Question: What does the star emit during the AGB phase?
About the article "Nebula", section "Protoplanetary nebula".

Candidate passages:
 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.  
A protoplanetary nebula (PPN) is an astronomical object which is at the short-lived episode during a star's rapid stellar evolution between the late asymptotic giant branch (LAGB) phase and the following planetary nebula (PN) phase.
During the AGB phase, the star undergoes mass loss, emitting a circumstellar shell of hydrogen gas.
When this phase comes to an end, the star enters the PPN phase.
The PPN is energized by the central star, causing it to emit strong infrared radiation and become a reflection nebula.
Collaminated stellar winds from the central star shape and shock the shell into an axially symmetric form, while producing a fast moving molecular wind.
The exact point when a PPN becomes a planetary nebula (PN) is defined by the temperature of the central star.
The PPN phase continues until the central star reaches a temperature of 30,000 K, after which is it hot enough to ionize the surrounding gas.
After a star passes through the asymptotic giant branch (AGB) phase, the short planetary nebula phase of stellar evolution begins as gases drift away from the central star at speeds of a few kilometers per second.
The central star is the remnant of its AGB progenitor, an electron-degenerate carbon-oxygen core that has lost most of its hydrogen envelope due to mass loss on the AGB.
As the gases expand, the central star undergoes a two stage evolution, first growing hotter as it continues to contract and hydrogen fusion reactions occur in the shell around the core and then slowly cooling once the hydrogen shell is exhausted through fusion and mass loss.
In the second phase, it radiates away its energy and fusion reactions cease, as the central star is not heavy enough to generate the core temperatures required for carbon and oxygen to fuse.
During the first phase, the central star maintains constant luminosity, while at the same time it grows ever hotter, eventually reaching temperatures around 100,000 K. In the second phase, it cools so much that it does not give off enough ultraviolet radiation to ionize the increasingly distant gas cloud.
The star becomes a white dwarf, and the expanding gas cloud becomes invisible to us, ending the planetary nebula phase of evolution.
For a typical planetary nebula, about 10,000 years passes between its formation and recombination of the star. 
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. 
As many as a quarter of all post-AGB stars undergo what is dubbed a "born-again" episode.
The carbon–oxygen core is now surrounded by helium with an outer shell of hydrogen.
If the helium is re-ignited a thermal pulse occurs and the star quickly returns to the AGB, becoming a helium-burning, hydrogen-deficient stellar object.
If the star still has a hydrogen-burning shell when this thermal pulse occurs, it is termed a "late thermal pulse".
Otherwise it is called a "very late thermal pulse".
The outer atmosphere of the born-again star develops a stellar wind and the star once more follows an evolutionary track across the Hertzsprung–Russell diagram.
However, this phase is very brief, lasting only about 200 years before the star again heads toward the white dwarf stage.
Observationally, this late thermal pulse phase appears almost identical to a Wolf–Rayet star in the midst of its own planetary nebula.