The proto-planetary nebula phase of stellar evolution is undergone by intermediate mass stars (0.8 Msolar < M < 8 Msolar) between the end of the Asymptotic Giant Branch phase and that of the planetary nebula. It is a rapid transition lasting at most a few thousand years and in some cases possibly as little as about 100 years. The term is often used interchangeably with 'pre-planetary nebula' and 'post-AGB star'.
When stars leave the tip of the AGB, the prodigious mass-loss of up to 10-4 Msolar year-1 moving at 10-20 km s-1 that characterises that phase drops dramatically to something of order 10-7 Msolar year-1. The result is that the extended circumstellar envelope that developed during the AGB phase detaches from the stellar photosphere and the proto-planetary nebula phase begins. The detached dust shell in these objects is apparent in their infrared spectra energy distributions, which typically display a 'double-humped' profile with the shorter-wavelength component due to the photosphere and the longer-wavelength component due to the dust shell. It is thought that a fast wind of order 100s km s-1 is then triggered and emanates from the stellar core, although the launching mechanism of this wind is unclear. The interaction with the fast wind is believed to be the mechanism by which any asymmetries in the remnant, slow AGB wind are amplified, eventually resulting in the vast array of morphologies displayed by planetary nebulae once the central star has become sufficiently hot (Teff > 30,000 K) to commence photoionisation of the surrounding material. Once photoionisation commences, the star enters the planetary nebula phase.