thermal pulse: Periodic eruptions in the helium fusion shell in an aging giant star; thought to aid in ejecting the surface layers of the stars to form planetary nebulae.
Thermal Pulse - The rapid consumption of helium in a shell within an asymptotic giant branch star ...
Thermal Pulse: A sudden increase in temperature caused by a dramatic increase in a nuclear burning rate. Although it falls short of what we would call an explosion, it does drive a dynamic readjustment in the star.
The interval between successive thermal pulses decreases as the AGB star ages. For solar-mass stars such pulses dramatically increase the luminosity for several decades.
the Sun climbs the AGB, encountering four thermal pulses .... the Sun reaches its largest extent at 0.99 AU ... However, at this point the Sun's mass has been reduced to 0.591 M(sun) and the orbits of Venus and Earth have moved out to 1.22 and 1.
However, the hydrogen shell eventually dumps enough helium "ash" onto the helium shell that the latter undergoes an explosive event called a thermal pulse.
However, over periods of 10,000 to 100,000 years, the helium shell switches on again, and the hydrogen shell switches off, a process known as a helium shell flash or thermal pulse.
Meanwhile, each thermal pulse causes the outer layers of the star to expand, resulting in a period of mass loss. Eventually, the outer layers of the star are ejected completely and ionised by the white dwarf to form a planetary nebula.
As new regions of helium start to burn, the star can experience Helium Shell Flashes (also called thermal pulses).
 See also: Red Giant, Giant, Energy, Luminosity, Mass
  
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