Editing Planetary nebula (section)

==Origins==
[[File:Stellar nebula simulation.jpg|thumb|alt=Central star has elongated S shaped curve of white emanating in opposite directions to the edge. A butterfly-like area surrounds the S shape with the S shape corresponding to the body of the butterfly. |Computer simulation of the formation of a planetary nebula from a star with a warped disk, showing the complexity which can result from a small initial asymmetry]]

Stars greater than 8&nbsp;[[solar mass]]es (M<sub>⊙</sub>) will probably end their lives in dramatic [[supernova]]e explosions, while planetary nebulae seemingly only occur at the end of the lives of intermediate and low mass stars between 0.8&nbsp;M<sub>⊙</sub> to 8.0&nbsp;M<sub>⊙</sub>.<ref name="Macieletal2009">{{harvnb|Maciel|Costa|Idiart|2009|pp=127–37}}</ref> Progenitor stars that form planetary nebulae will spend most of their lifetimes converting their [[hydrogen]] into [[helium]] in the star's core by [[nuclear fusion]] at about 15&nbsp;million [[Kelvin|K]]. This generates energy in the core, which creates outward pressure that balances the crushing inward pressures of gravity.<ref name=Harpaz4>{{harvnb|Harpaz|1994|pp=55–80}}</ref> This state of equilibrium is known as the [[main sequence]], which can last for tens of millions to billions of years, depending on the mass.

When the hydrogen in the core starts to run out, nuclear fusion generates less energy and gravity starts compressing the core, causing a rise in temperature to about 100&nbsp;million&nbsp;K.<ref name=Harpaz6>{{harvnb|Harpaz|1994|pp=99–112}}</ref> Such high core temperatures then make{{how|date=December 2023}} the star's cooler outer layers expand to create much larger red giant stars. This end phase causes a dramatic rise in stellar luminosity, where the released energy is distributed over a much larger surface area, which in fact causes the average surface temperature to be lower. In [[stellar evolution]] terms, stars undergoing such increases in luminosity are known as [[Asymptotic giant branch|asymptotic giant branch stars]] (AGB).<ref name=Harpaz6/> During this phase, the star can lose 50–70% of its total mass from its [[stellar wind]].<ref name=wood>{{cite journal
 | last1=Wood | first1=P. R. | last2=Olivier | first2=E. A.
 | last3=Kawaler | first3=S. D. | year=2004
 | title=Long Secondary Periods in Pulsating Asymptotic Giant Branch Stars: An Investigation of Their Origin
 | journal=[[The Astrophysical Journal]]
 | volume=604 | issue=2 | pages=800
 | bibcode=2004ApJ...604..800W | doi=10.1086/382123 | doi-access= | s2cid=121264287 }}</ref>

For the more massive asymptotic giant branch stars that form planetary nebulae, whose progenitors exceed about 0.6M<sub>⊙</sub>, their cores will continue to contract. When temperatures reach about 100&nbsp;million K, the available [[helium|helium nuclei]] fuse into [[carbon]] and [[oxygen]], so that the star again resumes radiating energy, temporarily stopping the core's contraction. This new helium burning phase (fusion of helium nuclei) forms a growing inner core of inert carbon and oxygen. Above it is a thin helium-burning shell, surrounded in turn by a hydrogen-burning shell. However, this new phase lasts only 20,000 years or so, a very short period compared to the entire lifetime of the star.

The venting of atmosphere continues unabated into interstellar space, but when the outer surface of the exposed core reaches temperatures exceeding about 30,000&nbsp;K, there are enough emitted [[ultraviolet]] [[photon]]s to [[ionisation|ionize]] the ejected atmosphere, causing the gas to shine as a planetary nebula.<ref name=Harpaz6/>