It is well known that PNe are a rather heterogeneous group of em.-line objects having different morphology or being star-like, especially when we consider their evolutionary stage. There are altogether 245 objects which are no longer classified as PNe (Table 3 and Table 1).
It is relatively easy to recognize extragalactic objects (ad 1) according to their morphology on direct photographs or even better from the red-shift of the spectral lines.
As to the galactic objects the largest group of misclassified PNe are actually red stars (ad 2) with strong continua near H. The red stellar continua simulate the existence of emission lines H and [NII] on low-dispersion spectra. Objects with no emission lines (ad 5) belong also to this category. The reflection nebulae (ad 5) may in principle also be objects in the evolutionary phase before PNe, i.e. pre-PNe. Galactic objects with emission lines are most similar to PNe. Several types of em.-line stars (ad 3) without nebulae look like young PNe; symbiotic stars (ad 6) as well as compact HII regions (ad 4) can be extremely similar to PNe. Also novae with their nebulae could match PNe.
The last category represents objects which are actually not real nebulae (ad 7,8) or objects which are identical with already known PNe or their condensations (ad 9).
Four objects given also in Table 3 (MaC 1-1, 217-00.1*; Cn 1-1, 330+04.1*; PC 11, 331-05.1*; M 1-27, 356-02.2*) were once removed from the list of PNe as em.-line stars but have now been included again in the list of PNe.
The theoretical definition of ``what is a planetary nebula'' plays a great role for the decision as to whether we include the object in the list of PNe or not. This definition depends of course on the present state of knowledge.
|Former confusion with another object-type:||N|
|galaxy, em.-line galaxy||53|
|em.-line star (H em. only, Be, BQ[ ], WR, OB, peculiar)||40|
|object with no em. lines, reflection nebula||24|
|plate fault, nova||17|
|several faint stars, no FC and no observations||4|
|identity with another PN or a knot in another object||7|