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Vorontsov-Velyaminov scheme

A system for classifying planetary nebulae according to their visual appearance. It recognizes the following main types:
  1. Stellar image (I).
  2. Smooth disk (II) (a, brighter toward center; b, uniform brightness; c, traces of a ring structure).
  3. Irregular disk (III) (a, very irregular brightness distribution; b, traces of ring structure).
  4. Ring structure (IV).
  5. Irregular form, similar to a diffuse nebula (V).
  6. 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).

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