The classification used in the present investigation is essentially the detailed formulation of a preliminary classification published in a previous paper.[6] It was developed in 1923, from a study of photographs of several thousand nebulae, including practically all the brighter objects and a thoroughly representative collection of the fainter ones.[7] It is based primarily on the structural forms of photographic images, although the forms divide themselves naturally into two groups: those found in or near the Milky Way and those in moderate or high galactic latitudes. In so far as possible, the system is independent of the orientation of the objects in space. With minor changes in the orig…
Your support pays for translating these classics into more languages.
Support for $12/month DonateSubscribers read with instant translation, without waiting for the free daily limit.