Editing Orion Nebula (section)

== Physical characteristics ==
[[File:Hubble Snaps View of the Orion Nebula.ogv|thumb|300px|right|Discussing the location of the Orion Nebula, what is seen within the star-formation region, and the effects of interstellar winds in shaping the nebula]]
[[File:Orion composite1.jpg|thumb|200px|The constellation of Orion with the Orion Nebula (lower middle)]]

The Orion Nebula is visible with the naked eye even from areas affected by [[light pollution]]. It is seen as the middle "star" in the "sword" of Orion, which are the three stars located south of Orion's Belt. The "star" appears fuzzy to sharp-eyed observers, and the nebulosity is obvious through [[binoculars]] or a small [[telescope]]. The peak surface brightness of the central region of M42 is about 17 Mag/arcsec<sup>2</sup> and the outer bluish glow has a peak surface brightness of 21.3 Mag/arcsec<sup>2</sup>.<ref>{{cite web
 |url        = http://www.clarkvision.com/astro/surface-brightness-profiles/introduction.html
 |title      = Surface Brightness of Deep Sky Objects
 |date       = March 28, 2004
 |first      = Roger
 |last       = Clark
 |access-date = June 29, 2013
}}. </ref>

The Orion Nebula contains a very young [[open cluster]], known as the [[Trapezium Cluster]] due to the [[Asterism (astronomy)|asterism]] of its primary four stars within a diameter of 1.5 light years. Two of these can be resolved into their component binary systems on nights with good [[astronomical seeing|seeing]], giving a total of six stars. The stars of the Trapezium Cluster, along with many other stars, are still in their [[star formation|early years]]. The Trapezium Cluster is a component of the much larger Orion Nebula cluster, an association of about 2,800 stars within a diameter of 20 light years.<ref>
{{Cite journal
 | first1 = L. A. | last1 = Hillenbrand
 | first2 = L. W. | last2 = Hartmann
 | date = 1998
 | title =  Preliminary Study of the Orion Nebula Cluster Structure and Dynamics
 | journal = [[Astrophysical Journal]]
 | volume = 492 | issue = 2
 | pages = 540–553
 | bibcode=1998ApJ...492..540H
 | doi = 10.1086/305076
| s2cid = 43038127
 | url = https://authors.library.caltech.edu/75192/1/Hillenbrand_1998_ApJ_492_540.pdf
 }}</ref> The Orion Nebula is in turn surrounded by the much larger [[Orion molecular cloud complex]], which is hundreds of light years across, spanning the whole Orion Constellation. Two million years ago the Orion Nebula cluster may have been the home of the [[runaway star]]s [[AE Aurigae]], [[53 Arietis]], and [[Mu Columbae]], which are currently moving away from the nebula at speeds greater than {{cvt|100.|km/s}}.<ref>
{{Cite journal
 | display-authors = 1 
 | first1 = A. | last1 = Blaauw
 | first2 = W. W. | last2 = Morgan
 | date = 1954
 | title = The Space Motions of AE Aurigae and μ Columbae with Respect to the Orion Nebula
 | journal = [[Astrophysical Journal]]
 | volume = 119 | pages = 625
 | bibcode=1954ApJ...119..625B
 | doi = 10.1086/145866
| doi-access = free
 }}</ref>

=== Coloration ===
Observers have long noted a distinctive greenish tint to the nebula, in addition to regions of red and of blue-violet. The red hue is a result of the [[H-alpha|Hα]] recombination line [[radiation]] at a [[wavelength]] of 656.3&nbsp;[[nanometer|nm]]. The blue-violet coloration is the reflected radiation from the massive [[Stellar classification|O-class]] stars at the core of the nebula.

The green hue was a puzzle for astronomers in the early part of the 20th century because none of the known [[spectral line]]s at that time could explain it. There was some speculation that the lines were caused by a new element, and the name [[nebulium]] was coined for this mysterious material. With better understanding of [[atomic physics]], however, it was later determined that the green spectrum was caused by a low-probability [[electron]] transition in [[doubly ionized oxygen]], a so-called "[[Forbidden line|forbidden transition]]". This radiation was impossible to reproduce in the laboratory at the time, because it depended on the [[wiktionary:quiescence#English|quiescent]] and nearly collision-free environment found in the high vacuum of deep space.<ref>
{{Cite journal
 | last1 = Bowen | first1 = Ira Sprague
 | date = 1927
 | title = The Origin of the Nebulium Spectrum
 | journal = [[Nature (journal)|Nature]]
 | volume = 120 | pages = 473
 | doi = 10.1038/120473a0
|bibcode = 1927Natur.120..473B
 | issue=3022| doi-access = free
 }}</ref>