Editing Planetary nebula (section)

=== 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>