M51 is another very High Surface Brightness galaxy.  It is
interacting with a small companion galaxy.  That interaction serves
to redistribute and compress the gas which facilitates star 
formation.  In addition, the symmetry of tidal encounters often
induces spiral structure in the more massive of the two galaxies.
This spiral structure is clearly seen in this blue image.  M51 is
the host galaxy for SN 1994 I.