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Additional info for Astronomy (April 2016)
5 Luminosity Eﬀects Luminosity effects are evident in the morphology of galaxies through surface brightness differences between giants and dwarfs and through the sophistication of structure such as spiral arms. Van den Bergh (1998) describes his classification system which takes luminosity effects into account using a set of luminosity classes that are analogous to those used for stars. The largest, most massive spirals have long and well-developed arms, while less massive spirals have less well-defined arms.
In the same manner as bars, a strong spiral often has dust lanes on the concave sides of the inner arms. This is shown for NGC 1566 in the upper right panels of > Fig. 1-25. Both bar and spiral dust lanes are face-on patterns. Another type of face-on pattern is the dust ring. The inner dust ring of NGC 7217 is shown in the right middle frames of > Fig. 1-25, and it appears as the dark, inner edge of a stellar ring having the same shape. Dust rings can also be detected in more inclined galaxies.
This is a type of spiral that is deficient in neutral atomic hydrogen gas, and, as a consequence, it has a lower amount of dust and star formation activity. The arms of NGC 4921 resemble those of an Sb or Sbc galaxy in pitch angle and extensiveness but are as smooth as those typically seen in Sa galaxies. Anemic spirals were first recognized as galaxies with “fuzzy” arms (see van den Bergh, 1998) where star formation has been suppressed due to ram-pressure stripping in the cluster environment. In the case of NGC 4921, the environment is the Coma Cluster.