The Whirlpool Nebula, also known as Messier 51, Arp~85, VV~1 and NGC 5194/NGC 5195, was the first object in the night sky with a spiral structure to be discovered, way back in 1845. It can be found in the constellation Canes Venatici. Messier 51 really consists of two galaxies, NGC 5194 and NGC 5195. NGC 5194 is a large spiral galaxy, while its companion is a smaller barred galaxy.
Messier 51 is not as big and massive as the Milky Way, but it is brighter as a result of being populated by bright, hot, young O and B type stars. It is approximately 31 million light-years distant.