Shell Burning

An interesting property of shell burning in stars arises for AGB stars. To understand what happens consider the following point:

Suppose that nuclear reactions are ignited in the shell. This causes the temperature of the shell to increase ===> pressure of the shell goes up.
This causes the shell to expand a little. However, since the shell is initially very thin, i.e., dR very small, even a small expansion leads to a significant change in the volume of the shell and hence to a large decrease in the density of the shell. This spells trouble for the star as we now discuss.
As T increases, the shell starts to expand in response to the higher pressure. This is good as the expansion will tend to lower the temperature and to keep the nuclear reaction rate reasonable. What happens is that the pressure will cause the region to expand to the point where the reaction rate is just large enough to keep the region hot. This is usually what happens in core burning stars.
In shell sources, because the density decreases as the shell expands, we find that the pressure drops very quickly too. This slows and stops the expansion before the temperature can drop very much. The nuclear burning region will thus not cool in response to the nuclear energy generation and the rate of the nuclear reactions runs away. We get a flash.
However, in this case, unlike the helium flash, the runaways are not huge [pulses of 100 to 100,000 L(Sun)] and are cyclical. They lead to what are referred to as thermal pulses:

The relevant point for planetary nebulae is that it is beleived that these pulses will lead to an enahanced rate of mass loss from the pulsing AGB star. The mass loss can take the form of the ejection of a series of shells or it can simply lead to an enhanced stellar wind. In any event, thermal pulsing could lead to the ejection of the envelope of the AGB star at speeds of 10 to 30 kilometers per second.