thermal pulse

Deep inside an aging AGB star, two nuclear furnaces sit one inside the other: a helium-burning shell wrapped by a hydrogen-burning shell. They do not burn together in harmony. Instead the inner helium shell behaves like a faulty pilot light that keeps cutting out and then relighting in sudden, violent bursts. Each of these bursts is a thermal pulse — a periodic flare-up that shakes the whole star from the inside, roughly once every ten thousand to a hundred thousand years.

Here is the cycle. For most of the time, only the hydrogen shell burns, dropping fresh helium 'ash' down onto the dormant helium shell beneath it. That helium layer is thin and slightly degenerate, so when it finally gets hot and dense enough to ignite, it does so explosively, releasing energy thousands of times faster than the star's normal output for a few hundred years. This flash pushes the layers above outward, cooling the hydrogen shell until it switches off. Then everything settles, the hydrogen shell relights, and the slow buildup begins again — pulse after pulse, each one a little stronger than the last.

Thermal pulses are the engine of an AGB star's chemistry. Each flash stirs up the region between the shells, where the slow neutron capture process is quietly building heavy elements, and the resulting convection can dredge this freshly made material up to the surface. Pulse by pulse, the star manufactures and exposes carbon and exotic heavy atoms, then loses them to space in its wind. The number of pulses a star undergoes — usually only a few to a few dozen — helps set how much carbon and how many heavy elements it ultimately contributes to the galaxy.