Theory of Evolution of Central Stars of Planetary Nebulae

Abstract

Young central stars of planetary nebulae burn hydrogen and helium in the shell sources. Within less than 104 years nuclear fuel is exhausted and old central stars cool off at almost constant radius to the white dwarf stage.Young, i.e., luminous central stars should follow complicated loops on the H-R diagram as a result of helium shell flashes. FG Sagittae is the example. This should be a typical behaviour and a systematic search for light variations on a time scale of years or decades among other luminous entral stars should be undertaken. Because of rapid evolution of their nuclei many planetary nebulae may be far from the ionization equilibrium. This should be taken into account when the effective temperature and bolometric luminosity of the central stars are estimated.