Nuclei of Planetary Nebulae

Abstract

In 1969 Aller and I collected information about the spectra of emission-line nuclei of planetary nebulae. (See also Aller, 1967, for classification of both emission and absorption spectra.) We classified them and compared them to the spectra of Population I stars. Our idea was to follow this general classification by detailed observation and comparison of stars in each of the classes. Most of the detailed studies are not complete, but already the picture has changed from the way Aller and I saw it.