Editing Stellar evolution (section)

====Asymptotic-giant-branch phase====
{{Main|Asymptotic giant branch}}
After a star has consumed the helium at the core, hydrogen and helium fusion continues in shells around a hot core of [[carbon]] and [[oxygen]]. The star follows the [[asymptotic giant branch]] on the Hertzsprung–Russell diagram, paralleling the original red-giant evolution, but with even faster energy generation (which lasts for a shorter time).<ref>{{Cite journal | last1 = Sackmann | first1 = I. -J. | last2 = Boothroyd | first2 = A. I. | last3 = Kraemer | first3 = K. E. | title = Our Sun. III. Present and Future | doi = 10.1086/173407 | journal = The Astrophysical Journal | volume = 418 | pages = 457 | year = 1993 |bibcode = 1993ApJ...418..457S | doi-access = free }}</ref>  Although helium is being burnt in a shell, the majority of the energy is produced by hydrogen burning in a shell further from the core of the star.  Helium from these hydrogen burning shells drops towards the center of the star and periodically the energy output from the helium shell increases dramatically.  This is known as a [[thermal pulse]] and they occur towards the end of the asymptotic-giant-branch phase, sometimes even into the post-asymptotic-giant-branch phase. Depending on mass and composition, there may be several to hundreds of thermal pulses.

There is a phase on the ascent of the asymptotic-giant-branch where a deep convective zone forms and can bring carbon from the core to the surface.  This is known as the second dredge up, and in some stars there may even be a third dredge up.  In this way a [[carbon star]] is formed, very cool and strongly reddened stars showing strong carbon lines in their spectra.  A process known as hot bottom burning may convert carbon into oxygen and nitrogen before it can be dredged to the surface, and the interaction between these processes determines the observed luminosities and spectra of carbon stars in particular clusters.<ref name=hbb>{{cite journal|author1=van Loon |author2=Zijlstra |author3=Whitelock|author4=Peter te Lintel Hekkert|author5=Chapman|author6=Cecile Loup|author7=Groenewegen|author8=Waters|author9=Trams|title=Obscured Asymptotic Giant Branch stars in the Magellanic Clouds IV. Carbon stars and OH/IR stars|date=1998|volume=329|issue=1|pages=169–85|journal= Monthly Notices of the Royal Astronomical Society|arxiv=astro-ph/9709119v1 |citeseerx=10.1.1.389.3269 |bibcode = 1996MNRAS.279...32Z |doi=10.1093/mnras/279.1.32 |doi-access=free |url=https://pure.uva.nl/ws/files/978044/1833_20357y.pdf}}</ref>

Another well known class of asymptotic-giant-branch stars is the [[Mira variable]]s, which pulsate with well-defined periods of tens to hundreds of days and large amplitudes up to about 10 magnitudes (in the visual, total luminosity changes by a much smaller amount). In more-massive stars the stars become more luminous and the pulsation period is longer, leading to enhanced mass loss, and the stars become heavily obscured at visual wavelengths.  These stars can be observed as [[OH/IR star]]s, pulsating in the infrared and showing OH [[maser]] activity.  These stars are clearly oxygen rich, in contrast to the carbon stars, but both must be produced by dredge ups.