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ThroughtheOrionNebulato.. TheOrionNebula(alsoknownasMessier 42,M42,orNGC1976)isadifusenebula situatedintheMilkyWay,beingsouthof Orion'sBeltintheconstelationofOrion.[b]It isoneofthebrightestnebulae,andisvisible Thenebulaisvisiblewiththenakedeyeeven fromareasafectedbysomelightpolution.It isseenasthemiddle"star"inthe"sword"of Orion,whicharethethreestarslocatedsouth Stelarwindandefects Onceformed,thestarswithinthenebulaemit

astreamofchargedparticlesknownasa stelarwind.Massivestarsandyoungstars havemuchstrongerstelarwindsthanthe Sun.[40]Thewindformsshockwavesor hydrodynamicalinstabilitieswhenit encountersthegasinthenebula,whichthen shapesthegasclouds.Theshockwaves fromstelarwindalsoplayalargepartin stelarformationbycompactingthegas clouds,creatingdensityinhomogeneitiesthat leadtogravitational colapseofthecloud. Viewoftheripples(Kelvin–Helmholtz instability)formedbytheactionofstelar windsonthecloud. Therearethreediferentkindsofshocksin theOrionNebula.Manyarefeaturedin Herbig–Haroobjects:[41] Bowshocksarestationaryandareformed whentwoparticlestreamscolidewitheach other.Theyarepresentnearthehoteststars inthenebulawherethestelarwindspeedis

ofOrion'sBelt.Thestarappearsfuzzyto sharp-eyedobservers,andthenebulosityis obviousthroughbinocularsorasmal telescope.Thepeaksurfacebrightnessof thecentral regionisabout17Mag/arcsec2 (about14milinits)andtheouterbluishglow hasapeaksurfacebrightnessof21.3 Mag/arcsec2(about0.27milinits).[9](Inthe photosshownherethebrightnes,or luminance,isenhancedbyalargefactor.) Coloration Observershavelongnotedadistinctive greenishtinttothenebula,inadditionto regionsofredandofblue-violet.Theredhue isaresultoftheHαrecombinationline radiationatawavelengthof656.3nm.The blue-violetcolorationisthereflectedradiation fromthemassiveO-classstarsatthecoreof thenebula.

tothenakedeyeinthenightsky.M42is locatedatadistanceof1,344±20light years[3][6]andistheclosestregionof masivestarformationtoEarth.TheM42 nebulaisestimatedtobe24lightyears across.Ithasamassofabout2,000times thatoftheSun.Oldertextsfrequentlyreferto theOrionNebulaastheGreatNebulain OrionortheGreatOrionNebula.[7] TheOrionNebulaisoneofthemost scrutinizedandphotographedobjectsinthe nightsky,andisamongthemostintensely studiedcelestialfeatures.[8]Thenebulahas revealedmuchabouttheprocessofhow starsandplanetarysystemsareformedfrom colapsingcloudsofgasanddust. Astronomershavedirectlyobserved protoplanetarydisks,browndwarfs,intense andturbulentmotionsofthegas,andthe photo-ionizingefectsofmasivenearby

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