barred spiral galaxy

Barred spiral galaxies are relatively common, with surveys showing that up to two-thirds of all spiral galaxies contain a bar. The current hypothesis is that the bar structure acts as a type of stellar nursery, fueling star birth at their centers. The bar is thought to act as a mechanism that channels gas inwards from the spiral arms through orbital resonance, in effect funneling the flow to create new stars. This process is also thought to explain why many barred spiral galaxies have active galactic nuclei, such as that seen in the Southern Pinwheel Galaxy.
The creation of the bar is generally thought to be the result of a density wave radiating from the center of the galaxy whose effects reshape the orbits of the inner stars. This effect builds over time to stars orbiting further out, which creates a self-perpetuating bar structure.Another possible cause of bar creation is gravitational disruptions between galaxies or a collision of two galaxies.

Bars are thought to be a temporary phenomenon in the life of spiral galaxies, the bar structure decaying over time, transforming the galaxy from a barred spiral to a "regular" spiral pattern. Past a certain size the accumulated mass of the bar compromises the stability of the overall bar structure. Barred spiral galaxies with high mass accumulated in their center tend to have short, stubby bars.Since so many spiral galaxies have a bar structure, it is likely that it is a recurring phenomenon in spiral galaxy development. The oscillating evolutionary cycle from spiral galaxy to barred spiral galaxy is thought to take on the average about two billion years.
Recent studies have confirmed the idea that bars are a sign of galaxies reaching full maturity as the "formative years" end. A team led by Kartik Sheth of the Spitzer Science Center at the California Institute of Technology in Pasadena discovered that only 20 percent of the spiral galaxies in the distant past possessed bars, compared with nearly 70 percent of their modern counterparts. [wiki]