Editing Orion Nebula (section)

==Star formation==
[[File:M42proplyds.jpg|thumb|right|View of several [[proplyd]]s within the Orion Nebula taken by the [[Hubble Space Telescope]]]]
[[File:Star Formation Fireworks in Orion.jpg|thumb|right|Star Formation Fireworks in Orion]]

The Orion Nebula is an example of a [[stellar nursery]] where new stars are being born. Observations of the nebula have revealed approximately 700 stars in various stages of formation within the nebula.

In 1979 observations with the [[André Lallemand|Lallemand electronic camera]] at the [[Pic du Midi Observatory|Pic-du-Midi Observatory]] showed six unresolved high-ionization sources near the [[Trapezium Cluster]]. These sources were interpreted as partly ionized globules (PIGs). The idea was that these objects are being ionized from the outside by M42.<ref>{{Cite journal|last1=Laques|first1=P.|last2=Vidal|first2=J. L.|date=March 1979|title=Detection of a new kind of condensations in the center of the Orion Nebula, by means of S 20 photocathodes associated with a Lallemand electronic camera.|journal=Astronomy & Astrophysics|language=en|volume=73|pages=97–106|bibcode=1979A&A....73...97L|issn=0004-6361}}</ref> Later observations with the [[Very Large Array]] showed solar-system-sized condensations associated with these sources. Here the idea appeared that these objects might be low-mass stars surrounded by an evaporating protostellar accretion disk.<ref>{{Cite journal|last1=Churchwell|first1=E.|last2=Felli|first2=M.|last3=Wood|first3=D. O. S.|last4=Massi|first4=M.|date=October 1987|title=Solar System--sized Condensations in the Orion Nebula|journal=Astrophysical Journal|language=en|volume=321|pages=516|doi=10.1086/165648|bibcode=1987ApJ...321..516C|issn=0004-637X|doi-access=free}}</ref> In 1993 observations with the Hubble Space Telescope have yielded the major confirmation of [[protoplanetary disk]]s within the Orion Nebula, which have been dubbed ''[[proplyds]]''.<ref>
{{Cite journal
 | first1 = Mark J. | display-authors = 1 | last1 = McCaughrean
 | first2 = C. Robert | last2 = O'dell
 | date = 1996
 | title = Direct Imaging of Circumstellar Disks in the Orion Nebula
 | journal = [[Astronomical Journal]]
 | volume = 111 | pages = 1977
 | bibcode = 1996AJ....111.1977M
 | doi = 10.1086/117934
| s2cid = 122335780 | doi-access = free
 }}</ref><ref>{{Cite journal|last1=O'dell|first1=C. R.|last2=Wen|first2=Zheng|last3=Hu|first3=Xihai|date=June 1993|title=Discovery of New Objects in the Orion Nebula on HST Images: Shocks, Compact Sources, and Protoplanetary Disks|journal=Astrophysical Journal|language=en|volume=410|pages=696|doi=10.1086/172786|bibcode=1993ApJ...410..696O|issn=0004-637X|doi-access=free}}</ref> HST has revealed more than 150 of these within the nebula, and they are considered to be systems in the earliest stages of [[solar system formation]]. The sheer number of them have been used as evidence that the formation of planetary systems is fairly common in the [[universe]].

[[Stellar evolution|Stars form]] when clumps of [[hydrogen]] and other gases in an [[H II region]] contract under their own gravity. As the gas collapses, the central clump grows stronger and the gas heats to extreme temperatures by converting [[gravitational potential energy]] to [[thermal energy]]. If the temperature gets high enough, [[nuclear fusion]] will ignite and form a [[protostar]]. The protostar is 'born' when it begins to emit enough radiative energy to balance out its gravity and halt [[gravitational collapse]].

Typically, a cloud of material remains a substantial distance from the star before the fusion reaction ignites. This remnant cloud is the protostar's protoplanetary disk, where planets may form. Recent [[infrared]] observations show that protoplanetary disks in the Orion Nebula contain dust grains that are growing, beginning the process of forming [[planetesimal]]s.<ref>{{Cite journal
 | display-authors=etal | author=Kassis, Marc
 | date = 2006
 | title = Mid-Infrared Emission at Photodissociation Regions in the Orion Nebula
 | journal = [[Astrophysical Journal]]
 | volume = 637 | issue = 2 | pages = 823–837
 | bibcode = 2006ApJ...637..823K
 | doi = 10.1086/498404
| doi-access = free}} See also the [http://www.keckobservatory.org/news/science/060109_orion/index.html press release] {{Webarchive|url=https://web.archive.org/web/20061024081423/http://www.keckobservatory.org/news/science/060109_orion/index.html |date=October 24, 2006 }}</ref>

Once the protostar enters into its [[main sequence]] phase, it is classified as a star. Even though most planetary disks can form planets, observations show that intense stellar radiation should have destroyed any proplyds that formed near the Trapezium group, if the group is as old as the low mass stars in the cluster.<ref name="salisbury" /> Since proplyds are found very close to the Trapezium group, it can be argued that those stars are much younger than the rest of the cluster members.{{efn|C. Robert O'Dell commented about this Wikipedia article, "The only egregious error is the last sentence in the Stellar Formation section. It should actually read:{{br}}'Even though most planetary disks can form planets, observations show that intense stellar radiation should have destroyed any proplyds that formed near the Trapezium group, if the group is as old as the low mass stars in the cluster. Since proplyds are found very close to the Trapezium group, it can be argued that those stars are much younger than the rest of the cluster members.{{'"}}}}

===Stellar wind and effects===
Once formed, the stars within the nebula emit a stream of charged particles known as a [[stellar wind]]. [[OB star|Massive stars]] and [[T Tauri star|young stars]] have much stronger stellar winds than the [[Sun]].<ref>Ker Than, January 11, 2006, "[http://www.space.com/scienceastronomy/060111_orion_news.html The Splendor of Orion: A Star Factory Unveiled]", Space.com</ref> The wind forms shock waves or hydrodynamical instabilities when it encounters the gas in the nebula, which then shapes the gas clouds. The shock waves from stellar wind also play a large part in stellar formation by compacting the gas clouds, creating density inhomogeneities that lead to gravitational collapse of the cloud.

[[File:Ripples.png|thumb| View of the ripples ([[Kelvin–Helmholtz instability]]) formed by the action of stellar winds on the cloud.]]

There are three different kinds of shocks in the Orion Nebula. Many are featured in [[Herbig–Haro objects]]:<ref>"[http://www.vanderbilt.edu/news/releases?id=23966 Mapping Orion's Winds]", January 16, 2006, ''Vanderbilt News Service''</ref>
* [[Bow shock]]s are stationary and are formed when two particle streams collide with each other. They are present near the hottest stars in the nebula where the stellar wind speed is estimated to be thousands of kilometers per second and in the outer parts of the nebula where the speeds are tens of kilometers per second. Bow shocks can also form at the front end of stellar jets when the jet hits [[interstellar medium|interstellar particles]].
* Jet-driven shocks are formed from jets of material sprouting off newborn [[T Tauri stars]]. These narrow streams are traveling at hundreds of kilometers per second and become shocks when they encounter relatively stationary gases.
* Warped shocks appear bow-like to an observer. They are produced when a jet-driven shock encounters gas moving in a cross-current.
* The interaction of the stellar wind with the surrounding cloud also forms "waves" which are believed to be due to the hydrodynamical [[Kelvin-Helmholtz instability]].<ref>Denise Chow. [https://web.archive.org/web/20130626131145/http://www.nbcnews.com/id/38757647 Young stars blamed for space cloud ripples], ''[[NBC News]]''</ref>

The dynamic gas motions in M42 are complex, but are trending out through the opening in the bay and toward the Earth.<ref name=Balick/> The large neutral area behind the ionized region is currently contracting under its own gravity.

There are also [[supersonic]] 'bullets' of gas piercing the hydrogen clouds of the Orion Nebula. Each bullet is ten times the diameter of [[Pluto]]'s orbit and tipped with iron atoms glowing in the infrared. They were probably formed one thousand years earlier from an unknown violent event.<ref>{{cite web |url=http://www.gemini.edu/index.php?q=node/226 |title=Gemini's Laser Vision Reveals Striking New Details in Orion Nebula |date=March 22, 2007 |work=Gemini Observatory |access-date=June 1, 2010}}</ref>