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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