M51 - The Whirlpool galaxy
The famous Whirlpool galaxy M51 was one of Messier's original discoveries: He discovered it on October 13, 1773, when observing a comet. Its companion, NGC 5195, was discovered in 1781 by his friend, Pierre Mechain, so that it is mentioned in his 1784 catalog: `It is double, each has a bright center, which are separated 4'35". The two "atmospheres" touch each other, the one is even fainter than the other.' NGC 5195 was assigned its own number by William Herschel: H I.186.
M-51 is the Whirlpool galaxy. This pair of galaxies are on a collision course.