Galaxies can have various shapes , from elliptical to spiral. Among the great family of spiral galaxies, about two third have a central stellar bar. Bars are complex systems and thanks to new observations and sophisticated numerical models developed in the last 20 years, considerable progress has been made in understanding their morphology, dynamics, formation and evolution. Bars are, of course, principally formed from stars but some of them have also large amounts of interstellar dust, neutral hydrogen, ionized and cold molecular gas. Our own Galaxy, the Milky Way, has a small bar in its center.
Bars affect galaxy evolution.
Introducing a bar in a spiral disk, for instance from an interaction with another galaxy, will result in some dramatic changes for the host galaxy. When the bar is formed, motions of a large fraction of the stars in the galaxy become chaotic. For the interstellar gas clouds, these chaotic motions are also induced. Collisions between the clouds in the system disk become frequent and violent. In particular, a large quantity of gas will fall along the bar and might eventually reach the galaxy center.