The Whirlpool Galaxy, also known as Messier 51a, M51a, or NGC 5194, is an interacting[7] grand-design[8] spiral galaxy with a Seyfert 2 active galactic nucleus[9] in the constellation Canes Venatici. It was the first galaxy to be classified as a spiral galaxy.[10] Recently it was estimated to be 23 ± 4 million light-years from the Milky Way,[3] but different methods yield distances between 15 and 35 million light-years. Messier 51 is one of the best known galaxies in the sky.[11] The galaxy and its companion, NGC 5195, are easily observed by amateur astronomers, and the two galaxies may even be seen with binoculars.[12] The Whirlpool Galaxy is also a popular target for professional astronomers, who study it to further understand galaxy structure (particularly structure associated with the spiral arms) and galaxy interactions. What later became known as the Whirlpool Galaxy was discovered on October 13, 1773 by Charles Messier while hunting for objects that could confuse comet hunters, and was designated in Messier's catalogue as M51.[13] Its companion galaxy, NGC 5195, was discovered in 1781 by Pierre Méchain, although it was not known whether it was interacting or merely another galaxy passing at a distance. It was, however, not until 1845 that William Parsons, 3rd Earl of Rosse, employing a 72-inch (1.8 m) reflecting telescope at Birr Castle, Ireland, found the Whirlpool possessed a spiral structure, the first known "nebula" to be known to have one. These "spiral nebulae" were not recognized as galaxies until Edwin Hubble was able to observe Cepheid variables in some of these spiral nebulae, which provided evidence that these objects were indeed separate galaxies.[14] The advent of radio astronomy and subsequent radio images of M51 unequivocally demonstrated that the Whirlpool and its companion galaxy are indeed interacting. Sometimes the designation M51 is used to refer to the pair of galaxies, in which case the individual galaxies may be referred to as M51A (NGC 5194) and M51B (NGC 5195).

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