The helium core is eventually exhausted leaving a carbon core with a helium shell.|
Once again, increasing core temperatures result in shell fuel ignition and the helium shell begins fusion depositing more carbon ash into the core.
Once again the sun will have expanded, this time it will grow massive, as much as 1000 times it current size.
Within the core of the massive giant, instabilities cause the sun to pulsate, contracting and expanding a number of times, allowing the outer layers of atmosphere to be cast outward from sun.
Now without a fuel to burn, the core compacts, shrinking away from the expanding outer layers of gas.
These layers of gas are excited by the particles and radiation given off the hot central exposed core and glow as a planetary nebula.