Although PN are well known for their ring-shape appearance, they in fact have a diverse range of morphologies. Using photographs that he took at the Lick Observatory, Curtis (1918) was the first to arrange PN into different classes based on their appearances. The origin of such diverse shapes has remained a mystery for a long time. For example, the well known Ring Nebula (NGC 6720) has an elliptical ring appearance. The most obvious interpretation is that this represents a three-dimensional hollow shell projected onto the sky. However, the actual observed surface brightness of the “hole” in comparison to the shell is too low (∽1:20) to be consistent with this model (Minkowski and Osterbrock, 1960). The observed intensity distribution is in fact more compatible with an open-ended toroid viewed end on (Khromov and Kohoutek, 1968). Although this model gives a good approximation to the observed image, the origin of such a toroid is not explained. The physical origin of the different morphologies of PN and how they evolve to such forms therefore represents one of the greatest challenges in PN research.

Morphological classifications

Curtis (1918) classified his sample of 78 PN into helical, annular, disk (uniform and centrally bright), amorphous, and stellar. Subsequent classification schemes often use similar descriptive forms: stellar, disk, irregular, ring, anomalous (Perek & Kohoutek, 1967); elliptical, rings, bipolar, interlocking, peculiar, and doubtful (Greig, 1971; Westerlund and Henize, 1967); and round, elliptical, and butterfly (Balick, 1987).