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As stars follow orbits of different sizes and at different rates, accidental alignments lead to regions of greater mass, which attract more stars. In this way, flocculent galaxies form. Flocculence consists of spiral segments, formed out of alignments in orbital rosettes. This will tend to happen more on the outer part of stellar orbits, where orbital velocity is less and stars spend more time. In consequence, trailing spirals are found in galaxies.
Deletions:
As stars follow orbits of different sizes and at different rates, accidental alignments lead to regions of greater mass, which attract more stars. In this way, flocculent galaxies form. Focculence consists of spiral segments, formed out of alignments in orbital rosettes. This will tend to happen more on the outer part of stellar orbits, where orbital velocity is less and stars spend more time. In consequence, trailing spirals are found in galaxies.
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Our paper, ""Galactic Spiral Structure"", [[http://arxiv.org/abs/0901.3503 arXiv:0901.3503]], shows how galaxies naturally evolve to form grand-design two-arm spirals. Here we give a brief description of the workings of spiral arms.
====""""Spiral Potential====
The gravitational potential of a spiral galaxy can be compared to a giant, spiral-grooved, funnel. The gravity of the spiral arms creates the grooves in the potential. A star near apocentre, the slowest part of its orbit, will tend to fall into a groove and then follow the groove toward the centre of the spiral, picking up momentum as it goes. Eventually, the star gains enough momentum to jump free of its groove. It crosses over the next-highest groove, then falls back to a higher point in its original groove. At the same time, the funnel rotates slowly backwards due to orbital precession.
As stars are drawn into an arm, the gravitational field of the arm grows stronger, making the groove deeper and drawing greater number of stars into the arm. Thus, mutual gravity between stars reinforces spiral structure. The potential field of the arms locks the rate of orbital precession to spiral pattern speed for a wide range of orbits.
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Our paper, ""Galactic Spiral Structure"", [[http://arxiv.org/abs/0901.3503 arXiv:0901.3503]], shows how galaxies naturally evolve to form grand-design two-arm spirals. Here we give a brief description of the workings of spiral arms.
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