With some complications glossed over, the envelope and as much as 50% of the stellar mass is detached from the star and expelled into space leaving the AGB star very hot core exposed.
Note that the name is a misnomer. There are several PN that can be barely resolved with the naked eye as is the case for the brighter planets. So these objects do not "twinkle" like the stars and they (PN and planets) got lumped together.
Lots of emission lines from the e- cascades
Some electron orbits are "meta-stable" and rather than immediately de-exciting via emission of a photon from these starts the e- sit in these particular excited states for a long time before dropping down and emitting a photon. On Earth, before the "forbidden" photon can be emitted, the atom is collisionally de-excited. It is only because the density in the PN is very low that an atom can float around long enough in the excited state for a photon-emission de-excitation to occur.
It is a geometrical effect.
In the RGB and AGB phases of evolution the convection zone of stars extends deep into the center of the star and the freshly minted He, Carbon and Oxygen products of fusion reactions are mixed up to the surface of stars. PN envelopes are therefore composed partly of "new" elements.
PN return between 0.05 and 0.6 MSun of partly enriched material back to the ISM.
For these stars it never gets hot enough in the core to have Carbon or Oxygen fusion occur. So the "central star" of PN is not a star at all.
|Equilibrium||Gravity vs. e- degeneracy|
|Temperature||Hot! (100,000 K to 200,000 K)|
|Size||R = 6000 km (Earth-sized)|
|Energy Source||None (residual heat)|
The core is hot and therefore blue and small with a correspondingly low luminosity.
H fusion in core
He core contraction & H shell fusion
He core fusion & H shell fusion
Carbon core fusion & He shell & H shell
|5 x 107 years
9 x 109 years
5 x 108 years
5 x 107 years
1 x 106 years
long, long, long time