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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7$\begingroup$ 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. $\endgroup$– zephyrCommented Aug 30, 2017 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."
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2$\begingroup$ Please provide some more detail than a single quote. $\endgroup$ Commented Sep 5, 2017 at 20:46
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$\begingroup$ Are you sure this is up-to-date? It has been known since the 70's that stars in a galaxy have neither purely circular nor purely elliptical orbits. $\endgroup$ Commented Sep 6, 2017 at 1:04
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$\begingroup$ found a couple of physics sims here youtube.com/watch?v=rcQiLAktC-M and youtube.com/watch?v=lSymkkrbYZI $\endgroup$ Commented Sep 10, 2017 at 19:25