  Strange modes have been found in the radial spectrum of many luminous stars,
most recently in Cepheids and RR Lyrae. We show that there is nothing strange
about these modes -- they must exist even in the adiabatic limit.
  With a change of variables and without approximation the adiabatic linear
pulsation equation is reduced to a Schroedinger equation in which the radial
coordinate is the local sound-traversal time. In this formulation, the narrow
hydrogen partial ionization region is seen to act as a potential barrier,
separating the star into two regions. Resonances between the two regions result
in the strange modes, for which the ratio of interior to exterior amplitude is
at a minimum. The relative location of the barrier changes with the stellar
parameters, and this gives rise to avoided level-crossings along a sequence of
models. 2 The appearance of strange modes and the associated level crossings
are exhibited with an analytic toy-model with the potential barrier
approximated by a delta function. This toy-model is readily extensible to
nonadiabatic modes.
  Hydrodynamical calculations find that pure strange mode limit-cycles have
extremely small photospheric velocities and luminosity variations in the
milli-magnitude range. They are therefore expected to be difficult to observe.

Astrophysics;Chaotic Dynamics;Chaotic Dynamics