M51 - Whirlpool Galaxy

The Whirlpool Galaxy was originally characterized in 1773 by Charles Messier long before it's spiral structure was known.  It was not until 1845 that William Parsons with the aid of a 72-inch reflector discerned the spiral arms and sketched it's interacting companion galaxy, M51b (NGC5195).  Both arms of this Grand Design galaxy owe their compact, defined structure to the interaction with the smaller companion, which has induced gravitational tides (density waves) that contract the arms.  The result is an enormous and active period of stellar formation that is evident in the image by compacted dust lanes encircled by vibrant blue light from newborn stars.  The lesser companion is theorized to have passed through the plane of M51 and is now on the far side just behind the trailing tip of one of the arms.