Editing Orion Nebula (section)

=== 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>