[link|http://www-thphys.physics.ox.ac.uk/users/SimonBerman/gasdyn.html| [link|http://www-thphys.physics.ox.ac.uk/users/SimonBerman/gasdyn.html|http://www-thphys.p.../gasdyn.html]]

From spectroscopic Doppler shift measurements, the bar structure is observed to rotate as a solid body in the same sense as the galaxy about an axis normal to the plane of the disk. This is unlike the differential rotation seen in the main body of the disk. The axial ratios of bars vary from one when a bulge is present without a bar up to about five, whereas the actual shape of a bar is better described as boxy than elliptical. From observing bars in edge on galaxies it is known that bars are a three dimensional phenomena with thickness generally larger than that of their host galaxy.

Given that bars are so common, it seems safe to assume that they are both easy to 'make' and fairly long lived structures. Numerical simulations have shown that bars can form in one of two ways, either spontaneously through global instabilities in an unbarred disk galaxy or induced by a close interaction with a companion galaxy. Once formed, bars can have a significant effect on the evolution of the host galaxy, channeling gas flows into the galactic nucleus to ignite starbursts or AGN's or feeding a central black hole.

[...]

But bars probably do not live for ever. Although a pure stellar bar seems, in simulations, to remain robust for a Hubble time or more, the accumulation of large quantities of gas at a bar's centre can lead to its disruption, by modifying the orbit's of the constituent stars. This could happen if the host galaxy swallowed a satellite galaxy or it could be the result of the gas flows mentioned above.

So it seems to be caused by a gravitational effect and isn't stable forever but is for a rather long time. (A [link|http://www.astro.virginia.edu/~jh8h/glossary/hubbletime.htm|"Hubble Time"] is the inverse of the Hubble Time Constant and is the estimated age of the universe (assuming uniform expansion)).

HTH. Maybe deS can elaborate.

Cheers,
Scott.