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How does the bar in a barred spiral galaxy form? What prevents it from being spirals all the way down like water spiralling down a plughole?

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    $\begingroup$ For one, spirals in a spiral galaxy are not the same as spirals in draining water. You can't expect the same behavior because the same physics is not governing both concepts. $\endgroup$ – zephyr Aug 30 '17 at 14:33
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"Galactic bars develop when stellar orbits in a spiral galaxy become unstable and deviate from a circular path. The tiny elongations in the stars’ orbits grow and get locked into place, forming a bar. The bar becomes even more pronounced as it collects more and more stars in elliptical orbits. Eventually, a high fraction of the stars in the galaxy’s inner region join the bar. This process has been demonstrated repeatedly with computer-based simulations."

Source: http://hubblesite.org/hubble_discoveries/science_year_in_review/pdf/2008/barred_spiral_galaxies_and_galactic_evolution.pdf

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