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		<subject index="0" >Re(5): SECOND IMAGE - M51 WHIRLPOOL GALAXY</subject>

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

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There is a little more material in the arms than outside them.  Material - in particular clouds of gas and dust - orbiting the centre of the galaxy arrives in a spiral arm and tends to get bunched up there (a bit like a slight jam in motorway traffic).  This helps some of the clouds collapse to form stars.  The most massive stars that form are very, very bright.   But they only last a few million years.  These very bright stars are responsible for giving us a view of the spiral arms.  

Fainter stars also form and live for immense periods of tme - but they are relatively insignificant and they simply spread out over time to form a disc of stars.  The spiral arms are embedded in this disc.   
James R. Downes on 15 May 2006 at 22:55 +0000 wrote:
David A. Edwards writes:
>The presence of these young stars illuminates the arms, and the
>presence of the arms helps or makes new stars form.  

How so?