quigley wrote:Thank you Ann and Neufer for the great explanations of the origins of galactic bars. Upon viewing this image, the first question that came to mind was, "What causes such distinctive formations to occur?" You answered that. So the spiral arms "peel" off stars from the ends of the bars over time?
Again, I'm not the best person to elaborate, but...
I think that basically two things are happening. First and most importantly, the orbits of the stars near the center of the galaxy start following elliptical (elongated) orbits around the center of the galaxy. They then synchronize their orbits so that all or most stars in that part of the galaxy follow more or less the same elliptical orbit. Think of it like this. An elliptical orbit could be "lying down" or "standing up". To get a bar, all or most of the stars in the bar must orbit in the same direction, "lying down" or "standing up". The stars do indeed synchronize their orbits.
NGC 1097. ESA/Hubble & NASA
Another very important thing about barred galaxies is that they have dust lanes running from a little ring in their centers out along the bar all the way to the first spiral arm. On one side, the dust lane is running "above" the bar (not really, but sort of) and on the other side, it is running "below" the bar. Personally I don't understand math, but I will hazard a guess that the dust lanes along the bar are "shepherding" the stars in the bar along their elliptical orbits.
I believe that the dust lanes also move starmaking material like gas and dust into two places in a barred galaxy: it is sent into the ring around the nucleus, where we often find a lot of star formation in barred galaxies. And it is sent to the ends of the bar. Here we often find star formation, too. I believe that star formation at the ends of a bar will make that bar grow in size.
So what happens when a barred galaxy stops making new stars? That happens when the galaxy has used up its available gas and dust, or when the gas and dust becomes too turbulent to settle down into star making. Well, for one thing, the dust lanes probably disappear from the bar, because the dust lanes are the mechanisms for funneling star making material into the ring around the nucleus and to the ends of the bar.
NGC 936 is a barred galaxy that gave up star formation long ago. It is a very "red and dead" galaxy. But you can still see a "ring" around its nucleus, and you can see that there used to be quite a lot of star formation at the ends of the bar. The bar ends are bright.
And that's what I can say about the formation of bars, I think.