Messier 51, commonly known as the Whirlpool Galaxy, is a spiral galaxy located in the constellation Canes Venatici, at a distance of 31 million light-years. It was discovered by French astronomer Charles Messier on October 13, 1773, while observing a comet. Years later, in 1781, Messier's disciple, Pierre Méchain, discovered M51's companion, which they named M51B. M51 Whirlpool Galaxy.