Each week, discover a new spectacular image obtained at CFHT .
Bars are density waves generated by gravitational instabilites or galaxy collisions.
They are made of stars and gas and play an essential role both in the dynamics and evolution of galaxies. In particular, they may help fuel the central regions of galaxies with gas and dust by allowing these components to lose part of their angular momentum. However there seems to be no link between the presence of an active nucleus and the existence of a bar. One of the reason invoked to explain this is that a large-scale bar can usually drag the gas within the central kpc or so, but then other processes should take the relay. Recent studies have revealed bars within bars, which may play such a role.