M51, the Whirlpool Galaxy, was one of Charles Messier's original discoveries. First catalogued in 1773, M51 is the dominant member of a small group of galaxies located over 37 million lightyears away. M51 is easy to see under dark sky conditions, but it is quite sensitive to light pollution.

The smaller galaxy, NGC5195, is sometimes referred to as "M51B", while the larger galaxy is identified as "M51A". According to current theories, the spiral structure of M51A is a result of its encounter with the smaller galaxy. The interaction of the two objects, compressed and disturbed the gas in several regions, resulting the formation of new young stars.