Editing Planetary nebula (section)

===Physical characteristics===
[[File:M57 The Ring Nebula.JPG|thumb|alt=Elliptical shell with fine red outer edge surrounding region of yellow and then pink around a nearly circular blue area with the central star at its center. A few background stars are visible.|NGC 6720, the [[Ring Nebula]]]]
[[File:IC3568.jpg|thumb|[[Lemon slice nebula]] (IC 3568)]]

A typical planetary nebula is roughly one [[light year]] across, and consists of extremely rarefied gas, with a density generally from 100 to 10,000 particles {{nowrap|per cm<sup>3</sup>}}.<ref name=Osterbrock1>{{harvnb|Osterbrock|Ferland|2005|p=10}}</ref> (The Earth's atmosphere, by comparison, contains 2.5{{e|19}} particles {{nowrap|per cm<sup>3</sup>}}.) Young planetary nebulae have the highest densities, sometimes as high as 10<sup>6</sup> particles {{nowrap|per cm<sup>3</sup>}}. As nebulae age, their expansion causes their density to decrease. The masses of planetary nebulae range from 0.1 to 1&nbsp;[[solar mass]]es.<ref name=Osterbrock1/>

Radiation from the central star heats the gases to temperatures of about 10,000&nbsp;[[Kelvin|K]].<ref name=Gurzadyan2>{{harvnb|Gurzadyan|1997|p=238}}</ref> The gas temperature in central regions is usually much higher than at the periphery reaching 16,000–25,000&nbsp;K.<ref name=Gurzadyan3>{{harvnb|Gurzadyan|1997|pp=130–7}}</ref> The volume in the vicinity of the central star is often filled with a very hot (coronal) gas having the temperature of about 1,000,000&nbsp;K. This gas originates from the surface of the central star in the form of the fast stellar wind.<ref name=Osterbrock261>{{harvnb|Osterbrock|Ferland|2005|pp=261–2}}</ref>

Nebulae may be described as ''matter bounded'' or ''radiation bounded''. In the former case, there is not enough matter in the nebula to absorb all the UV photons emitted by the star, and the visible nebula is fully ionized. In the latter case, there are not enough UV photons being emitted by the central star to ionize all the surrounding gas, and an ionization front propagates outward into the circumstellar envelope of neutral atoms.<ref name=Osterbrock2>{{harvnb|Osterbrock|Ferland|2005|p=207}}</ref>