Bar and ring structure in NGC 1433

The stars orbiting in the disks of spiral galaxies can, under the right conditions, exhibit a wealth of patterns caused by the complex orbital mechanics of a widespread mass distribution, and these patterns can in fact be used to diagnose internal motions. NGC 1433 shows a wide range of these phenomena. A small elliptical ring occurs close to the nucleus; this ring is a site of star formation, and hence appears blue. This ring really is elliptical rather than a circle, since the outer parts of the galaxy are nearly round as we see them. Next outward is a bar, with delicate dust lanes along one edge. The theory of stellar orbits in fact predicts that bars and rings should either align parallel or perpendicular to one another depending on the relative speeds ofindividual stars and the overall pattern. There is a prominent outer ring, beyond which are two prominent spiral arms. Between the two outer arms are shorter spur arms, which may be locked in place by the gravitational disturbance produced by the overall two-sided pattern.

This true-color composite was produced from CCD images taken by Deborah Crocker and Lauren Jones (University of Alabama) at the 1.5-meter telescope of Cerro Tololo Inter-American Observatory, Chile.

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