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=[[:en:Solar System|Solar System]]=
[[Image:Sun in X-Ray.png|thumb|right|The Sun as seen in the [[x-ray]] region of the [[electromagnetic spectrum]]]]
==Heliopause==
[[Image:Voyager 1 entering heliosheath region.jpg|left|thumb|300px|The [[Voyager program|Voyagers]] entering the [[heliosheath]].]]
The Sun is the Solar System's parent star, and far and away its chief component. Its large mass gives it an interior [[density]] high enough to sustain [[nuclear fusion]], which releases enormous amounts of [[energy]], mostly [[radiant energy|radiated]] into [[outer space|space]] as [[electromagnetic radiation]] such as [[visible spectrum|visible light]].
L'[[eliosfera]] è divisa in due regioni distinte. Il [[vento solare]] viaggia a circa 40.000 km/s fino a quando non si scontra con i flussi di plasma nel [[mezzo interstellare]]. La collisione si verifica presso il [[termination shock]], che si trova tra 80 e 100 [[Unità Astronomica|UA]] dal Sole in direzione sopravvento, e fino a circa 200 [[Unità Astronomica|UA]] dal Sole sottovento.<ref name="fahr">{{cite journal |year=2000 |author=Fahr, H. J.; Kausch, T.; Scherer, H. |title=A 5-fluid hydrodynamic approach to model the Solar System-interstellar medium interaction |journal=Astronomy & Astrophysics | volume=357 | pages=268 |url=http://aa.springer.de/papers/0357001/2300268.pdf | format=PDF | bibcode=2000A&A...357..268F }} See Figures 1 and 2.</ref> Qui il vento rallenta drasticamente, condensa e diviene più turbolento,<ref name=fahr /> formando una grande struttura ovale conosciuta con il nome di [[heliosheath]], la quale sembra si comporti come la coda di una cometa: esso si estende verso l'esterno per altri 40 [[Unità Astronomica|UA]] sul lato sopravvento, mentre si estende molto meno nella direzione opposta. Entrambe le sonde [[Voyager 1]] e [[Voyager 2]] hanno superato il termination shock ed sono entrate nell'heliosheath, e distano rispettivamente 94 e 84 [[Unità Astronomica|UA]] dal Sole.<ref>{{cite journal | doi=10.1126/science.1117684 | year=2005 | month=Sep | author=Stone, E. C.; Cummings, A. C.; Mcdonald, F. B.; Heikkila, B. C.; Lal, N.; Webber, W. R. | title=Voyager 1 explores the termination shock region and the heliosheath beyond | volume=309 | issue=5743 | pages=2017–20 | issn=0036-8075 | pmid=16179468 | journal=Science (New York, N.Y.)}}</ref><ref>{{cite journal | doi=10.1038/nature07022 | year=2008 | month=Jul | author=Stone, E. C.; Cummings, A. C.; Mcdonald, F. B.; Heikkila, B. C.; Lal, N.; Webber, W. R. | title=An asymmetric solar wind termination shock | volume=454 | issue=7200 | pages=71–4 | issn=0028-0836 | pmid=18596802 | journal=Nature }}</ref> Il limite esterno dell'[[eliosfera]], l'[[eliopausa]], è il punto in cui il vento solare si esaurisce ed inizia dello spazio interstellare.<ref name="Voyager">{{cite web |url=http://www.nasa.gov/vision/universe/solarsystem/voyager_agu.html |title=Voyager Enters Solar System's Final Frontier |work=NASA |accessdate=2007-04-02}}</ref>
The shapeSun andis formclassified ofas thea outermoderately edgelarge of[[yellow thedwarf]], heliospherebut this name is likelymisleading affectedas, bycompared theto stars in [[fluidMilky dynamicsWay|our galaxy]], ofthe interactionsSun withis therather interstellarlarge medium<refand name=fahrbright. />Stars asare wellclassified asby solarthe magnetic[[Hertzsprung-Russell fieldsdiagram]], prevailinga tograph thewhich south,plots e.g.the itbrightness isof bluntlystars shapedagainst withtheir thesurface northern[[temperature]]s. hemisphereGenerally, extendinghotter 9stars AUare (roughlybrighter. 900Stars millionfollowing miles)this fartherpattern thanare thesaid southernto hemisphere.be Beyondon the heliopause,[[main atsequence]]; aroundthe 230 AU,Sun lies right in the [[bowmiddle of shock]]it. However, astars plasmabrighter "wake"and lefthotter bythan the Sun asare itrare, travelswhile throughstars thedimmer [[Milkyand Way]]cooler are common.<ref>{{cite web |year=2001 date|author=JuneSmart, 24R. L.; Carollo, 2002D.; |author=PLattanzi, M. CG.; FrischMcLean, (UniversityB.; ofSpagna, Chicago)A. |title=The Sun'sSecond HeliosphereGuide &Star HeliopauseCatalogue |and Cool Stars |work=[[AstronomyPerkins Picture of the Day]]Observatory | url=http://antwrpadsabs.gsfcharvard.nasa.govedu/apodabs/ap0206242001udns.conf..html119S |accessdate=2006-0612-2326}}</ref>
It is believed that the Sun's position on the main sequence puts it in the "prime of life" for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion. The Sun is growing brighter; early in its history it was 75 percent as bright as it is today.<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 |doi=10.1126/science.11539665 |pmid=11539665}}</ref>
No spacecraft have yet passed beyond the heliopause, so it is impossible to know for certain the conditions in local interstellar space. It is expected that [[NASA]]'s [[Voyager program|Voyager spacecraft]] will pass the heliopause some time in the next decade and transmit valuable data on radiation levels and solar wind back to the Earth.<ref>{{cite web | year=2007 | title=Voyager: Interstellar Mission | work=NASA Jet Propulsion Laboratory | url=http://voyager.jpl.nasa.gov/mission/interstellar.html |accessdate=2008-05-08}}</ref> How well the heliosphere shields the Solar System from cosmic rays is poorly understood. A NASA-funded team has developed a concept of a "Vision Mission" dedicated to sending a probe to the heliosphere.<ref>{{cite conference |title=Innovative Interstellar Explorer |author=R. L. McNutt, Jr. et al. | booktitle= Physics of the Inner Heliosheath: Voyager Observations, Theory, and Future Prospects |publisher=AIP Conference Proceedings |volume=858 |pages=341–347 |year=2006 |url=http://adsabs.harvard.edu/abs/2006AIPC..858..341M |doi=10.1063/1.2359348}}</ref><ref>{{cite web |year=2007 |title=Interstellar space, and step on it! |work=New Scientist |url=http://space.newscientist.com/article/mg19325850.900-interstellar-space-and-step-on-it.html |date=2007-01-05 |accessdate=2007-02-05 | author=Anderson, Mark}}</ref>
The Sun is a [[metallicity|population I star]]; it was born in the later stages of the [[Universe#Evolution|universe's evolution]]. It contains more elements heavier than hydrogen and helium ("[[metallicity|metals]]" in astronomical parlance) than older population II stars.<ref>{{cite journal |author=T. S. van Albada, Norman Baker |title=On the Two Oosterhoff Groups of Globular Clusters |journal=Astrophysical Journal |volume=185 |year=1973 |pages=477–498 |doi=10.1086/152434}}</ref> Elements heavier than hydrogen and helium were formed in the [[solar core|cores]] of ancient and exploding stars, so the first generation of stars had to die before the universe could be enriched with these atoms. The oldest stars contain few metals, while stars born later have more. This high metallicity is thought to have been crucial to the Sun's developing a [[planetary system]], because planets form from accretion of metals.<ref> {{cite web |title=An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect |author=Charles H. Lineweaver |work=University of New South Wales |date=2001-03-09 |url=http://arxiv.org/abs/astro-ph/0012399 |accessdate=2006-07-23}}</ref>
==Oort cloud==
[[Image:Kuiper oort.jpg|thumb|Artist's rendering of the Kuiper Belt and hypothetical [[Oort cloud]].]]
[[Image:Heliospheric-current-sheet.gif|left|thumb|The [[heliospheric current sheet]].]]
The hypothetical Oort cloud is a great mass of up to a trillion icy objects that is believed to be the source for all long-period comets and to surround the Solar System at roughly 50,000 AU (around 1 [[light-year]] (LY)), and possibly to as far as 100,000 AU (1.87 LY). It is believed to be composed of comets which were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events such as collisions, the gravitational effects of a passing star, or the [[galactic tide]], the [[tidal force]] exerted by the [[Milky Way]].<ref>{{cite web |year=2001 |author= Stern SA, Weissman PR. |title=Rapid collisional evolution of comets during the formation of the Oort cloud. |work=Space Studies Department, Southwest Research Institute, Boulder, Colorado| url=http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11214311&dopt=Citation |accessdate=2006-11-19}}</ref><ref>{{cite web |year=2006 |author=Bill Arnett |title=The Kuiper Belt and the Oort Cloud |work=nineplanets.org |url=http://www.nineplanets.org/kboc.html |accessdate=2006-06-23}}</ref>
===Interplanetary medium===
Along with [[light]], the Sun radiates a continuous stream of charged particles (a [[Plasma (physics)|plasma]]) known as the [[solar wind]]. This stream of particles spreads outwards at roughly 1.5 million kilometres per hour,<ref>{{cite web |title=Solar Physics: The Solar Wind |work=Marshall Space Flight Center |date=2006-07-16<!--Internet Archive estimate--> |url=http://solarscience.msfc.nasa.gov/SolarWind.shtml |accessdate=2006-10-03}}</ref> creating a tenuous atmosphere (the [[heliosphere]]) that permeates the Solar System out to at least 100 AU (see [[#Heliopause|heliopause]]). This is known as the [[interplanetary medium]]. [[Geomagnetic storm]]s on the Sun's surface, such as [[solar flare]]s and [[coronal mass ejection]]s, disturb the heliosphere, creating [[space weather]].<ref name="SunFlip">{{cite web |url=http://science.nasa.gov/headlines/y2001/ast15feb_1.htm |title=The Sun Does a Flip |accessdate=2007-02-04 |last=Phillips |first=Tony |date=2001-02-15 |work=Science@NASA}}</ref> The Sun's rotating magnetic field acts on the interplanetary medium to create the [[heliospheric current sheet]], the largest structure in the Solar System.<ref>{{cite web |title=Artist's Conception of the Heliospheric Current Sheet |work=Wilcox Solar Observatory |url=http://quake.stanford.edu/~wso/gifs/HCS.html |accessdate=2006-06-22}}</ref>
;Sedna and the inner Oort cloud
:[[90377 Sedna]] (525.86 AU average) is a large, reddish Pluto-like object with a gigantic, highly elliptical orbit that takes it from about 76 AU at perihelion to 928 AU at aphelion and takes 12,050 years to complete. [[Michael E. Brown|Mike Brown]], who discovered the object in 2003, asserts that it cannot be part of the [[scattered disc]] or the [[Kuiper belt]] as its perihelion is too distant to have been affected by Neptune's migration. He and other astronomers consider it to be the first in an entirely new population, which also may include the object {{mpl|2000 CR|105}}, which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3420 years.<ref>{{cite web |year=2004 |author=David Jewitt |title=Sedna – 2003 VB<sub>12</sub> |work=University of Hawaii |url=http://www.ifa.hawaii.edu/~jewitt/kb/sedna.html |accessdate=2006-06-23}}</ref> Brown terms this population the "Inner Oort cloud," as it may have formed through a similar process, although it is far closer to the Sun.<ref>{{cite web |title=Sedna |author=Mike Brown |url=http://www.gps.caltech.edu/~mbrown/sedna/ |work=CalTech |accessdate=2007-05-02}}</ref> Sedna is very likely a dwarf planet, though its shape has yet to be determined with certainty.
[[Image:Aurora-SpaceShuttle-EO.jpg|thumb|right|[[Aurora australis]] seen from orbit.]]
==Boundaries==
[[Earth's magnetic field]] protects [[Earth's atmosphere|its atmosphere]] from interacting with the solar wind. Venus and Mars do not have magnetic fields, and the solar wind causes their atmospheres to gradually bleed away into space.<ref>{{cite science |last=Lundin |first=Richard |date=2001-03-09 |title=Erosion by the Solar Wind |author=Rickard Lundin |journal=Science |volume=291 |issue=5510 |pages=1909 |doi=10.1126/science.1059763 |url=http://sciencemag.org/cgi/content/full/291/5510/1909 |accessdate=2006-12-26|abstract=http://sciencemag.org/cgi/content/summary/291/5510/1909}}</ref> The interaction of the solar wind with Earth's magnetic field creates the [[Aurora (astronomy)|aurorae]] seen near the [[Earth's magnetic field#Magnetic poles|magnetic poles]].
[[Cosmic ray]]s originate outside the Solar System. The heliosphere partially shields the Solar System, and planetary magnetic fields (for those planets that have them) also provide some protection. The density of cosmic rays in the [[interstellar medium]] and the strength of the Sun's magnetic field change on very long timescales, so the level of cosmic radiation in the Solar System varies, though by how much is unknown.<ref name="Langner_et_al_2005">{{cite journal |last=Langner |first=U. W. |coauthors=M.S. Potgieter |year=2005 |title=Effects of the position of the solar wind termination shock and the heliopause on the heliospheric modulation of cosmic rays |journal=Advances in Space Research |volume=35 |issue=12 |pages=2084–2090 |doi=10.1016/j.asr.2004.12.005 |url=http://adsabs.harvard.edu/abs/2005AdSpR..35.2084L |accessdate=2007-02-11}}</ref>
The interplanetary medium is home to at least two disc-like regions of [[cosmic dust]]. The first, the [[interplanetary dust cloud|zodiacal dust cloud]], lies in the inner Solar System and causes [[zodiacal light]]. It was likely formed by collisions within the asteroid belt brought on by interactions with the planets.<ref>{{cite web |year=1998 |title=Long-term Evolution of the Zodiacal Cloud |url=http://astrobiology.arc.nasa.gov/workshops/1997/zodiac/backman/IIIc.html |accessdate=2007-02-03}}</ref> The second extends from about 10 AU to about 40 AU, and was probably created by similar collisions within the Kuiper belt.<ref>{{cite web |year=2003 |title=ESA scientist discovers a way to shortlist stars that might have planets |work=ESA Science and Technology |url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=29471 |accessdate=2007-02-03}}</ref><ref>{{cite journal |last=Landgraf |first=M. |coauthors=Liou, J.-C.; Zook, H. A.; Grün, E. |month=May |year=2002 |title=Origins of Solar System Dust beyond Jupiter |journal=The Astronomical Journal |volume=123 |issue=5 |pages=2857–2861 |doi=10.1086/339704 |url=http://www.iop.org/EJ/article/1538-3881/123/5/2857/201502.html |accessdate=2007-02-09}}</ref>
Much of our Solar System is still unknown. The Sun's gravitational field is estimated to dominate the gravitational forces of [[List of nearest stars|surrounding stars]] out to about two light years (125,000 AU). The outer extent of the Oort cloud, by contrast, may not extend farther than 50,000 AU.<ref>{{cite book |title=The Solar System: Third edition |author=T. Encrenaz, JP. Bibring, M. Blanc, MA. Barucci, F. Roques, PH. Zarka |publisher=Springer |year=2004 |pages=1}}</ref> Despite discoveries such as Sedna, the region between the Kuiper belt and the Oort cloud, an area tens of thousands of AU in radius, is still virtually unmapped. There are also ongoing studies of the region between Mercury and the Sun.<ref>{{cite web |year=2004 |author=Durda D.D.; Stern S.A.; Colwell W.B.; Parker J.W.; Levison H.F.; Hassler D.M. |title=A New Observational Search for Vulcanoids in SOHO/LASCO Coronagraph Images |url=http://www.ingentaconnect.com/search/expand?pub=infobike://ap/is/2000/00000148/00000001/art06520&unc=ml |accessdate=2006-07-23}}</ref> Objects may yet be discovered in the Solar System's uncharted regions.
==Note==
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