solar apex]], is the direction that the Sun travels through space in the Milky Way. The general direction of the Sun's galactic motion is towards the star [[Vega]] near the constellation of [[Hercules (constellation)|Hercules]], at an angle of roughly 60 sky degrees to the direction of the [[Galactic Center]]. The Sun's orbit around the Galaxy is expected to be roughly elliptical with the addition of perturbations due to the galactic spiral arms and non-uniform mass distributions. In addition the Sun oscillates up and down relative to the galactic plane approximately 2.7 times per orbit. This is very similar to how a [[simple harmonic oscillator]] works with no drag force (dampening) term. Due to the relatively higher density of stars close to the galactic plane, these oscillations often coincide with [[mass extinction]] periods on earth, presumably due to increased [[impact events]].<ref name="extinction">{{cite journal
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| − | | title = The galactic cycle of extinction
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| − | | author = Gillman, M. and Erenler, H.
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| − | | journal = International Journal of Astrobiology
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| − | | volume =
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| − | | issue =
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| − | | pages =
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| − | | year = 2008
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| − | | url = http://journals.cambridge.org/action/displayAbstract?aid=1804088
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| − | | accessdate = 2008-04-11 | volume = 386
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| − | | pages = 155
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| − | | doi = 10.1017/S1473550408004047 <!--Retrieved from CrossRef by DOI bot-->
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| − | }}</ref>
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| − | It takes the Solar System about 225–250 million years to complete one orbit of the galaxy (a ''[[galactic year]]''),<ref name="fn10">{{cite web
| + | |
| − | | last =Leong
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| − | | first =Stacy
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| − | | year=2002
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| − | | url =http://hypertextbook.com/facts/2002/StacyLeong.shtml
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| − | | title =Period of the Sun's Orbit around the Galaxy (Cosmic Year)
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| − | | work=The Physics Factbook
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| − | | accessdate =2007-05-10 }}</ref> so it is thought to have completed 20–25 orbits during the lifetime of the Sun and 1/1250th of a revolution since the [[origin of humans]]. The [[orbital speed]] of the Solar System about the center of the Galaxy is approximately 220 km/s. At this speed, it takes around 1400 years for the Solar System to travel a distance of 1 light-year, or 8 days to travel 1 [[Astronomical unit|AU]].<ref>{{cite book
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| − | | last = Garlick
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| − | | first = Mark Antony
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| − | | title = The Story of the Solar System
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| − | | publisher = [[Cambridge University]]
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| − | | date = 2002
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| − | | pages = 46
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| − | | isbn = 0521803365
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| − | }}</ref>
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| − | ==Life cycle==
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| − | {{main|Formation and evolution of the solar system|Stellar evolution}}
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| − | The Sun's current [[main sequence]] age, determined using [[computer simulation|computer models]] of [[stellar evolution]] and [[nucleocosmochronology]], is thought to be about 4.57 billion years.<ref name="Bonanno">{{cite journal
| + | |
| − | |last=Bonanno
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| − | |first=A.
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| − | |coauthors=Schlattl, H.; Patern, L.
| + | |
| − | |year= 2002
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| − | |url=http://arxiv.org/PS_cache/astro-ph/pdf/0204/0204331.pdf
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| − | |format=PDF
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| − | |title=The age of the Sun and the relativistic corrections in the EOS
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| − | |journal=Astronomy and Astrophysics
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| − | |volume=390
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| − | |pages=1115–1118}}</ref>
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| − | It is thought that about 4.59 billion years ago, the rapid collapse of a [[hydrogen]] [[molecular cloud]] led to the formation of a third generation [[T Tauri star|T Tauri]] [[Population I star]], the Sun. The nascent star assumed a nearly circular orbit about 26,000 light-years from the centre of the [[Milky Way Galaxy]].<ref>According to [http://www.psrd.hawaii.edu/Sept02/isotopicAges.html isotopicAges], the Ca-Al-I's (= [[Ca-Al-rich inclusion]]s) here formed in a [[proplyd]] (= protoplanetary disk]). The page [[Protoplanetary disk]] says that proplyds are never older than 25 Ma. If 4567 Ma is given for the age of the Earth, then 4567 + 25 = 4592. But 25 Ma is the "maximum age" of [[proplyd]]s. If proplyds slowly decay from the influence of the Sun and from planetesimal formation, then most Ca-Al-I's must have been formed some time within the range of 0 Ma and 25 Ma after the formation of the proplyd. If the median of Ca-Al-I ages are about 10 Ma after the proplyd formation, then we get 4565 + 10 = 4575, but this figure is created by speculating twice. Since it is assumed that planetary formation occurs over a period of about 100,000 years, that is the date given here</ref>
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| − | The Sun is about halfway through its [[main sequence|main-sequence]] [[stellar evolution|evolution]], during which [[stellar nucleosynthesis|nuclear fusion]] reactions in its core fuse hydrogen into helium. Each second, more than 4 million [[tonne]]s of matter are converted into energy within the Sun's core, producing [[neutrino]]s and [[solar radiation]]; at this rate, the Sun will have so far converted around 100 Earth-masses of matter into energy. The Sun will spend a total of approximately 10 [[1000000000 (number)|billion]] years as a main sequence star.
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| − | The Sun does not have enough mass to explode as a [[supernova]]. Instead, in 5–6 billion years, it will enter a [[red giant]] phase, its outer layers expanding as the hydrogen fuel in the core is consumed and the core contracts and heats up. Helium fusion will begin when the core temperature reaches around 100 MK and will produce carbon, entering the [[asymptotic giant branch]] phase.<ref name=zeilik>{{cite book | last=Zeilik | first=Michael A. | coauthors=Gregory, Stephan A. | title=Introductory Astronomy & Astrophysics | edition=4th ed. | year=1998 | publisher=Saunders College Publishing | isbn=0030062284 | pages=322 }}</ref>
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| − | [[Image:Sun Life.png|thumb|300px|left|Life-cycle of the Sun; sizes are not drawn to scale.]]
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| − | Earth's fate is not clear. As a red giant, the Sun will have a maximum radius beyond the Earth's current orbit, {{convert|1|AU|m| abbr=on | lk=on}}, 250 times the present radius of the Sun.<ref name=Schroeder /> However, by the time it is an asymptotic giant branch star, the Sun will have lost roughly 30% of its present mass due to a [[stellar wind]], so the orbits of the planets will move outward. If it were only for this, Earth would probably be spared, but new research suggests that Earth will be swallowed by the Sun due to tidal interactions.<ref name=Schroeder>{{cite journal | first=K.-P. | last=Schröder | coauthors=Smith, Robert Connon | year=2008 | title=Distant future of the Sun and Earth revisited |
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| − | doi=10.1111/j.1365-2966.2008.13022.x | journal=Monthly Notices of the Royal Astronomical Society | pages=in press | id={{arxiv|0801.4031}} }}<br />See also {{cite news | url=http://space.newscientist.com/article/dn13369-hope-dims-that-earth-will-survive-suns-death.html?feedId=online-news_rss20 | title=Hope dims that Earth will survive Sun's death | date=[[22 February]] [[2008]] | work=NewScientist.com news service | first=Jason | last=Palmer | accessdate=2008-03-24 }}</ref> Even if Earth escapes incineration in the Sun, its water will be boiled away and most of its atmosphere would escape into space. In fact, even during its life in the main sequence the Sun is gradually becoming more luminous, its surface temperature slowly rising. The increase in solar temperatures is such that in about 900 million years, the surface of the Earth will become too hot for the survival of life as we know it.<ref>{{cite journal
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| − | | author=Guillemot, H.; Greffoz, V.
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| − | | title=Ce que sera la fin du monde
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| − | | journal=Science et Vie
| + | |
| − | | date=March 2002
| + | |
| − | | volume=N° 1014
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| − | | language=French }}</ref> After another billion years the surface water will have completely disappeared.<ref>{{cite news
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| − | | first=Damian
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| − | | last=Carrington
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| − | | title=Date set for desert Earth
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| − | | publisher=BBC News
| + | |
| − | | date=[[February 21]], [[2000]]
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| − | | url=http://news.bbc.co.uk/1/hi/sci/tech/specials/washington_2000/649913.stm
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| − | | accessdate=2007-03-31 }}</ref>
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| − | Following the red giant phase, intense thermal pulsations will cause the Sun to throw off its outer layers, forming a [[planetary nebula]]. The only object that will remain after the outer layers are ejected is the extremely hot stellar core, which will slowly cool and fade as a [[white dwarf]] over many billions of years. This [[stellar evolution]] scenario is typical of low- to medium-mass stars.<ref name="future-sun">{{cite web
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| − | |author=Pogge, Richard W.
| + | |
| − | |year=1997
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| − | |url=http://www.astronomy.ohio-state.edu/~pogge/Lectures/vistas97.html
| + | |
| − | |title=The Once and Future Sun
| + | |
| − | |publisher=The Ohio State University (Department of Astronomy)
| + | |
| − | |format=lecture notes
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| − | |work=[http://www-astronomy.mps.ohio-state.edu/Vistas/ New Vistas in Astronomy]
| + | |
| − | |accessdate=2005-12-07}}</ref><ref name="Sackmann">{{cite journal
| + | |
| − | |last=Sackmann
| + | |
| − | |first=I.-Juliana
| + | |
| − | |coauthors=Arnold I. Boothroyd; Kathleen E. Kraemer
| + | |
| − | |year=1993
| + | |
| − | |month=11
| + | |
| − | |url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993ApJ%2E%2E%2E418%2E%2E457S&db_key=AST&high=24809&nosetcookie=1
| + | |
| − | |title=Our Sun. III. Present and Future
| + | |
| − | |journal=Astrophysical Journal
| + | |
| − | |volume=418
| + | |
| − | |pages=457}}</ref>
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| − | | + | |
| − | ==Structure==
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| − | [[Image:Solar internal structure.svg|thumb|right|300px|An illustration of the structure of the Sun]]
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| − | The Sun is a yellow dwarf star. It comprises approximately 99% of the total mass of the solar system. The Sun is a near-perfect [[sphere]], with an [[oblateness]] estimated at about 9 millionths,<ref name="Godier">{{cite journal
| + | |
| − | |last=Godier
| + | |
| − | |first=S.
| + | |
| − | |coauthors=Rozelot J.-P.
| + | |
| − | |year=2000
| + | |
| − | |url=http://aa.springer.de/papers/0355001/2300365.pdf
| + | |
| − | |format=PDF
| + | |
| − | |title=The solar oblateness and its relationship with the structure of the tachocline and of the Sun's subsurface
| + | |
| − | |journal=Astronomy and Astrophysics
| + | |
| − | |volume=355
| + | |
| − | |pages=365–374}}</ref> which means that its polar diameter differs from its equatorial diameter by only 10 km (6 mi). As the Sun exists in a [[Plasma (physics)|plasmatic state]] and is not solid, it undergoes differential [[Solar rotation|rotation]] as it spins on its [[Axis of rotation|axis]] (i.e. the Sun rotates faster at its [[equator]] than at its [[poles]]). The period of this ''actual rotation'' is approximately 25 days at the equator and 35 days at the poles. However, due to our constantly changing vantage point from the [[Earth]] as it orbits the Sun, the ''apparent rotation'' of the Sun at its equator is about 28 days. The centrifugal effect of this slow rotation is 18 million times weaker than the surface gravity at the Sun's equator. Also, the tidal effect from the planets does not significantly affect the shape of the Sun.
| + | |
| − | | + | |
| − | The Sun does not have a definite boundary as rocky planets do; in its outer parts the density of its gases drops approximately [[exponential distribution|exponentially]] with increasing distance from the center of the Sun. Nevertheless, the Sun has a well-defined interior structure, described below. The Sun's radius is measured from its center to the edge of the [[photosphere]]. This is simply the layer above which the gases are too cool or too thin to radiate a significant amount of light; the photosphere is the surface most readily visible to the [[naked eye]]. The solar core comprises 10 percent of its total volume, but 40 percent of its total mass.<ref>{{cite web | url=http://fusedweb.pppl.gov/CPEP/Chart_Pages/5.Plasmas/SunLayers.html | title=From Core to Corona: Layers of the Sun | author=Hannah Cohen | publisher=Princeton Plasma Physics Laboratory (PPPL) | date=[[2007-05-16]]}}</ref>
| + | |
| − | | + | |
| − | The solar interior is not directly observable, and the Sun itself is opaque to [[electromagnetic radiation]]. However, just as [[seismology]] uses waves generated by [[earthquake]]s to reveal the interior structure of the Earth, the discipline of [[helioseismology]] makes use of pressure waves ([[infrasound]]) traversing the Sun's interior to measure and visualize the Sun's inner structure. [[Computer modeling]] of the Sun is also used as a theoretical tool to investigate its deeper layers.
| + | |
| − | | + | |
| − | ===Core===
| + | |
| − | [[Image:Sun parts big.jpg|thumb|right|300px|Cross-section of a solar-type star. (NASA)]]
| + | |
| − | The [[Solar core| core]] of the Sun is considered to extend from the center to about 0.2 solar radii. It has a density of up to 150,000 kg/m³ (150 times the density of water on Earth) and a temperature of close to 13,600,000 [[kelvin]] (by contrast, the surface of the Sun is around 5,800 kelvin). Recent analysis of [[Solar and Heliospheric Observatory|SOHO]] mission data favors a faster rotation rate in the core than in the rest of the radiative zone.<ref>Garcia R. A. et al. "[http://www.sciencemag.org/cgi/content/short/316/5831/1591 Tracking Solar Gravity Modes: The Dynamics of the Solar Core]", ''Science'', '''316''', 5831, 1591 - 1593 (2007)</ref> Through most of the Sun's life, energy is produced by [[nuclear fusion]] through a series of steps called the [[Proton-proton chain reaction|p–p (proton–proton) chain]]; this process converts [[hydrogen]] into [[helium]]. The core is the only location in the Sun that produces an appreciable amount of [[heat]] via fusion: the rest of the star is heated by energy that is transferred outward from the core. All of the energy produced by fusion in the core must travel through many successive layers to the solar photosphere before it escapes into space as [[sunlight]] or [[kinetic energy]] of particles.
| + | |
| − | | + | |
| − | About 3.4{{e|38}} [[proton]]s (hydrogen nuclei) are converted into helium nuclei every second (out of ~8.9{{e|56}} total amount of free protons in the Sun), releasing energy at the matter–energy conversion rate of 4.26 million [[tonnes]] per second, 383 [[Yotta-|yotta]][[watt]]s (3.83{{e|26}} W) or 9.15{{e|10}} [[megaton]]s of [[Trinitrotoluene|TNT]] per second. This actually corresponds to a surprisingly low rate of energy production in the Sun's core—about 0.3 µW/cm³ (microwatts per cubic cm), or about 6 µW/kg of matter. For comparison, the human body produces heat at approximately the rate 1.2 W/kg, millions of times greater per unit mass. The use of plasma with similar parameters for energy production on Earth would be completely impractical—even a modest 1 GW fusion power plant would require about 170 billion tonnes of plasma occupying almost one cubic mile. Hence, terrestrial fusion reactors utilize far higher plasma temperatures than those in Sun's interior.
| + | |
| − | | + | |
| − | The rate of nuclear fusion depends strongly on density and temperature, so 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 [[thermal expansion|expand]] slightly against the [[weight]] of the outer layers, reducing the fusion rate and correcting the [[Perturbation (astronomy)|perturbation]]; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the fusion rate and again reverting it to its present level.
| + | |
| − | | + | |
| − | The high-energy [[photon]]s (gamma rays) released in [[nuclear fusion|fusion]] reactions are absorbed in only few millimetres of solar plasma and then re-emitted again in random direction (and at slightly lower energy)—so 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.<ref name="NASA">{{cite web
| + | |
| − | |url=http://sunearthday.nasa.gov/2007/locations/ttt_sunlight.php
| + | |
| − | |title=The 8-minute travel time to Earth by sunlight hides a thousand-year journey that actually began in the core
| + | |
| − | }}</ref>
| + | |
| − | | + | |
| − | After a final trip through the convective outer layer to the transparent "surface" of the photosphere, the photons escape as [[visible light]]. Each gamma ray in the Sun's core is converted into several million visible light photons before escaping into space. [[Neutrino]]s 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 [[Solar neutrino problem| lower than theories predicted]] by a factor of 3. This discrepancy was recently resolved through the discovery of the effects of [[neutrino oscillation]]: the Sun in fact emits the number of neutrinos predicted by the theory, but neutrino detectors were missing 2/3 of them because the neutrinos had changed [[flavor (particle physics)|flavor]].
| + | |
| − | | + | |
| − | ===Radiative zone===
| + | |
| − | From about 0.2 to about 0.7 solar radii, solar material is hot and dense enough that [[thermal radiation]] is sufficient to transfer the intense heat of the core outward. In this zone there is no thermal [[convection]]; while the material grows cooler as altitude increases, this [[temperature gradient]] is less than the value of [[adiabatic lapse rate]] and hence cannot drive convection. Heat is transferred by [[radiation]]—[[ions]] of [[hydrogen]] and [[helium]] emit [[photons]], which travel a brief distance before being reabsorbed by other ions. In this way energy makes its way very slowly (see above) outward.
| + | |
| − | | + | |
| − | Between the radiative zone and the convection zone is a transition layer called 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 -- i.e. a condition where successive vertical layers slide past one another.
| + | |
| − | | + | |
| − | ===Convection zone===
| + | |
| − | [[Image:SunLayers.png|thumb|right|220px|Structure of the Sun]]
| + | |
| − | In the Sun's outer layer (down to approximately 70% of the solar radius), the solar plasma is not dense enough or hot enough to transfer the heat energy of the interior outward via radiation. As a result, thermal convection occurs as [[thermal|thermal columns]] carry hot material to the surface (photosphere) of the Sun. Once the material cools off at the surface, it plunges back downward to the base of the convection zone, to receive more heat from the top of the radiative zone. [[Convective overshoot]] is thought to occur at the base of the convection zone, carrying turbulent downflows into the outer layers of the radiative zone.
| + | |
| − | | + | |
| − | The thermal columns in the convection zone form an imprint on the surface of the Sun, in the form of the [[granule (solar physics)|solar granulation]] and [[supergranulation]]. The turbulent convection of this outer part of the solar interior gives rise to a "small-scale" dynamo that produces magnetic north and south poles all over the surface of the Sun.
| + | |
| − | | + | |
| − | ===Photosphere===
| + | |
| − | The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opaque to visible light. Above the photosphere visible sunlight is free to propagate into space, and its energy escapes the Sun entirely.
| + | |
| − | The change in opacity is due to the decreasing amount of H<sup>-</sup> ions, which absorb visible light easily. Conversely, the visible light we see is produced as electrons react with [[hydrogen]] atoms to produce H<sup>-</sup> ions.<ref name="Gibson">{{cite book
| + | |
| − | |last=Gibson
| + | |
| − | |first=Edward G.
| + | |
| − | |year=1973
| + | |
| − | |title=The Quiet Sun
| + | |
| − | |publisher=NASA}}</ref><ref name="Shu">{{cite book
| + | |
| − | |last=Shu
| + | |
| − | |first=Frank H.
| + | |
| − | |year=1991
| + | |
| − | |title=The Physics of Astrophysics
| + | |
| − | |publisher=University Science Books}}</ref>
| + | |
| − | The photosphere is actually tens to hundreds of kilometers thick, being 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]]. Sunlight has approximately a [[black-body]] spectrum that indicates its temperature is about 6,000 [[kelvin|K]], interspersed with atomic [[absorption line]]s from the tenuous layers above the photosphere. The photosphere has a particle density of about 10<sup>23</sup> m<sup>−3</sup> (this is about 1% of the particle density of [[Earth's atmosphere]] at sea level).
| + | |
| − | | + | |
| − | During early studies of the [[optical spectrum]] of the photosphere, some absorption lines were found that did not correspond to any [[chemical element]]s then known on Earth. In 1868, [[Norman Lockyer]] hypothesized that these absorption lines were because of a new element which he dubbed "[[helium]]", after the Greek Sun god [[Helios]]. It was not until 25 years later that helium was isolated on Earth.<ref name="Lockyer">{{cite web
| + | |
| − | |url=http://www-solar.mcs.st-andrews.ac.uk/~clare/Lockyer/helium.html
| + | |
| − | |work=Solar and Magnetospheric MHD Theory Group
| + | |
| − | |publisher=University of St Andrews
| + | |
| − | |title=Discovery of Helium
| + | |
| − | |accessdate=2006-03-22}}</ref>
| + | |
| − | | + | |
| − | ===Atmosphere===
| + | |
| − | [[Image:Solar eclips 1999 4 NR.jpg|thumb|right|200px|During a total [[solar eclipse]], the solar [[corona]] can be seen with the naked eye.]]
| + | |
| − | The parts of the Sun above the photosphere are referred to collectively as the ''solar atmosphere''. They can be viewed with telescopes operating across the [[electromagnetic spectrum]], from radio through [[visible light]] to [[gamma rays]], and comprise five principal zones: the ''temperature minimum'', the [[chromosphere]], the [[solar transition region|transition region]], the [[corona]], and the [[heliosphere]]. The heliosphere, which may be considered the tenuous outer atmosphere of the Sun, extends outward past the orbit of [[Pluto]] to the [[heliopause]], where it forms a sharp [[shock wave|shock front]] boundary with the [[interstellar medium]]. The chromosphere, transition region, and corona are much hotter than the surface of the Sun. The reason why has not been conclusively proven; evidence suggests that [[Alfvén wave]]s may have enough energy to heat the corona.<ref>{{cite journal |last=De Pontieu |first=Bart |coauthors= ''et al'' |date=[[2007-12-07]] |title=Chromospheric Alfvénic Waves Strong Enough to Power the Solar Wind |journal=[[Science (journal)|Science]] |volume=318 |issue=5856 |pages=1574 - 77 |doi=10.1126/science.1151747 |url=http://www.sciencemag.org/cgi/content/full/318/5856/1574 |accessdate= 2008-01-22 }}</ref>
| + | |
| − | | + | |
| − | The coolest layer of the Sun is a temperature minimum region about 500 km above the photosphere, with a temperature of about 4,000 [[Kelvin|K]]. This part of the Sun is cool enough to support simple molecules such as [[carbon monoxide]] and water, which can be detected by their absorption spectra.
| + | |
| − | | + | |
| − | Above the temperature minimum layer is a thin layer about 2,000 km thick, dominated by a spectrum of emission and absorption lines. It is called the ''chromosphere'' from the Greek root ''chroma'', meaning color, because the chromosphere is visible as a colored flash at the beginning and end of [[solar eclipse|total eclipses of the Sun]]. The temperature in the chromosphere increases gradually with altitude, ranging up to around 100,000 K near the top.
| + | |
| − | | + | |
| − | [[Image:171879main LimbFlareJan12 lg.jpg|thumb|left|350px|Taken by [[Hinode]]'s Solar Optical Telescope on [[January 12]], [[2007]], this image of the Sun reveals the filamentary nature of the plasma connecting regions of different magnetic polarity.]]
| + | |
| − | | + | |
| − | Above the [[chromosphere]] is a [[solar transition region|transition region]] in which the temperature rises rapidly from around 100,000 [[kelvin|K]] to coronal temperatures closer to one million K. The increase is because of a [[phase transition]] as [[helium]] within the region becomes fully [[ionization|ionized]] by the high temperatures. The transition region does not occur at a well-defined altitude. Rather, it forms a kind of [[Halo (optical phenomenon)|nimbus]] around chromospheric features such as [[Spicule (solar physics)|spicule]]s and [[Solar filament|filament]]s, and is in constant, chaotic motion. The transition region is not easily visible from Earth's surface, but is readily observable from [[outer space|space]] by instruments sensitive to the [[ultraviolet|far ultraviolet]] portion of the [[electromagnetic spectrum|spectrum]].
| + | |
| − | | + | |
| − | The [[corona]] is the extended outer atmosphere of the Sun, which is much larger in volume than the Sun itself. The corona merges smoothly with the [[solar wind]] that fills the [[solar system]] and [[heliosphere]]. The low corona, which is very near the surface of the Sun, has a particle density of 10<sup>14</sup>–10<sup>16</sup> m<sup>−3</sup>. (Earth's atmosphere near sea level has a particle density of about 2{{e|25}} m<sup>−3</sup>.) The temperature of the corona is several million kelvin. While 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]].
| + | |
| − | | + | |
| − | The [[heliosphere]] extends from approximately 20 solar radii (0.1 AU) to the outer fringes of the solar system. Its inner boundary is defined as the layer in which the flow of the [[solar wind]] becomes ''superalfvénic''—that is, where the flow becomes faster than the speed of Alfvén waves. Turbulence and dynamic forces outside this boundary 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, forming the solar magnetic field into a [[Parker spiral|spiral]] shape, until it impacts the [[heliopause]] more than 50 AU from the Sun. In December 2004, the [[Voyager program|Voyager 1 probe]] passed through a shock front that is thought to be part of the heliopause. Both of the Voyager probes have recorded higher levels of energetic particles as they approach the boundary.<ref>{{cite web |url=http://www.spaceref.com/news/viewpr.html?pid=16394 |title=The Distortion of the Heliosphere: our Interstellar Magnetic Compass |date=[[2005-03-15]] |author=[[European Space Agency]] |accessdate=2006-03-22}}</ref>
| + | |
| − | | + | |
| − | == Chemical composition ==
| + | |
| − | | + | |
| − | The Sun is composed of [[chemical element]]s. Various scientists have analysed these elements to find out their abundances, their relations to planetary elements, and their diffusion (distribution) within the solar interior.
| + | |
| − | | + | |
| − | === Element abundances ===
| + | |
| − | | + | |
| − | According to Bahcal (1990)<ref name="bahcal1990">Bahcall, J. N. 1990, Neutrino Astrophysics (Cambridge University Press, Cambridge)</ref> cited in Thoul (1993:15),<ref name="thoul1993">Thoul ''et al'' 1993: http://arxiv.org/abs/astro-ph/9304005</ref> the characteristic mass fractions of some elements in the solar interior (r < 0.7R) are:
| + | |
| − | * [[Hydrogen]]: 34[[percentage|%]]
| + | |
| − | * [[Helium]]: 64%
| + | |
| − | * [[Oxygen]]: 1%
| + | |
| − | | + | |
| − | ==== Lithium, Beryllium, and Boron ====
| + | |
| − | | + | |
| − | In 1968, a [[Belgium|Belgian]] academic found that the abundances of [[lithium]], [[beryllium]], and [[boron]] are higher than previously thought (Grevesse 1968<ref name="grevesse1968">Nicolas Grevesse 1968, Solar abundances of lithium, beryllium and boron, Solar Physics Journal, Volume 5, Number 2 / October, 1968, DOI 10.1007/BF00147963, pp 159-180, Springer Netherlands, ISSN 0038-0938 (Print) ISSN 1573-093X (Online), http://www.springerlink.com/content/l37qghqnm7345247/</ref>).
| + | |
| − | | + | |
| − | ==== Neon ====
| + | |
| − | | + | |
| − | In 2005, three academics claimed that the [[neon]] abundance in the Sun may be higher than previously thought, based on [[helioseismology|helioseismological]] observations (Bahcall ''et al'' 2005<ref name="bahcall-etal2005">Bahcall John N., Basu Sarbani, Sereneli Aldo M. 2005: What Is the Neon Abundance of the Sun?, The Astrophysical Journal, 631:1281–1285, 2005 October 1, DOI: 10.1086/431926, The American Astronomical Society (USA), http://www.journals.uchicago.edu/doi/abs/10.1086/431926</ref>).
| + | |
| − | | + | |
| − | ==== Helium ====
| + | |
| − | | + | |
| − | Until at least 1986 the generally accepted initial helium content of the Sun was Y=0.25, but two academics in 1986 claimed that the value Y=0.279 is more correct (Lebreton and Maeder 1986:119<ref name="lebreton-maeder1986">Lebreton, Y. & Maeder, A. (1986), The evolution and helium content of the sun, Astronomy and Astrophysics (ISSN 0004-6361), vol. 161, no. 1, June 1986, p. 119-124., http://articles.adsabs.harvard.edu//full/1986A%26A...161..119L/0000119.000.html</ref>).
| + | |
| − | | + | |
| − | ==== Singly-ionised iron group elements ====
| + | |
| − | | + | |
| − | In 1970s, much research focused on the abundances of [[iron group]] elements in the Sun(Biemont 1978;<ref name="biemont1978">Biemont Emile, 1978: Abundances of singly-ionized elements of the iron group in the sun, Royal Astronomical Society, Monthly Notices, vol. 184, Sept. 1978, p. 683-694, http://adsabs.harvard.edu/abs/1978MNRAS.184..683B</ref> and Ross and Aller 1976, Withbroe 1976, Hauge and Engvold 1977, cited in Biemont 1978<ref name="biemont1978"/>).
| + | |
| − | | + | |
| − | The first largely complete set of ''gf'' values of singly-ionised iron group elements were made available first by Corliss and Bozman (1962 cited in Biemont 1978<ref name="biemont1978"/>) and Warner (1967 cited in Biemont 1978<ref name="biemont1978"/>), and improved ''f'' values were computed by Smith (1976 cited in Biemont 1978<ref name="biemont1978"/>). In 1978 the abundances of [[singly-ionised]] elements of the iron group were derived by Biemont (1978<ref name="biemont1978"/>).
| + | |
| − | | + | |
| − | The abundance determination of some iron group elements is difficult because of their hyperfine structures, eg [[cobalt]] and [[manganese]] (Biemont 1978<ref name="biemont1978"/>).
| + | |
| − | | + | |
| − | === Solar and planetary mass fractionation relationship ===
| + | |
| − | | + | |
| − | Various authors have considered the existence of a mass fractionation relationship between the isotopic compositions of solar and planetary [[noble gas]]es (Signer and Suess 1963; Manuel 1967; Marti 1969; Kuroda and Manuel 1970;
| + | |
| − | Srinivasan and Manuel 1971, all cited in Manuel and Hwaung 1983<ref name="manuel1983"/>), for example correlations between isotopic compositions of planetary and solar [[Ne]] and [[Xe]] (Kuroda and Manuel 1970 cited in Manuel and Hwaung 1983:7<ref name="manuel1983"/>). Nevertheless, the belief that the whole Sun has the same composition as the solar atmosphere was still widespread, at least until 1983 (Manuel and Hwaung 1983:7<ref name="manuel1983"/>).
| + | |
| − | | + | |
| − | In 1983, two academics claimed that it was the fractionation in the Sun itself that caused the fractionation relationship between the isotopic compositions of planetary and solar wind implanted noble gases (Manuel and Hwaung 1983:7<ref name="manuel1983"/>).
| + | |
| − | | + | |
| − | === Element diffusion ===
| + | |
| − | | + | |
| − | The Sun is composed of [[chemical element]]s. Of particular scientific interest is the '''diffusion''' of these elements inside the Sun, i.e. their distribution inside the star's interior. The diffusion of solar elements is determined by many variables, including [[gravity]], which causes the heavier elements (e.g. [[helium]] in absence of other heavier elements) to stick to the centre of the solar mass while the non-heavy elements (e.g. [[hydrogen]]) diffuse towards the exterior of the Sun (Thoul ''et al'' 1993:3).<ref name="thoul1993"/>
| + | |
| − | | + | |
| − | ==== Helium diffusion ====
| + | |
| − | | + | |
| − | Of specialist scientific interest is the diffusion of [[helium]] in the solar interior. It has been found that the diffusion process of helium speeds up with time (Noerdlinger 1977<ref name="noerdlinger1977">Noerdlinger, P. D., Diffusion of helium in the Sun, Astronomy and Astrophysics, vol. 57, no. 3, May 1977, p. 407-415, online: http://adsabs.harvard.edu/full/1977A&A....57..407N</ref>).
| + | |
| − | | + | |
| − | === Composition of the photosphere ===
| + | |
| − | | + | |
| − | The composition of the [[photosphere]], ie the surface layers of the Sun, is usually taken as representative of the [[chemical composition of the primordial solar system]], except for [[deuterium]], [[lithium]], [[boron]], and [[beryllium]] (Aller 1968<ref name="aller1968">Aller L. H. (1968): The chemical composition of the Sun and the solar system, Proceedings of the Astronomical Society of Australia, Vol. 1, p.133, http://adsabs.harvard.edu/full/1968PASAu...1..133A</ref>).
| + | |
| − | | + | |
| − | ==Solar cycles==
| + | |
| − | {{Main|Sunspots}}
| + | |
| − | ===Sunspots and the sunspot cycle===
| + | |
| − | | + | |
| − | [[Image:Solar-cycle-data.png|thumb|right|250px|Measurements of solar cycle variation during the last 30 years.]]
| + | |
| − | When observing the Sun with appropriate filtration, the most immediately visible features are usually its [[sunspot]]s, which are well-defined surface areas that appear darker than their surroundings because of lower temperatures. Sunspots are regions of intense magnetic activity where [[convection]] is inhibited by strong magnetic fields, reducing energy transport from the hot interior to the surface. The magnetic field gives rise to strong heating in the corona, forming [[active region]]s that are the source of intense [[solar flare]]s and [[coronal mass ejection]]s. The largest sunspots can be tens of thousands of kilometers across.
| + | |
| − | | + | |
| − | The number of sunspots visible on the Sun is not constant, but varies over an 11-year cycle known as the [[Solar cycle]]. At a typical solar minimum, few sunspots are visible, and occasionally none at all can be seen. Those that do appear are at high solar latitudes. As the sunspot cycle progresses, the number of sunspots increases and they move closer to the equator of the Sun, a phenomenon described by [[Spörer's law]]. Sunspots usually exist as pairs with opposite magnetic polarity. The magnetic polarity of the leading sunspot alternates every solar cycle, so that it will be a north magnetic pole in one solar cycle and a south magnetic pole in the next.
| + | |
| − | [[Image:Sunspot-number.png|thumb|right|250px|History of the number of observed sunspots during the last 250 years, which shows the ~11-year solar cycle.]]
| + | |
| − | | + | |
| − | The solar cycle has a great influence on [[space weather]], and is a significant influence on the Earth's climate. Solar activity minima tend to be correlated with colder temperatures, and longer than average solar cycles tend to be correlated with hotter temperatures. In the 17th century, the solar cycle appears to have stopped entirely for several decades; very few sunspots were observed during this period. During this era, which is known as the [[Maunder minimum]] or [[Little Ice Age]], Europe experienced very cold temperatures.<ref name="Lean">{{cite journal
| + | |
| − | |last=Lean
| + | |
| − | |first=J.
| + | |
| − | |coauthors=Skumanich A.; White O.
| + | |
| − | |year=1992
| + | |
| − | |title=Estimating the Sun's radiative output during the Maunder Minimum
| + | |
| − | |journal=Geophysical Research Letters
| + | |
| − | |volume=19
| + | |
| − | |pages=1591–1594}}</ref> Earlier extended minima have been discovered through analysis of [[tree ring]]s and also appear to have coincided with lower-than-average global temperatures.
| + | |
| − | | + | |
| − | ===Possible long term cycle===
| + | |
| − | A recent theory claims that there are magnetic instabilities in the core of the Sun which cause fluctuations with periods of either 41,000 or 100,000 years. These could provide a better explanation of the [[ice age]]s than the [[Milankovitch cycles]]. Like many theories in astrophysics, this theory cannot be tested directly.<ref>
| + | |
| − | {{cite journal
| + | |
| − | |last=Ehrlich
| + | |
| − | |first=Robert
| + | |
| − | |year=2007
| + | |
| − | |title=Solar Resonant Diffusion Waves as a Driver of Terrestrial Climate Change
| + | |
| − | |journal=Journal of Atmospheric and Solar-Terrestrial Physics
| + | |
| − | |url=http://arxiv.org/abs/astro-ph/0701117}}</ref><ref>{{cite journal
| + | |
| − | |year=2007
| + | |
| − | |title=Sun's fickle heart may leave us cold
| + | |
| − | |journal=[[New Scientist]]
| + | |
| − | |volume=2588
| + | |
| − | |pages=12
| + | |
| − | |date=27 Jan. 2007
| + | |
| − | |url=http://environment.newscientist.com/channel/earth/mg19325884.500-suns-fickle-heart-may-leave-us-cold.html}}
| + | |
| − | </ref>
| + | |
| − | | + | |
| − | ==Theoretical problems==
| + | |
| − | ===Solar neutrino problem===
| + | |
| − | | + | |
| − | For many years the number of solar [[electron neutrino]]s detected on Earth was one third to one half of the number predicted by the [[standard solar model]]. This anomalous result was termed the '''[[solar neutrino problem]]'''. Theories proposed to resolve the problem either tried to reduce the temperature of the Sun's interior to explain the lower neutrino flux, or posited that electron neutrinos could [[neutrino oscillation|oscillate]]—that is, change into undetectable [[tau neutrino|tau]] and [[muon neutrino]]s as they traveled between the Sun and the Earth.<ref name="Haxton">{{cite journal
| + | |
| − | |last=Haxton
| + | |
| − | |first=W. C.
| + | |
| − | |year=1995
| + | |
| − | |url=http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1995ARA%26A..33..459H&data_type=PDF_HIGH&type=PRINTER&filetype=.pdf
| + | |
| − | |format=PDF
| + | |
| − | |title=The Solar Neutrino Problem
| + | |
| − | |journal=Annual Review of Astronomy and Astrophysics
| + | |
| − | |volume=33
| + | |
| − | |pages=459–504}}</ref> Several neutrino observatories were built in the 1980s to measure the solar neutrino flux as accurately as possible, including the [[Sudbury Neutrino Observatory]] and [[Kamiokande]]. Results from these observatories eventually led to the discovery that neutrinos have a very small [[rest mass]] and do indeed oscillate.<ref name="Schlattl">{{cite journal
| + | |
| − | |last=Schlattl
| + | |
| − | |first=H.
| + | |
| − | |year=2001
| + | |
| − | |title=Three-flavor oscillation solutions for the solar neutrino problem
| + | |
| − | |journal=Physical Review D
| + | |
| − | |volume=64
| + | |
| − | |issue=1}}</ref> Moreover, in 2001 the Sudbury Neutrino Observatory was able to detect all three types of neutrinos directly, and found that the Sun's ''total'' neutrino emission rate agreed with the Standard Solar Model, although depending on the neutrino energy as few as one-third of the neutrinos seen at Earth are of the electron type. This proportion agrees with that predicted by the [[MSW effect|Mikheyev-Smirnov-Wolfenstein effect]] (also known as the matter effect), which describes neutrino oscillation in matter. Hence, the problem is now resolved.
| + | |
| − | | + | |
| − | ===Coronal heating problem===
| + | |
| − | The optical surface of the Sun (the [[photosphere]]) is known to have a temperature of approximately 6,000 [[Kelvin|K]]. Above it lies the solar corona at a temperature of 1,000,000 K. The high temperature of the corona shows that it is heated by something other than direct heat [[Heat conduction|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 and 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 gas in the form of heat. The other is [[magnetic field|magnetic]] heating, in which magnetic energy is continuously built up by photospheric motion and released through [[magnetic reconnection]] in the form of large [[solar flare]]s and myriad similar but smaller events.<ref name="Alfven">{{cite journal
| + | |
| − | |last=Alfvén
| + | |
| − | |first=H.
| + | |
| − | |year=1947
| + | |
| − | |title=Magneto-hydrodynamic waves, and the heating of the solar corona
| + | |
| − | |journal=Monthly Notices of the Royal Astronomical Society
| + | |
| − | |volume=107
| + | |
| − | |pages=211}}</ref>
| + | |
| − | | + | |
| − | Currently, it is unclear whether waves are an efficient heating mechanism. All waves except [[Alfvén wave]]s have been found to dissipate or refract before reaching the corona.<ref name="Sturrock">{{cite journal
| + | |
| − | |last=Sturrock
| + | |
| − | |first=P. A.
| + | |
| − | |coauthors=Uchida, Y.
| + | |
| − | |year=1981
| + | |
| − | |url=http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1981ApJ...246..331S&data_type=PDF_HIGH&type=PRINTER&filetype=.pdf
| + | |
| − | |format=PDF
| + | |
| − | |title=Coronal heating by stochastic magnetic pumping
| + | |
| − | |journal=Astrophysical Journal
| + | |
| − | |volume=246
| + | |
| − | |pages=331}}</ref> In addition, Alfvén waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms. One possible candidate to explain coronal heating is continuous flaring at small scales,<ref name="Parker2">{{cite journal
| + | |
| − | |last=Parker
| + | |
| − | |first=E. N.
| + | |
| − | |year=1988
| + | |
| − | |url=http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1988ApJ...330..474P&data_type=PDF_HIGH&type=PRINTER&filetype=.pdf
| + | |
| − | |format=PDF
| + | |
| − | |title=Nanoflares and the solar X-ray corona
| + | |
| − | |journal=Astrophysical Journal
| + | |
| − | |volume=330
| + | |
| − | |pages=474}}</ref> but this remains an open topic of investigation.
| + | |
| − | | + | |
| − | ===Faint young Sun problem===
| + | |
| − | {{main|Faint young Sun paradox}}
| + | |
| − | | + | |
| − | Theoretical models of the Sun's development suggest that 3.8 to 2.5 billion years ago, during the [[Archean|Archean period]], 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 the Earth's surface, and thus life should not have been able to develop. However, the geological record demonstrates that the Earth has remained at a fairly constant temperature throughout its history, and in fact that the young Earth was somewhat warmer than it is today. The consensus among scientists is that the young Earth's atmosphere contained much larger quantities of [[greenhouse gas]]es (such as [[carbon dioxide]], [[methane]] and/or [[ammonia]]) than are present today, which trapped enough heat to compensate for the lesser amount of solar energy reaching the planet.<ref name="Kasting">{{cite journal
| + | |
| − | |last=Kasting
| + | |
| − | |first=J. F.
| + | |
| − | |coauthors=Ackerman, T. P.
| + | |
| − | |year=1986
| + | |
| − | |title=Climatic Consequences of Very High Carbon Dioxide Levels in the Earth’s Early Atmosphere
| + | |
| − | |journal=Science
| + | |
| − | |volume=234
| + | |
| − | |pages=1383–1385}}</ref>
| + | |
| − | | + | |
| − | ==Magnetic field==
| + | |
| − | {{seealso|Stellar magnetic field}}
| + | |
| − | [[Image:Heliospheric-current-sheet.gif|thumb|right|220px|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 (physics)|plasma]] in the [[interplanetary medium]].<ref>{{cite web |url=http://wso.stanford.edu/#MeanField |title=The Mean Magnetic Field of the Sun |publisher=The Wilcox Solar Observatory |accessdate=2007-08-01}}</ref>]]
| + | |
| − | | + | |
| − | All matter in the Sun is in the form of [[gas]] and [[plasma (physics)|plasma]] because of its high temperatures. This makes it possible for the Sun to rotate faster at its equator (about 25 days) than it does at higher latitudes (about 35 days near its poles). The [[solar rotation|differential rotation]] of the Sun's latitudes causes its [[magnetic field]] lines to become twisted together over time, causing [[Coronal loop|magnetic field loops]] to erupt from the Sun's surface and trigger the formation of the Sun's dramatic [[sunspot]]s and [[solar prominence]]s (see [[magnetic reconnection]]). This twisting action gives rise to the [[solar dynamo]] and an 11-year [[sunspot cycle|solar cycle]] of magnetic activity as the Sun's magnetic field reverses itself about every 11 years.
| + | |
| − | | + | |
| − | The influence of the Sun's rotating magnetic field on the plasma in the [[interplanetary medium]] creates the [[heliospheric current sheet]], which separates regions with magnetic fields pointing in different directions. The plasma in the interplanetary medium is also responsible for the strength of the Sun's magnetic field at the orbit of the Earth. If space were a vacuum, then the Sun's 10<sup>-4</sup> [[tesla (unit)|tesla]] magnetic dipole field would reduce with the cube of the distance to about 10<sup>-11</sup> tesla. But satellite observations show that it is about 100 times greater at around 10<sup>-9</sup> tesla. [[Magnetohydrodynamic]] (MHD) theory predicts that the motion of a conducting fluid (e.g., the interplanetary medium) in a magnetic field, induces electric currents which in turn generates magnetic fields, and in this respect it behaves like an [[MHD dynamo]].
| + | |
| − | | + | |
| − | == History of observation ==
| + | |
| − | === Early understanding ===
| + | |
| − | [[Image:Solvogn.jpg|thumb|200px|left|The [[Trundholm Sun chariot]] pulled by a horse is a sculpture believed to be illustrating an important part of [[Nordic Bronze Age]] mythology.]]
| + | |
| − | | + | |
| − | 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]] phenomenon. [[Sun worship|Worship]] of the Sun was central to civilizations such as the [[Inca]] of [[South America]] and the [[Aztec]]s of what is now [[Mexico]]. Many ancient monuments were constructed with solar phenomena in mind; for example, stone [[megalith]]s accurately mark the summer [[solstice]] (some of the most prominent megaliths are located in [[Nabta Playa]], [[Egypt]], and at [[Stonehenge]], [[England]]); [[Newgrange]], a prehistoric human-built mount in [[Ireland]], was designed to detect the winter solstice; the pyramid of [[El Castillo, Chichen Itza|El Castillo]] at [[Chichén Itzá]] in Mexico is designed to cast shadows in the shape of serpents climbing the [[pyramid]] at the vernal and autumn [[equinox]]es. With respect to the [[fixed star]]s, the Sun appears from Earth to revolve once a year along the [[ecliptic]] through the [[zodiac]], and so Greek astronomers considered it to be one of the seven [[planet]]s (Greek ''planetes'', "wanderer"), after which the seven days of the [[week]] are named in some languages.
| + | |
| − | | + | |
| − | === Development of scientific understanding ===
| + | |
| − | | + | |
| − | One of the first people to offer a scientific explanation for the Sun was the [[Ancient Greece|Greek]] [[philosopher]] [[Anaxagoras]], who reasoned that it was a giant flaming ball of metal even larger than the [[Peloponnese|Peloponnesus]], and not the [[chariot]] of [[Helios]]. For teaching this [[heresy]], he was imprisoned by the authorities and [[capital punishment|sentenced to death]], though he was later released through the intervention of [[Pericles]]. [[Eratosthenes]] might have been the first person to have accurately calculated the distance from the Earth to the Sun, in the 3rd century [[Common Era|BCE]], as 149 million kilometers, roughly the same as the modern accepted figure.
| + | |
| − | | + | |
| − | The theory that the Sun is the center around which the planets move was apparently proposed by the ancient Greek [[Aristarchus of Samos|Aristarchus]] and Indians (see [[Heliocentrism]]). This view was revived in the 16th century by [[Nicolaus Copernicus]]. In the early 17th century, the invention of the [[telescope]] permitted detailed observations of sunspots by [[Thomas Harriot]], [[Galileo Galilei]] and other astronomers. Galileo made some of the first known Western observations of sunspots and posited that they were on the surface of the Sun rather than small objects passing between the Earth and the Sun.<ref>{{cite web
| + | |
| − | |url=http://www.bbc.co.uk/history/historic_figures/galilei_galileo.shtml
| + | |
| − | |title=Galileo Galilei (1564–1642)
| + | |
| − | |publisher=BBC
| + | |
| − | |accessdate=2006-03-22}}</ref> Sunspots were also observed since the [[Han dynasty]] and Chinese astronomers maintained records of these observations for centuries.
| + | |
| − | In 1672 [[Giovanni Cassini]] and [[Jean Richer]] determined the distance to [[Mars]] and were thereby able to calculate the distance to the Sun.
| + | |
| − | [[Isaac Newton]] observed the Sun's light using a [[prism (optics)|prism]], and showed that it was made up of light of many colors,<ref>{{cite web
| + | |
| − | |url=http://www.bbc.co.uk/history/historic_figures/newton_isaac.shtml
| + | |
| − | |title=Sir Isaac Newton (1643–1727)
| + | |
| − | |publisher=BBC
| + | |
| − | |accessdate=2006-03-22}}</ref> while in 1800 [[William Herschel]] discovered [[infrared]] radiation beyond the red part of the solar spectrum.<ref>{{cite web
| + | |
| − | |url=http://coolcosmos.ipac.caltech.edu/cosmic_classroom/classroom_activities/herschel_bio.html
| + | |
| − | |title=Herschel Discovers Infrared Light
| + | |
| − | |publisher=Cool Cosmos
| + | |
| − | |accessdate=2006-03-22}}</ref> The 1800s saw spectroscopic studies of the Sun advance, and [[Joseph von Fraunhofer]] made the first observations of [[absorption lines]] in the spectrum, the strongest of which are still often referred to as Fraunhofer lines. When expanding the spectrum of light from the Sun, there are large number of missing colors can be found.
| + | |
| − | | + | |
| − | 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 was a gradually cooling liquid body that was radiating an internal store of heat.<ref>{{cite journal
| + | |
| − | |last=Thomson
| + | |
| − | |first=Sir William
| + | |
| − | |title=On the Age of the Sun’s Heat
| + | |
| − | |journal=Macmillan's Magazine
| + | |
| − | |year=1862
| + | |
| − | |volume=5
| + | |
| − | |pages=288–293
| + | |
| − | |url=http://zapatopi.net/kelvin/papers/on_the_age_of_the_suns_heat.html}}</ref> Kelvin and [[Hermann von Helmholtz]] then proposed the [[Kelvin-Helmholtz mechanism]] to explain the energy output. Unfortunately the resulting age estimate was only 20 million years, well short of the time span of several billion years suggested by geology. In 1890 [[Joseph Norman Lockyer|Joseph Lockyer]], who discovered helium in the solar spectrum, proposed a meteoritic hypothesis for the formation and evolution of the Sun.<ref>{{cite book
| + | |
| − | |last=Lockyer
| + | |
| − | |first=Joseph Norman
| + | |
| − | |title=The meteoritic hypothesis; a statement of the results of a spectroscopic inquiry into the origin of cosmical systems
| + | |
| − | |publisher=Macmillan and Co.
| + | |
| − | |location=London and New York
| + | |
| − | |year=1890
| + | |
| − | |url=http://adsabs.harvard.edu/abs/1890QB981.L78......}}</ref>
| + | |
| − | | + | |
| − | Not until 1904 was a substantiated 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.<ref>{{cite web
| + | |
| − | |last=Darden
| + | |
| − | |first=Lindley
| + | |
| − | |year=1998
| + | |
| − | |title=The Nature of Scientific Inquiry
| + | |
| − | |journal=Macmillan's Magazine
| + | |
| − | |url=http://www.philosophy.umd.edu/Faculty/LDarden/sciinq/}}</ref> 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 ''E'' = ''mc''².
| + | |
| − | | + | |
| − | 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.<ref>{{cite web
| + | |
| − | |date=June 15, 2005
| + | |
| − | |accessdate=2007-08-01
| + | |
| − | |title=Studying the stars, testing relativity: Sir Arthur Eddington
| + | |
| − | |publisher=ESA Space Science
| + | |
| − | |url=http://www.esa.int/esaSC/SEMDYPXO4HD_index_0.html}}</ref> The preponderance of hydrogen in the Sun was confirmed in 1925 by [[Cecilia Payne-Gaposchkin|Cecilia Payne]]. 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.<ref name="Bethe">{{cite journal
| + | |
| − | |last=Bethe
| + | |
| − | |first=H.
| + | |
| − | |year=1938
| + | |
| − | |title=On the Formation of Deuterons by Proton Combination
| + | |
| − | |journal=Physical Review
| + | |
| − | |volume=54
| + | |
| − | |pages=862–862}}</ref><ref name="Bethe2">{{cite journal
| + | |
| − | |last=Bethe
| + | |
| − | |first=H.
| + | |
| − | |year=1939
| + | |
| − | |title=Energy Production in Stars
| + | |
| − | |journal=Physical Review
| + | |
| − | |volume=55
| + | |
| − | |pages=434–456}}</ref>
| + | |
| − | | + | |
| − | Finally, a seminal paper was published in 1957 by [[Margaret Burbidge]], entitled "Synthesis of the Elements in Stars".<ref>{{cite journal
| + | |
| − | |author=E. Margaret Burbidge; G. R. Burbidge; William A. Fowler; F. Hoyle
| + | |
| − | |title=Synthesis of the Elements in Stars
| + | |
| − | |journal=Reviews of Modern Physics
| + | |
| − | |year=1957
| + | |
| − | |volume=29
| + | |
| − | |issue=4
| + | |
| − | |pages=547–650
| + | |
| − | |url=http://adsabs.harvard.edu/abs/1957RvMP...29..547B}}</ref> The paper demonstrated convincingly that most of the elements in the universe had been [[nucleosynthesis|synthesized]] by nuclear reactions inside stars, some like our Sun. This revelation stands today as one of the great achievements of science.
| + | |
| − | | + | |
| − | === Solar space missions ===
| + | |
| − | [[Image:I screenimage 30579.jpg|thumb|200px|right|Solar "[[Coronal mass ejection|fireworks]]" in sequence as recorded in November 2000 by four instruments onboard the [[Solar and heliospheric observatory|SOHO]] spacecraft.]]
| + | |
| − | | + | |
| − | The first satellites designed to observe the Sun were [[NASA]]'s [[Pioneer program|Pioneer]]s 5, 6, 7, 8 and 9, which were launched between 1959 and 1968. These probes orbited the Sun at a distance similar to that of the [[Earth]], and made the first detailed measurements of the solar wind and the solar magnetic field. Pioneer 9 operated for a particularly long period of time, transmitting data until 1987.<ref>{{cite web
| + | |
| − | |url=http://www.astronautix.com/craft/pio6789e.htm
| + | |
| − | |publisher=Encyclopedia Astronautica
| + | |
| − | |title=Pioneer 6-7-8-9-E
| + | |
| − | |accessdate=2006-03-22}}</ref>
| + | |
| − | | + | |
| − | In the 1970s, [[Helios probes|Helios 1]] and the [[Skylab]] [[Apollo Telescope Mount]] provided scientists with significant new data on solar wind and the solar corona. The Helios 1 satellite was a joint [[United States|U.S.]]-[[Federal Republic of Germany|German]] probe that studied the solar wind from an orbit carrying the spacecraft inside [[Mercury (planet)|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 ejection]]s, then called "coronal transients", and of [[coronal hole]]s, now known to be intimately associated with the [[solar wind]].
| + | |
| − | | + | |
| − | In 1980, the [[Solar Maximum Mission]] was launched by [[NASA]]. This spacecraft was designed to observe [[gamma ray]]s, [[X-ray]]s and [[UV]] radiation from [[solar flare]]s during a time of high solar activity. 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 [[reentry|re-entering]] the Earth's atmosphere in June 1989.<ref>{{cite web
| + | |
| − | |url=http://web.hao.ucar.edu/public/research/svosa/smm/smm_mission.html
| + | |
| − | |title=Solar Maximum Mission Overview
| + | |
| − | |first=Chris
| + | |
| − | |last=St. Cyr
| + | |
| − | |coauthors=Joan Burkepile
| + | |
| − | |accessdate=2006-03-22
| + | |
| − | |year=1998}}</ref>
| + | |
| − | | + | |
| − | [[Japan]]'s [[Yohkoh]] (''Sunbeam'') satellite, launched in 1991, observed solar flares at X-ray wavelengths. Mission data allowed scientists to identify several different types of flares, and also 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 [[solar eclipse|annular eclipse]] in 2001 caused it to lose its lock on the Sun. It was destroyed by atmospheric reentry in 2005.<ref>{{cite web
| + | |
| − | |url=http://www.jaxa.jp/press/2005/09/20050913_yohkoh_e.html
| + | |
| − | |title=Result of Re-entry of the Solar X-ray Observatory "Yohkoh" (SOLAR-A) to the Earth's Atmosphere
| + | |
| − | |year= 2005
| + | |
| − | |author=Japan Aerospace Exploration Agency
| + | |
| − | |accessdate=2006-03-22}}</ref>
| + | |
| − | | + | |
| − | 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 [[December 2]], [[1995]]. Originally a two-year mission, SOHO has now operated for over ten years (as of 2007). It has proved so useful that a follow-on mission, the [[Solar Dynamics Observatory]], is planned for launch in 2008. Situated at the [[Lagrangian point]] between the 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. In addition to its direct solar observation, SOHO has enabled the discovery of large numbers of comets, mostly very tiny [[sungrazing comet]]s which incinerate as they pass the Sun.<ref>{{cite web
| + | |
| − | |url=http://ares.nrl.navy.mil/sungrazer/
| + | |
| − | |title=SOHO Comets
| + | |
| − | |work=Large Angle and Spectrometric Coronagraph Experiment (LASCO)
| + | |
| − | |publisher=U.S. Naval Research Laboratory
| + | |
| − | |accessdate=2006-03-22}}</ref>
| + | |
| − | | + | |
| − | [[Image:174719main LEFTREDSouthPole304.jpg|thumb|200px|right|The Sun's south pole, taken by [[STEREO]] solar observation mission. Material can be seen erupting off the Sun in the lower right side of the image.]]
| + | |
| − | | + | |
| − | 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' past the planet into an orbit which would take it far above the plane of the ecliptic. Serendipitously, it was well-placed to observe the collision of [[Comet Shoemaker-Levy 9]] with Jupiter in 1994. 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 which scattered galactic [[cosmic ray]]s.<ref>{{cite web
| + | |
| − | |url=http://ulysses.jpl.nasa.gov/science/mission_primary.html
| + | |
| − | |title=Ulysses - Science - Primary Mission Results
| + | |
| − | |publisher=NASA
| + | |
| − | |accessdate=2006-03-22}}</ref>
| + | |
| − | | + | |
| − | Elemental abundances in the photosphere are well known from [[astronomical spectroscopy|spectroscopic]] studies, but the composition of the interior of the Sun is more poorly understood. A [[solar wind]] sample return mission, [[Genesis (spacecraft)|Genesis]], was designed to allow astronomers to directly measure the composition of solar material. Genesis returned to Earth in 2004 but was damaged by a crash landing after its [[parachute]] failed to deploy on reentry into Earth's atmosphere. Despite severe damage, some usable samples have been recovered from the spacecraft's sample return module and are undergoing analysis.
| + | |
| − | | + | |
| − | 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 the Earth. This enables [[stereoscopic]] imaging of the Sun and solar phenomena, such as [[coronal mass ejections]].
| + | |
| − | | + | |
| − | If one were to observe it from [[Alpha Centauri]], the closest star system, the Sun would appear to be in the constellation [[Cassiopeia (constellation)|Cassiopeia]].
| + | |
| − | | + | |
| − | == Observation and eye damage ==
| + | |
| − | | + | |
| − | [[Image:The sun1.jpg|thumb|right|The Sun as it appears through a camera [[Photographic lens|lens]] from the surface of Earth]]
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| − | Sunlight is very bright, and looking directly at the Sun with the [[naked eye]] for brief periods can be painful, but is not particularly hazardous for normal, non-dilated eyes.<ref>{{cite journal|title=Chorioretinal temperature increases from solar observation | author=T.J. White, M.A. Mainster, P.W. Wilson, and J.H.Tips | journal=Bulletin of Mathematical Biophysics | volume=33 | pages=1 | year=1971 | doi = 10.1007/BF02476660 <!--Retrieved from CrossRef by DOI bot-->}}</ref><ref>{{cite journal | author="M.O.M. Tso and F.G. La Piana | title=The Human Fovea After Sungazing | journal=Transactions of the American Academy of Ophthalmology & Otolaryngology | volume=79 | pages=OP-788 | year=1975}}</ref> 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.<ref>{{cite journal| title=Ultrastructural findings in solar retinopathy | author=Hopeross, M. W. | publication=EYE | volume=7 | page=29 | year=1993 }}</ref><ref>{{cite journal| title=Solar Retinopathy from Sun-Gazing Under Influence of LSD | author=Schatz, H. & Mendelbl, F. | publication=British Journal of Ophthalmology | volume=57 (4) | page=270 | year=1973}} </ref> [[ultraviolet|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, not on whether one looks directly at the Sun.<ref>{{cite journal | url=http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/safety2.html | title=Eye Safety During Solar Eclipses | author=Chou, B. Ralph, MSc, OD | publication=NASA RP 1383: Total Solar Eclipse of 1999 August 11 | month=April | year=1997 | page=19}}"''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.''"</ref> 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;<ref>{{cite journal | author=W.T. Ham Jr., H.A. Mueller, and D.H. Sliney | journal=Nature | title=Retinal sensitivity to damage from short wavelength light | volume=260 | pages=153}}</ref><ref>{{cite journal | author=W.T. Ham Jr., H.A. Mueller, J.J. Ruffolo Jr., and D. Guerry III | title=Solar Retinopathy as a function of Wavelength: its Significance for Protective Eyewear | journal="The Effects of Constant Light on Visual Processes", edited by T.P. Williams and B.N. Baker | publisher=Plenum Press, New York | year=1980 | pages=319-346}}</ref> 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.
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| − | Viewing the Sun through light-concentrating [[optics]] such as [[binoculars]] is very hazardous without an appropriate filter that blocks UV and substantially dims the sunlight. An [[neutral density filter|attenuating (ND) filter]] might not filter UV and so is still dangerous. Unfiltered binoculars can deliver over 500 times as much energy to the retina as using the naked eye, killing retinal cells almost instantly (even though the power per unit area of image on the retina is the same, the heat cannot dissipate fast enough because the image is larger). Even brief glances at the midday Sun through unfiltered binoculars can cause permanent blindness.<ref name="Marsh">{{cite journal
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| − | |last=Marsh
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| − | |first=J. C. D.
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| − | |url=http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1982JBAA...92..257M&data_type=PDF_HIGH&type=PRINTER&filetype=.pdf
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| − | |format=PDF
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| − | |title=Observing the Sun in Safety
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| − | |journal=J. Brit. Ast. Assoc.
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| − | |year=1982
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| − | |volume=92
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| − | |pages=6}}</ref> One way to view the Sun safely is by projecting its image onto a screen using a telescope and eyepiece without cemented elements. This should only be done with a small refracting telescope (or binoculars) with a clean eyepiece. Other kinds of telescopes can be damaged by this procedure.
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| − | Partial [[solar eclipse]]s 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 about 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.<ref name="Espenak">{{cite web
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| − | |last=Espenak
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| − | |first=F.
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| − | |title=Eye Safety During Solar Eclipses - adapted from NASA RP 1383 Total Solar Eclipse of 1998 February 26, April 1996, p. 17
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| − | |url=http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/safety.html
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| − | |accessdate=2006-03-22
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| − | |publisher=NASA}}</ref> 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.
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| − | During [[sunrise]] and [[sunset]], sunlight is attenuated due to [[Rayleigh scattering]] and [[Mie theory|Mie scattering]] from a particularly long passage through Earth's atmosphere and the direct 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.
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| − | Attenuating filters to view the Sun should be specifically designed for that use: some improvised filters pass UV or IR rays that can harm the eye at high brightness levels. Filters on telescopes or binoculars should be on the [[objective lens]] or [[aperture]], ''never'' on the [[eyepiece]], because eyepiece filters can suddenly crack or shatter due to high heat loads from the absorbed sunlight. Welding glass #14 is an acceptable solar filter, but "black" exposed photographic film is not (it passes too much infrared).
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| − | == In cultural history ==
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| − | Like other natural phenomena, the Sun has been an object of veneration in many cultures throughout human history. ''Sol'' ({{pronEng|sÉ’l}} in English) is the [[Latin]] word for "Sun". The Latin name is widely known, but not common in general English language use, although the adjectival form is the related word ''solar''. 'Sol' is more often used in [[science fiction]] writing (''[[Star Trek]]'' in particular) as a formal name for the specific [[star]], since in many stories the local Sun is a different star and thus the generic term "the Sun" would be ambiguous. By extension, the [[Solar System]] is often referred to in science fiction as the "Sol System". 'Sol' is sometimes used in scientific circles, but 'Sol' is not the "official" name of the Sun, and the word 'Sol' makes no appearances in common reference sources.<ref>{{cite web |url=http://www.straightdope.com/mailbag/msunname.html |title=The Straight Dope |accessdate=2007-12-05}}</ref>
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| − | The term ''sol'' is used by planetary astronomers to refer to the duration of a [[solar day]] on [[Mars (planet)|Mars]].<ref>{{cite web |url=http://www.nasa.gov/mission_pages/mer/images/pia01892.html |title=Opportunity's View, Sol 959 (Vertical) |publisher=NASA |date=November 15, 2006 |accessdate=2007-08-01}}</ref> A mean Earth solar day is approximately 24 hours, while a mean Martian sol, is 24 hours, 39 minutes, and 35.244 seconds.<ref>{{cite web |url=http://www.giss.nasa.gov/tools/mars24/help/notes.html |title=Technical Notes on Mars Solar Time as Adopted by the Mars24 Sunclock |publisher=NASA GISS |date=December 13, 2005 |accessdate=2007-08-01}}</ref> See also [[Timekeeping on Mars]].
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| − | ''Sol'' is also the modern word for "Sun" in [[Portuguese language|Portuguese]], [[Spanish language|Spanish]], [[Icelandic language|Icelandic]], [[Danish language|Danish]], [[Norwegian language|Norwegian]], [[Swedish language|Swedish]], [[Catalan language|Catalan]] and [[Galician language|Galician]]. The [[Peru]]vian [[currency]] [[Peruvian nuevo sol|nuevo sol]] is named after the Sun (in Spanish), like its successor (and predecessor, in use 1985–1991) the [[Peruvian inti|Inti]] (in [[Quechua]]). In [[Persian language|Persian]], ''sol'' means "[[solar year]]".
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| − | In [[East Asia]] the Sun is represented by the symbol æ—¥ (Chinese [[pinyin]] ''rì'') or 太阳 (''tà i yáng''). In [[Vietnamese language|Vietnamese]] these [[Chinese language|Han]] words are called ''nháºt'' and ''thái dÆ°Æ¡ng'' respectively, while the native Vietnamese word ''mặt trá»i'' literally means 'face of the heavens'. The [[Moon]] and the Sun are associated with the [[yin and yang]] where the Moon represents ''yin'' and the Sun ''yang'' as dynamic opposites.
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| − | ==See also==
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| − | {{portal|Solar System|Solar system.jpg}}
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| − | <div style="-moz-column-count:2; column-count:2;">
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| − | * [[Advanced Composition Explorer]]
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| − | * [[Ecliptic]]
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| − | * [[Energy Independence]]
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| − | * [[Formation and evolution of the Solar System]]
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| − | * [[List of solar cycles]]
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| − | * [[List of Solar System bodies formerly regarded as planets]]
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| − | * [[Solar deity]]
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| − | * [[Solar energy]]
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| − | * [[Stellar classification]]
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| − | * [[Sun-Earth Day]]
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| − | * [[The Sun in human culture]]
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| − | </div>
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| − | ==References==
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| − | {{reflist|2}}
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| − | {{refbegin}}
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| − | * Thompson, M. J. (2004), ''Solar interior: Helioseismology and the Sun's interior'', Astronomy & Geophysics, v. 45, p. 4.21-4.25
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| − | * T. J. White; M. A. Mainster; P. W. Wilson; and J. H. Tips, ''Chorioretinal temperature increases from solar observation'', Bulletin of Mathematical Biophysics 33, 1–17 (1971)
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| − | {{refend}}
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| − | ==External links==
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| − | {{sisterlinks|Sun}}
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| − | * [http://sohowww.nascom.nasa.gov/ Nasa SOHO (Solar & Heliospheric Observatory) satellite]
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| − | * [http://www.nso.edu National Solar Observatory]
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| − | * [http://www.astronomycast.com/astronomy/episode-30-the-sun-spots-and-all/ Astronomy Cast: The Sun]
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| − | {{The Sun}}
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| − | {{Sun spacecraft}}
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| − | {{Solar System|color=#ffffc0}}
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| − | {{featured article}}
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| − | [[Category:Plasma physics]]
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| − | [[Category:Space plasmas]]
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| − | [[Category:Sun| ]]
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| − | [[Category:Yellow dwarfs]]
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| − | [[Category:Stars with proper names]]
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