After considering my recent research into observations of supernova and planetary nebula, there might be a simple pattern. If this pattern's explanation is correct I hope to verify the details. None of this is new physics but I never find this pattern explained.
This pattern is consistent with the Donald Scott Electric Sun model and the Robitaille & Crothers liquid metallic hydrogen (LMH) solar model.
The Electric Sun model describes the overall solar radiation mechanism including the photosphere and corona as well as the external axial electric current from the galaxy. Internal electric currents are also described as the Sun is a form of an Alfven electrical circuit.
The LMH model describes many internal and outer layers, known from helioseismology.
There are 3 significant disruptions in a star's radiation mechanism:
1) the cataclysmic explosion of a supernova with ejected material,
2) the ejection of only the outer layers resulting in a spherical planetary nebula,
3) the periodic oscillation of the outer photosphere layer resulting in the periodic luminosity change seen as a pulsating variable star.
Each of the two models identifies a cause for a variation in brightness.
The Electric Sun model proposes Double Layers for the star's axial plasma current. The collapse of a double layer (DL) can be a catastrophic event. Ths internal DL collapse is the proposed trigger for the very rare supernova explosion. I expect others have the same conclusion; so this is not a surprise.
The central supernova remnant is usually a plasmoid, sometimes as a pulsar which remains very active, in X-ray or even gamma ray.
There must be different mechanisms defined for the 3 different events, so the reason for this post is to define the other 2.
I gather SAFIRE discovered the collapse of a DL in the anode's atmosphere has sufficient energy for fusion at the surface. This DL at the surface transfers less energy than a DL within the Sun.
The LMH model describes intercalate regions of non-hydrogen atoms between the hydrogen lattice layers. These atoms can get ejected with the fast solar wind, somewhat timed with solar cycles.
There is a youtube video by Sky Scholar titled "Intercalate Zones in the Sun and Stars!"
This description never mentions variable stars but these intercalate regions are connected to the periodic changes in solar activity like sunspots. They are an accumulation of non-hydrogen atoms; they don't belong between the hydrogen lattice layers; and there must be a mechanism for their removal.
from Wikipedia: "The pulsating stars swell and shrink, affecting their brightness and spectrum."
The cause of this periodic oscillation has been called the Kappa-mechanism to explain how the photosphere can oscillate at a considerable velocity during the variable star's luminosity cycle.
The photosphere is an outer layer lying on the convection zone. The material in the convection zone is in motion as it transfers heat from the layer below up to the photosphere.
The photosphere is the zone which actually radiates the solar energy.
During the observed Kappa mechanism of photosphere motion, the photosphere must be moving with or separate from the convection zone. The convection zone is always in motion. In a pulsating variable star, this cyclical mechanism for the photosphere consistently recovers to normal brightness.
I expect the LMH model can explain a variable star just as it does a normal star.
I propose the planetary nebula is the result of a severe disruption in this sub-surface cycle resulting in the ejection of only the outermost layer(s) of the star. The cycle of eliminating intercalate material is timed with changes in solar activity out at the solar surface.
The creation of a planetary nebula must be very rare. Perhaps there is a 'perfect storm' scenario when excessive intercalate material internally reaches a maximum at the same time as multiple energetic DL collapses occur above the photosphere resulting a major disruption in the major components in motion. When the intercalate material has more of the heavier elements this could contribute as an additional factor.
It is difficult to explain the velocity of the ejected plasma cloud.
The planetary nebula has a star remaining within the spherical nebula. This is a star, not a pulsar, nor a neutron star, nor a plasmoid.
These ejected layers were part of the active Alfven circuit and the ejected plasma sphere can maintain active plasma filaments to the central star.
Perhaps this rough explanation is missing critical details for creating a planetary nebula.
Perhaps others concluded the same cause but it is certainly not the popular explanation.
Regardless, I expect this explanation is better than the popular one proposing:
a) loss of equilibrium between the core's internal radiation's outward pressure against inward gravitational collapse,
b) this imbalance disrupts the inner fusion cycle,
c) the fusion cycle overheats,
d) requiring ejection of outer layers,
e) stellar winds drive the material of those layers away from the central star to create a gaseous (plasma) sphere, often symmetrical, moving at a high velocity,
f) the surviving central star is in a new fusion cycle.
Planetary nebula are certainly fascinating, however they are created.