XPost: sci.physics, sci.physics.relativity   
   From: paul.b.andersen@deletethishia.no.retro.com   
      
   Cray74@gmail.com wrote:   
   > It could also be fairly normal gas dynamics: densification and   
   > rarification in a rotating body of gas and dust. You get the pile-ups   
   > at the spiral arms and rarification between them.   
   >   
   > The outer edges "drag" behind because they have lower orbital   
   > velocities than material closer to the galactic core.   
      
   No, that doesn't add up.   
   (What you say is correct - but it isn't the explanation   
     of the spiral arms.)   
      
   Spirals seems to look more or less the same for billions   
   of years; the arms doesn't wind up as they would do if the reason   
   was as you say.   
   The matter density in a spiral galaxy isn't so different in the arms   
   and between the arms as a visible light picture makes it appear.   
   The spiral arms are moving through the matter of the galaxy.   
   What is seen as bright spiral arms is the star forming regions where   
   there are a lot of very bright blue giants (o and B stars). These only   
   live for few millions of years, so they are only present in the star   
   forming regions. In other words - star forming regions are bright.   
   The theory goes something along these lines:   
   The spiral arms are kind of pressure waves travelling through   
   the matter (gas and dust) of the galaxy. The compression of the gas   
   trigger the star formation, which in turn drives the the pressure wave on.   
      
   I think a lot of people are working of computer simulations   
   of this - which has been an enigma for a long time.   
      
   (Because it has been known for a long time that your rather - sorry -   
   naive explanation doesn't work.   
      
   Paul   
      
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