Editing Helium flash (section)

==Helium shell flash<span class="anchor" id="Shell helium flash"></span><span class="anchor" id="shell helium flash"></span>==
'''Helium shell flashes''' are a somewhat analogous but much less violent, nonrunaway helium ignition event, taking place in the absence of degenerate matter. They occur periodically in [[asymptotic giant branch]] stars in a shell outside the core. This is late in the life of a star in its giant phase. The star has burnt most of the helium available in the core, which is now composed of carbon and oxygen. Helium fusion continues in a thin shell around this core, but then turns off as helium becomes depleted. This allows hydrogen fusion to start in a layer above the helium layer. After enough additional helium accumulates, helium fusion is reignited, leading to a thermal pulse which eventually causes the star to expand and brighten temporarily (the pulse in luminosity is delayed because it takes a number of years for the energy from restarted helium fusion to reach the surface<ref name = "Wood"/>). Such pulses may last a few hundred years, and are thought to occur periodically every 10,000 to 100,000 years.<ref name = "Wood">{{Cite journal
| volume = 247
| issue = Part 1
| last = Wood
| first = P. R.
|author2=D. M. Zarro
| title = Helium-shell flashing in low-mass stars and period changes in mira variables
| journal = Astrophysical Journal
| date = 1981
|bibcode = 1981ApJ...247..247W
|doi = 10.1086/159032
| pages = 247 }}</ref> 
After the flash, helium fusion continues at an exponentially decaying rate for about 40% of the cycle as the helium shell is consumed.<ref name = "Wood"/> Thermal pulses may cause a star to shed circumstellar shells of gas and dust.{{citation needed|date=February 2016}}