Published online by Cambridge University Press: 14 August 2015
The nuclei of planetary nebulae are examined both observationally and theoretically. It is seen that the region occupied by these stars in the logT – logL diagram is quite wide but consistent with a general progression of stars from high to low luminosity, with a noticeable but not large increase in luminosity during the early phase. The ‘evolutionary path’ is intrinsically quite wide and may indicate the evolution of stars under different conditions or non-monotonic passage along the mean path. Among the several theoretical approaches to this subject, only the double shell burning models seem to offer enough luminosity and short enough timescales to match the observations.