Paradoxically, observations of PN in external galaxies offer several advantages over the study of galactic PN. Because of the heavy obscuration of the center of our Galaxy, our catalog of galactic PN is highly incomplete. However, PN in nearby galaxies, for example, M31, can be optically identified to within a few parsecs from the nucleus. PN were first detected in M31 by Baade (1955). Five PN southwest of M31's center were detected in [OIII] with a 1,500-å filter and the Palomar 5-m telescope. By using a narrower filter (23 å), Ford and Jacoby (1978) were able to go much fainter and detected 315 PN in the bulge of M31. Later surveys have cataloged PN in M32, NGC 205, NGC 185, and NGC 147.

A survey of PN in an external galaxy can in fact provide a much better picture of the spatial distribution of PN. As discussed in Chapter 16, the distances to galactic PN are highly uncertain. Since the PN in an external galaxy can be safely taken to be at the same distance, the identification of a large number of PN in a galaxy of known distance can lead to an accurate luminosity function.

CSPN are cores of AGB stars and are therefore very luminous objects. If the nebula is ionization bounded, the central star's radiation is reprocessed into line radiation. For example, in the nebular model shown in Fig. 4.3, ∼ 3% of the stellar luminosity (or ∼300 L⊙) comes out in the [OIII] line.