Question: What are a star's emissions in its AGB phase?
About the article "Nebula", section "Protoplanetary nebula".

Candidate passages:
The AGB phase is divided into two parts, the early AGB (E-AGB) and the thermally pulsing AGB (TP-AGB).
During the E-AGB phase the main source of energy is helium fusion in a shell around a core consisting mostly of carbon and oxygen.
During this phase the star swells up to giant proportions to become a red giant again.
The star's radius may become as large as one astronomical unit (~215R).
After the helium shell runs out of fuel, the TP-AGB starts.
Now the star derives its energy from fusion of hydrogen in a thin shell, which restricts the inner helium shell to a very thin layer and prevents it fusing stably.
However, over periods of 10,000 to 100,000 years, helium from the hydrogen shell burning builds up and eventually the helium shell ignites explosively, a process known as a helium shell flash.
The luminosity of the shell flash peaks at thousands of times the total luminosity of the star, but decreases exponentially over just a few years.
The shell flash causes the star to expand and cool which shuts off the hydrogen shell burning and causes strong convection in the zone between the two shells.
When the helium shell burning nears the base of the hydrogen shell, the increased temperature reignites hydrogen fusion and the cycle begins again.
The large but brief increase in luminosity from the helium shell flash produces an increase in the visible brightness of the star of a few tenths of a magnitude for several hundred years, a change unrelated to the brightness variations on periods of tens to hundreds of days that are common in this type of star.
During the thermal pulses, which last only a few hundred years, material from the core region may be mixed into the outer layers, changing the surface composition, a process referred to as "dredge-up".
Because of this dredge-up, AGB stars may show S-process elements in their spectra and strong dredge-ups can lead to the formation of carbon stars.
All dredge-ups following thermal pulses are referred to as third dredge-ups, after the first dredge-up, which occurs on the red-giant branch, and the second dredge up, which occurs during the E-AGB.
In some cases there may not be a second dredge-up but dredge-ups following thermal pulses will still be called a third dredge-up.
Thermal pulses increase rapidly in strength after the first few, so third dredge-ups are generally the deepest and most likely to circulate core material to the surface.AGB stars are typically long-period variables, and suffer mass loss in the form of a stellar wind.
Thermal pulses produce periods of even higher mass loss and may result in detached shells of circumstellar material.
A star may lose 50 to 70% of its mass during the AGB phase. 
 phase.
During the ensuing protoplanetary nebula phase, the central star's effective temperature will continue rising as a result of the envelope's mass loss as a consequence of the hydrogen shell's burning.
During this phase, the central star is still too cool to ionize the slow-moving circumstellar shell ejected during the preceding AGB phase.
However, the star does appear to drive high-velocity, collimated winds which shape and shock this shell, and almost certainly entrain slow-moving AGB ejecta to produce a fast molecular wind.
Observations and high-resolution imaging studies from 1998 to 2001, demonstrate that the rapidly evolving PPN phase ultimately shapes the morphology of the subsequent PN.
At a point during or soon after the AGB envelope detachment, the envelope shape changes from roughly spherically symmetric to axially symmetric.
The resultant morphologies are bipolar, knotty jets and Herbig–Haro-like "bow shocks".
These shapes appear even in relatively "young" PPN.  
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While most authors consider HD 179821 to be a supergiant star, others think it is actually a protoplanetary nebula or a post-AGB star at a distance of 1 kiloparsec (3,200 light years).
In that case the star's luminosity would be much lower, around 16,000 times that of our Sun and its initial mass would be equal to the current mass of our Sun.
This discrepancy arises because its distance is too great to be measured by parallax and it has some properties of both a yellow hypergiant and a protoplanetary nebula/Post-AGB star. 
Nearly all small and intermediate-mass stars (below about nine solar masses) will end up as white dwarfs once they have exhausted their supply of thermonuclear fuel.
Such stars spend most of their energy-producing life span as a main sequence star.
The amount of time that a star spends on the main sequence depends primarily on its mass, with the lifespan decreasing with increasing mass.
Thus, for IK Pegasi B to have become a white dwarf before component A, it must once have been more massive than component A.
In fact, the progenitor of IK Pegasi B is thought to have had a mass between 5 and 8 solar masses.
As the hydrogen fuel at the core of the progenitor of IK Pegasi B was consumed, it evolved into a red giant.
The inner core contracted until hydrogen burning commenced in a shell surrounding the helium core.
To compensate for the temperature increase, the outer envelope expanded to many times the radius it possessed as a main sequence star.
When the core reached a temperature and density where helium could start to undergo fusion this star contracted and became what is termed a horizontal branch star.
That is, it belonged to a group of stars that fall upon a roughly horizontal line on the H-R diagram.
The fusion of helium formed an inert core of carbon and oxygen.
When helium was exhausted in the core a helium-burning shell formed in addition to the hydrogen-burning one and the star moved to what astronomers term the asymptotic giant branch, or AGB.
(This is a track leading to the upper-right corner of the H-R diagram.)
If the star had sufficient mass, in time carbon fusion could begin in the core, producing oxygen, neon and magnesium.
The outer envelope of a red giant or AGB star can expand to several hundred times the radius of the Sun, occupying a radius of about (3 A.U.)
in the case of the pulsating AGB star Mira.
This is well beyond the current average separation between the two stars in IK Pegasi, so during this time period the two stars shared a common envelope.
As a result, the outer atmosphere of IK Pegasi A may have received an isotope enhancement.
Some time after an inert oxygen-carbon (or oxygen-magnesium-neon) core formed, thermonuclear fusion began to occur along two shells concentric with the core region; hydrogen was burned along the outermost shell, while helium fusion took place around the inert core.
However, this double-shell phase is unstable, so it produced thermal pulses that caused large-scale mass ejections from the star's outer envelope.
This ejected material formed an immense cloud of material called a planetary nebula.
All but a small fraction of the hydrogen envelope was driven away from the star, leaving behind a white dwarf remnant composed primarily of the inert core.