How is the bar formed in a Barred-Spiral galaxy? 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?
"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