Nebula 21664 224419491 2008-07-08T19:38:35Z Jagged 85 468111 /* Formation */ SN 1054 [[Image:Triangulum.nebula.full.jpg|thumb|250px|The Triangulum Emission Nebula [[NGC 604]]]] [[Image:Eagle nebula pillars.jpg|thumb|right|250px|The "Pillars of Creation" from the [[Eagle Nebula]]]] {{otheruses}} A '''nebula''' (from [[Latin]]: "mist" [http://www.etymonline.com/index.php?term=nebula]; pl. ''nebulae'' or ''nebulæ'', with [[Æ|ligature]] or ''nebulas'') is an [[interstellar cloud]] of [[cosmic dust|dust]], [[hydrogen]] gas and [[Plasma (physics)|plasma]]. It is the first stage of a [[star]]'s cycle. Originally ''nebula'' was a general name for any extended [[astronomy|astronomical]] [[astronomical object|object]], including [[galaxy|galaxies]] beyond the [[Milky Way]] (some examples of the older usage survive; for example, the [[Andromeda Galaxy]] was referred to as the ''[[Andromeda (mythology)|Andromeda]] Nebula'' before galaxies were discovered by [[Edwin Hubble]]). Nebulae often form star-forming regions, such as in the [[Eagle Nebula]]. This nebula is depicted in one of [[NASA]]'s most famous images, the "[[Eagle Nebula|Pillars of Creation]]". In these regions the formations of gas, dust and other materials 'clump' together to form larger masses, which attract further matter, and eventually will become big enough to form stars. The remaining materials are then believed to form [[planet]]s, and other [[planetary system]] objects. ==Formation== [[Image:Ngc2024 2mass.jpg|thumb|200px|left|NGC 2024, The [[Flame Nebula]]]] [[Image:Piotr_Wachowicz_eagle_nebula.jpg|thumb|200px|left|NGC 6611, or the [[Eagle Nebula]]]] Many nebulae form from the [[gravity|gravitational]] collapse of diffuse gas in the [[interstellar medium]] or ISM. As the material collapses under its own weight, massive stars may form in the center, and their [[ultraviolet radiation]] [[ion]]ises the surrounding gas, making it visible at optical [[wavelength]]s. An example of this type of nebula is the [[Rosette Nebula]] or the [[Pelican Nebula]]. The size of these nebulae, known as HII regions, varies depending on the size of the original cloud of gas, and the number of stars formed can vary too. As the sites of star formation, the formed stars are sometimes known as a young, loose cluster. Some nebulae are formed as the result of [[supernova]] explosions, the death throes of massive, short-lived stars. The material thrown off from the [[supernova]] explosion is ionised by the supernova remnant. One of the best examples of this is the [[Crab Nebula]], in [[Taurus (constellation)|Taurus]]. It is the result of a recorded supernova, [[SN 1054]], in the year 1054 and at the centre of the nebula is a [[neutron star]], created during the explosion. Other nebulae may form as [[planetary nebulae]]. This is the final stage of a low-mass star's life, like [[Earth]]'s [[Sun]]. [[Star]]s with a [[mass]] up to 8-10 solar masses evolve into [[red giant]]s and slowly lose their outer layers during pulsations in their atmospheres. When a star has lost a sufficient amount of material, its temperature increases and the ultraviolet radiation it emits is capable of [[ion]]izing the surrounding nebula that it has thrown off. ==Diffuse nebulae== [[Image:Omega Nebula.jpg|thumb|150px|The [[Omega Nebula]], an example of an [[emission nebula]].]] [[Image:Pleiades_large.jpg|thumb|150px|The [[Pleiades (star cluster)|Pleiades]]. The diffuse nebulae near the stars are examples of [[reflection nebula]].]] Most nebulae can be described as [[diffuse nebula]]e, which means that they are extended and contain no well-defined boundaries.<ref name="Messier">{{cite web|url=http://seds.lpl.arizona.edu/messier/diffuse.html|publisher=University of Illinois SEDS|title=The Messier Catalog: Diffuse Nebulae|accessdate=2007-06-12}}</ref> In visible light these nebulae may be divided into [[emission nebula]]e and [[reflection nebula]]e, a categorization that depends on how the light we see is created. Emission nebulae contain [[ion]]ized gas (mostly ionized [[hydrogen]]) that produces [[spectral line]] emission.<ref name="shu1982">{{cite book | author = F. H. Shu | title = The Physical Universe | publisher = University Science Books | date = 1982 | location = Mill Valley, California | id = ISBN 0-935702-05-9}}</ref> These emission nebulae are often called [[HII regions]]; the term "HII" is used in professional astronomy to refer to ionized hydrogen. In contrast to emission nebulae, reflection nebulae do not produce significant amounts of visible light by themselves but instead reflect light from nearby stars.<ref name="shu1982" /> [[Image:Horsehead-Hubble.jpg|left|thumb|150px|The [[Horsehead Nebula]], an example of a [[dark nebula]].]] Dark nebulae are similar to diffuse nebulae, but they are not seen by their emitted or reflected light. Instead, they are seen as dark clouds in front of more distant stars or in front of emission nebulae.<ref name="shu1982" /> Although these nebulae appear different at optical wavelengths, they all appear to be bright sources of emission at [[infrared]] wavelengths. This emission comes primarily from the [[cosmic dust|dust]] within the nebulae.<ref name="shu1982" /> {{clear}} ==Specific types of nebulae== While diffuse nebulae have poorly-defined boundaries, a few nebulae may actually be described as discrete objects with identifiable boundaries. ===Planetary nebulae=== [[Image:NGC6543.jpg|right|thumb|150px|The [[Cat's Eye Nebula]], an example of a [[planetary nebula]].]] [[Planetary nebula]]e are nebulae that form from the gaseous shells that are ejected from low-mass asymptotic giant branch stars when they transform into [[white dwarf]]s.<ref name="shu1982" /> These nebulae are emission nebulae with spectral emission that is similar to the emission nebulae found in [[star formation]] regions.<ref name="shu1982" /> Technically, they are a type of HII region because the majority of [[hydrogen]] will be ionised. However, planetary nebulae are denser and more compact than the emission nebulae in star formation regions.<ref name="shu1982" /> Planetary nebulae are so called because the first [[astronomy|astronomers]] who observed these objects thought that the nebulae resembled the disks of planets, although they are not at all related to planets.<ref name="s">{{cite book | author = E. Chaisson, S. McMillan | title = Astronomy: a beginner's guide to the universe | edition= 2nd | publisher = Prentice-Hall | date = 1995 | location = Upper Saddle River, New Jersey | id = ISBN 0-13-733916-X}}</ref> {{clear}} ====Protoplanetary nebula==== [[Image:PIA04533.jpg|thumb|100px|left|The [[Red Rectangle Nebula]], an example of a [[protoplanetary nebula]].]] A [[protoplanetary nebula]] (PPN) is an astronomical object which is at the short-lived episode during a star's rapid [[stellar evolution]] between the late [[asymptotic giant branch]] (LAGB) phase and the subsequent planetary nebula (PN) phase.<ref name="sahaietal2005">{{cite journal | author= R. Sahai, C. Sánchez Contreras, M. Morris | title=A Starfish Preplanetary Nebula: IRAS 19024+0044 | journal=Astrophysical Journal | year=2005 | volume=620 | pages=948–960 | url=http://adsabs.harvard.edu/abs/2005ApJ...620..948S | doi = 10.1086/426469 <!--Retrieved from CrossRef by DOI bot--> }}</ref> A PPN emits strong in infrared radiation, and is a kind of reflection nebula. The exact point when a PPN becomes a planetary nebula (PN) is defined by the temperature of the central star. {{clear}} ===Supernova remnants=== [[Image:Crab Nebula.jpg|thumb|150px|The [[Crab Nebula]], an example of a [[supernova remnant]].]] A [[supernova]] occurs when a high-mass star reaches the end of its life. When [[nuclear fusion]] ceases in the core of the star, the star collapses inward on itself. The gas falling inward either rebounds or gets so strongly heated that it expands outwards from the core, thus causing the star to explode.<ref name="shu1982"/> The expanding shell of gas form a [[supernova remnant]], a special type of [[diffuse nebula]].<ref name="shu1982"/> Although much of the optical and [[X-ray]] emission from supernova remnants originates from ionized gas, a substantial amount of the [[radio]] emission is a form of non-thermal emission called [[synchrotron emission]].<ref name="shu1982"/> This emission originates from high-velocity and [[electrons]] oscillating within [[magnetic field]]s.If the star cannot support itself, it can form a black hole, or also a gamma ray burst {{clear}} ==Notable named nebulae== *[[Eagle Nebula]] *[[Eta Carinae Nebula]] *[[Orion Nebula]] *[[Hourglass Nebula]] *[[Horsehead Nebula]] *[[Rosette Nebula]] *[[Ring Nebula]] *[[Crab Nebula]] *[[Cat's Eye Nebula]] *[[Barnard's Loop]] *[[Red Square Nebula]] *[[Tarantula Nebula]] *[[Boomerang Nebula]] ==See also== * [[Timeline of the interstellar medium and intergalactic medium]] * [[Orion Molecular Cloud Complex]] * [[Large Magellanic Cloud]] * [[Magellanic Clouds]] * [[Small Magellanic Cloud]] * [[H I region]] * [[H II region]] * [[Messier object]] ==References== <references/> ==Further reading== Lightner, G. Samuel. "Nebulae: Fuzzy Patches in Space." ''FusedWed.pppl.gov/CPEP'' 18 December 2000. ---. "Nebulae." ''FusedWed.pppl.gov/CPEP'' 17 November 2005. ---. "Reflection Nebulae." ''FusedWed.pppl.gov/CPEP'' 18 December 2000. ---. "Emission Nebulae." ''FusedWed.pppl.gov/CPEP'' 18 December 2000. ---. "Planetary Nebulae." ''FusedWed.pppl.gov/CPEP'' 18 December 2000. ---. "Supernova Remnants." ''FusedWed.pppl.gov/CPEP''18 December 2000. ==External links== {{commonscat|Nebula}} * [http://fusedweb.pppl.gov fusedweb.pppl.gov] * [http://www.geocities.com/altoid22001/starformation.html Information on star formation] * [http://www.space-and-telescope.com/MessierObjects.aspx Clickable table of Messier objects] [[Category:Nebulae]] [[Category:Space plasmas]] [[Category:Plasma physics]] [[ar:سديم]] [[bs:Maglica]] [[bg:Мъглявина]] [[ca:Nebulosa]] [[cs:Mlhovina]] [[da:Stjernetåge]] [[de:Nebel (Astronomie)]] [[el:Νεφέλωμα]] [[es:Nebulosa]] [[eo:Nebulozo]] [[eu:Nebulosa]] [[fa:سحابی]] [[fr:Nébuleuse]] [[gl:Nebulosa]] [[gu:નિહારિકા]] [[hr:Nebula]] [[io:Nebuloso]] [[id:Nebula]] [[it:Nebulosa]] [[he:ערפילית]] [[ka:ნისლეულები]] [[sw:Nebula]] [[la:Nebula (astronomia)]] [[lb:Niwwel (Astronomie)]] [[lt:Ūkas]] [[hu:Csillagköd]] [[ml:നെബുല]] [[nl:Nevels en gaswolken]] [[ja:星雲]] [[no:Stjernetåke]] [[nov:Nebula]] [[pl:Mgławica]] [[pt:Nebulosa]] [[ro:Nebuloasă]] [[ru:Туманность]] [[sk:Hmlovina]] [[sl:Meglica]] [[fi:Kaasusumu]] [[sv:Nebulosa]] [[th:เนบิวลา]] [[tr:Nebula (astronomi)]] [[uk:Галактична туманність]] [[ur:سحابیہ]] [[zh-yue:星雲]] [[zh:星云]]