The bar structure is generally considered a to be largely a region of older stars, with little obvious star formation visible. Stars within the bar tend to remain within the bar in non circular orbits which gives the bar its shape. The spiral arms, which often seem to extend from the ends of the central bar, are where star formation can be seem to be occuring, particularly when observed in the UV band. The bar structure is more prominant in the IR region and very inconspicuous, if visible at all in the UV region. It is thought that the bar structure has a similar thickness to the spiral arm structure, if it did not then you would expect to see bulges in many of the galaxies that we see side on, but this is not the case.
It is thought that the bar structure forms in galaxies where the mass of matter in the outer spiral is lower than in those galaxies which do not show it, this has been studied in computer simulations and gives rise to the belief that there must be a significant proportion of dark matter in the spirals which do not show the bar structure.
Gas flows in towards the ends of the bar from the spiral structures, the asymmetric gravitational field caused by the bar shape causing a decelaration of the gas. Often accumulation of matter can be seen here at the ends of the bar on the leading edge as bright regions or /and dust lanes. This is where gas is converging and getting compressed.
Image of M95 taken on 21 March 2009
Image of M109 taken on 21 March 2009
The picture to the left is a re-processed copy of the image of M109 above, which has been processed to bring out the faint sharp details. Within this picture you can more clearly see the hot star forming regions in the spiral arms. The bright regions at the end of the spiral arms also stands out more clearly in this image.
As a note here, you can also now see other faint galaxies more clearly in this image. This image was produced by blurring the original, then subtracting this blurred image from the original to give the sharp bright objects. This image was then added back into the original image to give the picture to the left.
At least half of all spiral galaxies have a central bar structure, and some authors estimate the number as high as two thirds of the spirals having a central bar. Two examples of barred spirals can be seen in the pictures below: