Stars on the main sequence (MS) obey a well-defined luminosity-spectral type relationship. The method of “spectroscopic parallax” assumes that a star of a certain spectral type will have a certain intrinsic luminosity, and by comparing with its visual magnitude, it is possible to derive its distance. However, the central stars of many PN cannot be observed, and even in the case where a visual magnitude is available, their high temperatures imply that large bolometric corrections are required. Whereas stars on the MS remain stationary in the same position on the H-R diagram, CSPN undergo large changes in temperature and luminosity over their lifetime. As a result, any derivations of distances from stellar properties are necessarily model dependent.

Except for a few cases of nearby PN where trigonometric parallaxes are possible, or in cases where there is a MS binary companion, most of the distances to PN have to be estimated from their nebular properties. By making certain assumptions on the nebular structure, distances can be derived by measurements of fluxes, angular sizes, electron densities, and so on. These methods are collectively called statistical distances.

A well-determined distance scale for PN is necessary for the investigation of the space density, galactic distribution, total number of PN, and the birth rate of PN in the Galaxy (see Chapter 18). Unfortunately, after many years of efforts, distances to PN remain controversial.