Messier 51 (better known as the Whirlpool Galaxy, M51 or NGC 5194) is a famous grand-design spiral galaxy located in the constellation Canis Venatici. It has an apparent magnitude of 8.4 and is located at an approximate distance of 23 million light-years from Earth. Messier 51 is classified as a Seyfert 2 galaxy, an active galaxy with a quasar-like nucleus, very high surface brightness and a characteristic bright core, one that appears particularly bright at infrared wavelengths.
The Whirlpool Galaxy is also known as the Question Mark Galaxy or Rosse’s Galaxy, after William Parsons, 3rd Earl of Rosse, who was the first to recognize the spiral nature of this "nebula" in 1845. It wasn’t until the 1920s, when Edwin Hubble proved that “spiral nebulae” were in fact distant galaxies, that Whirlpool and other objects of this type were recognized as independent galaxies and not nebulae inside the Milky Way.
The Whirlpool Galaxy is the brightest member of the M51 Group, a relatively small group of galaxies that also includes the famous Sunflower Galaxy (M63) and the fainter NGC 5023 and NGC 5229.
The tidal interaction with the dwarf galaxy NGC 5195 (Messier 51b), discovered only with the advent of radio astronomy, is thought to be triggering zones of star formation. Indeed, the interaction between the two leads to the compression of gaseous hydrogen which, in turn, leads to the formation of stellar "nurseries". The two galaxies are connected by a bridge of stardust and will eventually merge, but it will still take a long time for the merger to be complete.
The Whirlpool Galaxy is one of the original discoveries of Charles Messier, who first observed it on October 13, 1773, while NGC 5195 was discovered by Messier's friend and colleague Pierre Méchain on March 20, 1781. In his 1781 catalog, Messier it does not fully clarify whether the object referred to as M51 is just the larger galaxy or the interacting pair. Consequently, the Whirlpool Galaxy (NGC 5194) is sometimes referred to as Messier 51a and the smaller NGC 5195 as Messier 51b.