The flocculent spiral pattern in NGC 7217

This is an early-type spiral (type Sab), with a dominant bulge and tightly wound arms. In fact, if we were to look very deeply at this galaxy, we might consider it an elliptical galaxy or bulge with a weak spiral ring inside. The color difference betwen the old bulge population and the arms is quite noticeable. On close inspection, a tight spiral and ring pattern can be seen near the center of the galaxy, outlined solely by absorbing dust. The spiral arms in NGC 7217 are typical of so-called flocculent or filamentary arms. Although there is a clear impression of a spiral pattern, no single spiral feature can be traced very far around the galaxy - the situation is remniscent of twisted-rope optical illusions. This kind of pattern is produced by models which allow regions of star formation to trigger further nearby star formation, and having the whole complex smeared out with time by the galaxy's rotation. Grand-design patterns are most common in galaxies with bars or close neighbors; perhaps the dominant mode is controlled by a galaxy's overall form and surroundings.

This true-color composite was produced from CCD images taken by Ronald Buta and Guy Purcell (University of Alabama) at the 1.5-meter telescope of Cerro Tololo Inter-American Observatory, Chile.

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