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		<subject index="0" >Re(7): SECOND IMAGE - M51 WHIRLPOOL GALAXY</subject>

		<tonames index="0" >S194 06E real time observing</tonames>

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Spiral arms are pretty hard to understand!  I like to think of them as being like waves in my coffee. I can stir my coffee to get the whole lot revolving.  Then by tapping the cup I can create waves on the surface.  The waves correspond to the spiral, and the spinning coffee is the stars and gas.  But the waves do not spin with the coffee.
Carl Midgley 2 on 15 May 2006 at 23:25 +0000 wrote:
David A. Edwards on 15 May 2006 at 23:15 +0000 wrote:
They are almost static, with the individual stars orbiting the centre

I can understand that the stars move at different rates and that their relative positions change. However, what I'm not sure about is what are the spiral arms? If they are composed of dust and gas illuminated by the stars within them does the dust/gas itself not get pulled along by the gravitational pull of the stars moving around the centre?