Editing Stellar evolution (section)

====Post-AGB====
{{Main|Post-AGB}}
[[Image:NGC6543.jpg|thumb|The [[Cat's Eye Nebula]], a [[planetary nebula]] formed by the death of a star with about the same mass as the Sun]]
These mid-range stars ultimately reach the tip of the asymptotic-giant-branch and run out of fuel for shell burning. They are not sufficiently massive to start full-scale carbon fusion, so they contract again, going through a period of post-asymptotic-giant-branch superwind to produce a planetary nebula with an extremely hot central star. The central star then cools to a white dwarf. The expelled gas is relatively rich in heavy elements created within the star and may be particularly [[oxygen]] or [[carbon]] enriched, depending on the type of the star. The gas builds up in an expanding shell called a [[circumstellar envelope]] and cools as it moves away from the star, allowing [[Circumstellar dust|dust particles]] and molecules to form. With the high infrared energy input from the central star, ideal conditions are formed in these circumstellar envelopes for [[Astrophysical maser|maser]] excitation.

It is possible for thermal pulses to be produced once post-asymptotic-giant-branch evolution has begun, producing a variety of unusual and poorly understood stars known as born-again asymptotic-giant-branch stars.<ref name=bornagain>{{cite journal|bibcode=1991IAUS..145..363H|title=Atmospheres and Abundances of Blue Horizontal Branch Stars and Related Objects|journal=Evolution of Stars: The Photospheric Abundance Connection: Proceedings of the 145th Symposium of the International Astronomical Union|volume=145|pages=363|last1=Heber|first1=U.|year=1991}}</ref> These may result in extreme [[horizontal-branch]] stars ([[subdwarf B star]]s), hydrogen deficient post-asymptotic-giant-branch stars, variable planetary nebula central stars, and [[R Coronae Borealis variable]]s.