<Li> As clouds get swept up by the spiral arms, they collide with one another and drive shock waves through the gas, which in turn causes the gas to collapse and form stars . </Li> <P> The arms appear brighter because there are more young stars (hence more massive, bright stars). These massive, bright stars also die out quickly, which would leave just the darker background stellar distribution behind the waves, hence making the waves visible . </P> <P> While stars, therefore, do not remain forever in the position that we now see them in, they also do not follow the arms . The arms simply appear to pass through the stars as the stars travel in their orbits . </P> <P> Charles Francis and Erik Anderson showed from observations of motions of over 20,000 local stars (within 300 parsecs), that stars do move along spiral arms, and described how mutual gravity between stars causes orbits to align on logarithmic spirals . When the theory is applied to gas, collisions between gas clouds generate the molecular clouds in which new stars form, and evolution towards grand - design bisymmetric spirals is explained . </P>

Which particular feature of a spiral galaxy is evident even from this interior view of the milky way