In the earlier chapters, we discussed the radiative and mechanical processes in PN. These processes all contribute toward the interstellar radiation field and the return of stellarprocessed material to the interstellar medium. The return of mass to the interstellar medium is not only important in providing material for the formation of next generation of stars, but also in seeding the interstellar medium with CNO products that provide the cooling agents necessary for the collapse of interstellar clouds and star formation. In order for the relative importance between PN and other contributors (e.g. Wolf-Rayet stars and supernovae) to be assessed, it is necessary to know the total PN population in the Galaxy.

The first estimate of the total population of PN in the Galaxy (6 × 104) was given by Shklovsky (1956b), using the distance scale that he developed. This determination is instrumental in establishing the evolutionary status of PN as a stage that is passed through by all low-mass stars (Abell and Goldreich, 1966). This number also gives us an insight into the star-formation history of the Galaxy, as the progenitor stars of the PN that we are observing today were born billions of years ago.

Formation rate of PN in the Galaxy

In principle, the derivation of the formation rate of PN (x) is straightforward once the local number density of PN is known.