It was discovered in 1773, by Charles Messier while hunting for objects that could confuse comet hunters, and was designated in Messier's catalogue as M51. Its smaller 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. In 1845, William Parsons using a 72-inch (1.8 m) reflecting telescope in Ireland, found that the Whirlpool possessed a spiral structure, the first "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 they were so far away that they must be entirely separate galaxies.
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 and M51b.
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 and M51b.
The galaxy and its companion, NGC 5195, are easily observed by amateur astronomers, and the two galaxies may be seen from dark sky sites with binoculars. The Whirlpool Galaxy has been extensively observed by professional astronomers, who study it to understand galaxy structure (particularly that associated with the spiral arms) and galaxy interactions. This second version of my image is one that has been calibrated photometrically in Siril to give the true star colours.
Here's a widefield shot of the region which has been annotated in www.astrometry.net.
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