We now recognize that a planetary nebula is a dynamical system whose nebular evolution is closely coupled to the evolution of the central star. The existence of a planetary nebula depends on the nebular and central star components evolving in step with each other. A complete description of the PN phenomenon therefore requires the following elements:

1. Evolution model of the central star [L*(t) and T*(t)]. Other than the question of whether central stars of PN are predominately hydrogen or helium burning, one major uncertainty is the extent of mass loss in the post-AGB phase. Since the evolution time from the end of AGB to the beginning of photoionization is critical for the existence of PN, a better estimate on the mass-loss rate during the post-AGB phase is needed.

2. Winds from central stars of PN [睅(t) and v(t)]. Whereas mass loss during the post-AGB phase affects the transition time to PN, mass loss during the PN phase has crucial effects on the dynamics of the nebula. Not only does the wind from the central star compress and accelerate the nebular shell, it also shapes the morphology of the nebula. On the observational side, the line profiles can be used to measure the terminal velocity and the mass-loss rate. Since the winds are likely to be driven by radiation pressure on resonance lines, theoretical estimates on 睅 can also be made.

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