<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> <P> The stars in spirals are distributed in thin disks radial with intensity profiles such that </P> <Dl> <Dd> I (R) = I 0 e − R / h (\ displaystyle I (R) = I_ (0) e ^ (- R / h)) </Dd> </Dl> <Dd> I (R) = I 0 e − R / h (\ displaystyle I (R) = I_ (0) e ^ (- R / h)) </Dd>

Where are young stars found in a spiral galaxy