  Stars in the narrow mass range of about 2.5 and 3.5 solar masses can develop
a thermally unstable He-burning shell during its ignition phase. We study, from
the point of view secular stability theory, these so called thermal micropulses
and we investigate their properties; the thermal pulses constitute a convenient
conceptual laboratory to look thoroughly into the physical properties of a
helium-burning shell during the whole thermally pulsing episode. Linear
stability analyses were performed on a large number of 3 solar-mass star models
at around the end of their core helium-burning and the beginning of the
double-shell burning phase. The stellar models were not assumed to be in
thermal equilibrium. The thermal mircopulses, and we conjecture all other
thermal pulse episodes encountered by shell-burning stars, can be understood as
the nonlinear finite-amplitude realization of an oscillatory secular
instability that prevails during the whole thermal pulsing episode. Hence, the
cyclic nature of the thermal pulses can be traced back to a linear instability
concept.
