Editing Planetary nebula (section)

=== Spectra ===

The nature of planetary nebulae remained unknown until the first [[astronomical spectroscopy|spectroscopic]] observations were made in the mid-19th century. Using a [[Prism (optics)|prism]] to disperse their light, [[William Huggins]] was one of the earliest astronomers to study the [[optical spectrum|optical spectra]] of astronomical objects.<ref name=Moore2007/>

On August 29, 1864, Huggins was the first to analyze the spectrum of a planetary nebula when he observed [[Cat's Eye Nebula]].<ref name=Kwok1/> His observations of stars had shown that their spectra consisted of a [[Continuum (theory)|continuum]] of radiation with many [[absorption line|dark lines]] superimposed. He found that many nebulous objects such as the [[Andromeda Galaxy|Andromeda Nebula]] (as it was then known) had spectra that were quite similar. However, when Huggins looked at the Cat's Eye Nebula, he found a very different spectrum. Rather than a strong continuum with absorption lines superimposed, the Cat's Eye Nebula and other similar objects showed a number of [[Emission spectrum|emission lines]].<ref name=Moore2007/> Brightest of these was at a wavelength of 500.7&nbsp;[[nanometre]]s, which did not correspond with a line of any known element.<ref name=Huggins1864>{{harvnb|Huggins|Miller|1864|pp=437–44}}</ref>

At first, it was hypothesized that the line might be due to an unknown element, which was named [[nebulium]]. A similar idea had led to the discovery of [[helium]] through analysis of the [[Sun]]'s spectrum in 1868.<ref name=Kwok1/> While helium was isolated on Earth soon after its discovery in the spectrum of the Sun, "nebulium" was not. In the early 20th century, [[Henry Norris Russell]] proposed that, rather than being a new element, the line at 500.7&nbsp;nm was due to a familiar element in unfamiliar conditions.<ref name=Kwok1/>

Physicists showed in the 1920s that in gas at extremely low densities, [[electron]]s can occupy [[excited state|excited]] [[Metastability|metastable]] [[energy level]]s in atoms and ions that would otherwise be de-excited by collisions that would occur at higher densities.<ref name=Bowen1927>{{harvnb|Bowen|1927|pp=295–7}}</ref> Electron transitions from these levels in [[nitrogen]] and [[oxygen]] ions ({{nowrap|O<sup>+</sup>}}, [[Doubly ionized oxygen|{{nowrap|O<sup>2+</sup>}}]] (a.k.a. O&nbsp;{{Smallcaps|iii}}), and {{nowrap|N<sup>+</sup>}}) give rise to the 500.7&nbsp;nm emission line and others.<ref name=Kwok1/> These spectral lines, which can only be seen in very low-density gases, are called ''[[forbidden line]]s''. Spectroscopic observations thus showed that nebulae were made of extremely rarefied gas.<ref name=Gurzadyan>{{harvnb|Gurzadyan|1997}}</ref>

[[File:Potw1550a.tif|thumb|Planetary nebula NGC 3699 is distinguished by an irregular mottled appearance and a dark rift.<ref>{{cite web|title=A Planetary Nebula Divided|url=http://www.eso.org/public/images/potw1550a/|access-date=21 December 2015}}</ref>]]