Vorontsov-Velyaminov scheme
A system for classifying planetary nebulae according to their visual appearance. It recognizes the following main types:
Stellar image (I).
Smooth disk (II) (a, brighter toward center; b, uniform brightness; c, traces of a ring structure).
Irregular disk (III) (a, very irregular brightness distribution; b, traces of ring structure).
Ring structure (IV).
Irregular form, similar to a diffuse nebula (V).
Anomalous form (VI).
More complex structures are characterized by combinations such as “4+2” (ring and disk), or “4+4” (two rings). The scheme was devised by the Russian astrophysicist Boris Vorontsov-Velyaminov (1904-1994).