From the May 2011 issue

What causes the central bar in some spiral galaxies?

Dennis Shingler, Inverness, Florida
By | Published: May 23, 2011
A galaxy’s central bar may look like a solid structure, but it’s really a dense region that affects the galaxy as it rotates around the core. Gas near the galaxy’s center moves at faster speeds than that farther out. This, combined with density waves, helps create the bar. Astronomy: Roen Kelly, after Daisuke Namekata, et al.
Central bars occur in about two-thirds of spiral galaxies. We think they are regions of many overlapping stellar orbits. Bars are also density waves that rotate around the disk with a speed different from the rotation speed of individual stars, much like the waves that create the spiral structure of these galaxies.

Gravitational instabilities in the centers of galaxies, or gravitational disturbances from nearby galaxies, can cause density waves. As the waves rotate around the galaxy, they hold their shape like the blades of a fan. When a wave reaches a cloud of cold molecular gas, the density in the cloud may increase enough to cause the cloud to collapse under its own gravity, eventually forming stars.

Bars can also affect star formation by moving large amounts of gas toward (or away from) the galactic center. Gas in the inner region of the galaxy will orbit at a higher speed than the bar. When this gas catches up to the bar and passes through it, the bar’s stronger gravity slows it, so the material loses energy. As a result, the gas falls toward the galactic center. If this process funnels enough gas toward the galactic center, a tremendous period of star formation — called a nuclear starburst — follows.

Farther out in the galaxy, the gas moves more slowly than the bar. When the bar overtakes the gas, the bar pulls the material forward, speeding it up. The fast-moving gas will then gain energy and be flung outward, away from the galactic center. — Bradley W. Peterson, Iowa State University, Ames