Editing Planetary nebula (section)

== Observations ==
[[File:NGC7293 (2004).jpg|thumb|alt=Colorful shell which has an almost eye like appearance. The center shows the small central star with a blue circular area that could represent the iris. This is surrounded by an iris like area of concentric orange bands. This is surrounded by an eyelid shaped red area before the edge where plain space is shown. Background stars dot the whole image.|[[Helix Nebula|NGC 7293]], the [[Helix Nebula]]]]
[[File:Ngc2392.jpg|thumb|alt=Spherical shell of colored area against background stars. Intricate cometary-like knots radiate inwards from the edge to about a third of the way to the center. The center half contains brighter spherical shells that overlap each other and have rough edges. Lone central star is visible in the middle. No background stars are visible.|[[Eskimo Nebula|NGC 2392]], the [[Eskimo Nebula]]]]

=== Discovery ===

The first planetary nebula discovered (though not yet termed as such) was the [[Dumbbell Nebula]] in the constellation of [[Vulpecula]]. It was observed by [[Charles Messier]] on July 12, 1764 and listed as M27 in his [[Messier object|catalogue]] of nebulous objects.<ref name=Kwok1>{{harvnb|Kwok|2000|pp=1–7}}</ref> To early observers with low-resolution telescopes, M27 and subsequently discovered planetary nebulae resembled the giant planets like [[Uranus]].  As early as January 1779, the French astronomer [[Antoine Darquier de Pellepoix]] described in his observations of the [[Ring Nebula]], "a very dull nebula, but perfectly outlined; as large as Jupiter and looks like a fading planet".<ref name=Darquier/><ref name="Olsen2017"/><ref name="Steinicke2018"/> 

The nature of these objects remained unclear. In 1782, [[William Herschel]], discoverer of Uranus, found the [[Saturn Nebula]] (NGC 7009) and described it as "A curious nebula, or what else to call it I do not know". He later described these objects as seeming to be planets "of the starry kind".<ref>{{Cite journal|last=Zijlstra|first=A.|date=2015|title=Planetary nebulae in 2014: A review of research|url=http://www.astroscu.unam.mx/rmaa/RMxAA..51-2/PDF/RMxAA..51-2_azijlstra.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.astroscu.unam.mx/rmaa/RMxAA..51-2/PDF/RMxAA..51-2_azijlstra.pdf |archive-date=2022-10-09 |url-status=live|journal=Revista Mexicana de Astronomía y Astrofísica|volume=51|pages=221–230|arxiv=1506.05508|bibcode=2015RMxAA..51..221Z}}</ref>  As noted by Darquier before him, Herschel found that the disk resembled a planet but it was too faint to be one. In 1785, Herschel wrote to [[Jérôme Lalande]]:

<blockquote>These are celestial bodies of which as yet we have no clear idea and which are perhaps of a type quite different from those that we are familiar with in the heavens. I have already found four that have a visible diameter of between 15 and 30 seconds. These bodies appear to have a disk that is rather like a planet, that is to say, of equal brightness all over, round or somewhat oval, and about as well defined in outline as the disk of the planets, of a light strong enough to be visible with an ordinary telescope of only one foot, yet they have only the appearance of a star of about ninth magnitude.<ref>Quoted in {{cite journal|last1=Hoskin|first1=Michael|year=2014|title=William Herschel and the Planetary Nebulae|journal=Journal for the History of Astronomy|volume=45|issue=2|pages=209–225|bibcode=2014JHA....45..209H|doi=10.1177/002182861404500205|s2cid=122897343}}</ref></blockquote>

He assigned these to Class IV of his catalogue of "nebulae", eventually listing 78 "planetary nebulae", most of which are in fact galaxies.<ref>p. 16 in {{cite book|last1=Mullaney|first1=James|title=The Herschel Objects and How to Observe Them|year=2007|isbn=978-0-387-68124-5|series=Astronomers' Observing Guides|bibcode=2007hoho.book.....M|doi=10.1007/978-0-387-68125-2}}</ref>

Herschel used the term "planetary nebulae" for these objects. The origin of this term not known.<ref name="Kwok1" /><ref name="Moore2007">{{harvnb|Moore|2007|pp=279–80}}</ref> The label "planetary nebula" became ingrained in the terminology used by astronomers to categorize these types of nebulae, and is still in use by astronomers today.<ref name="seds2013">{{harvnb|SEDS|2013}}</ref><ref name="hubbleSite1997">{{harvnb|Hubblesite.org|1997}}</ref>

=== 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>]]

=== Central stars ===

The central stars of planetary nebulae are very hot.<ref name="Frankowskietal2009" /> Only when a star has exhausted most of its nuclear fuel can it collapse to a small size. Planetary nebulae are understood as a final stage of [[stellar evolution]]. Spectroscopic observations show that all planetary nebulae are expanding. This led to the idea that planetary nebulae were caused by a star's outer layers being thrown into space at the end of its life.<ref name=Kwok1/>

=== Modern observations ===

Towards the end of the 20th century, technological improvements helped to further the study of planetary nebulae.<ref name=KwokJun2005>{{harvnb|Kwok|2005|pp=271–8}}</ref> [[Space telescope]]s allowed astronomers to study light wavelengths outside those that the Earth's atmosphere transmits. The first UV observations of PNe ([[IC 2149]]) were performed from space, with the [[Orion 2 Space Observatory]] (see [[Orion 1 and Orion 2 Space Observatories]]) on board the [[Soyuz 13]] spacecraft in December 1973,<ref>{{cite journal | last1 = Gurzadyan | first1 = G.A. | year = 1975 | title = The ultraviolet spectrum of planetary nebula IC 2149| journal = MNRAS| volume = 172| pages = 249–256 |doi=10.1093/mnras/172.1.249| doi-access = free | bibcode= }}</ref> two photon emission from nebulae was detected for the first time.<ref>{{cite journal | last1 = Gurzadyan | first1 = G.A. | year = 1976 | title = Two-photon emission in planetary nebula IC 2149 | journal = Publications of the Astronomical Society of the Pacific | volume = 88 | page = 891 | doi=10.1086/130041 | bibcode= }}</ref>
[[Infrared]] and ultraviolet studies of planetary nebulae allowed much more accurate determinations of nebular [[temperature]]s, [[density|densities]] and elemental abundances.<ref name=Hora2004>{{harvnb|Hora|Latter|Allen|Marengo|2004|pp=296–301}}</ref><ref name=Kwoketal2006>{{harvnb|Kwok|Koning|Huang|Churchwell|2006|pp=445–6}}</ref> [[Charge-coupled device]] technology allowed much fainter spectral lines to be measured accurately than had previously been possible. The Hubble Space Telescope also showed that while many nebulae appear to have simple and regular structures when observed from the ground, the very high [[optical resolution]] achievable by telescopes above the [[Earth's atmosphere]] reveals extremely complex structures.<ref name=Reed1999>{{harvnb|Reed|Balick|Hajian|Klayton|1999|pp=2430–41}}</ref><ref name=Alleretal2003>{{harvnb|Aller|Hyung|2003|p=15}}</ref>

Under the [[Morgan-Keenan spectral classification]] scheme, planetary nebulae are classified as ''Type-'''P''''', although this notation is seldom used in practice.<ref name=Krause>{{harvnb|Krause|1961|p=187}}</ref>