M51 - Spiral Galaxy in Canes Venatici

Discovered 1773 by Charles Messier.

The famous Whirlpool galaxy Messier 51 (M51, NGC 5194) is one of the most conspicuous, and probably the most well known spiral galaxy in the sky.

M51 was one of Charles Messier's original discoveries: He discovered it on October 13, 1773, when observing a comet, and described it as a "very faint nebula, without stars" which is difficult to see. Its companion, NGC 5195, was discovered on March 21, 1781 by his friend Pierre Méchain, so that it is mentioned in Messier's 1781 catalog: `It is double, each has a bright center, which are separated 4'35". The two "atmospheres" touch each other, the one is even fainter than the other.' In addition to the description, in his personal copy of the catalog, Messier has added a small drawing, or sketch of the two "nebulae," M51 and NGC 5195, one of the rare cases a detailed drawing by Charles Messier of one of his objects is preserved. NGC 5195 was assigned an own catalog number by William Herschel: H I.186.
Occasionally, there is some confusion what is meant with the designation M51: The pair (justified by Messier's mention of both "nebulae") or the larger galaxy, NGC 5194. If the pair is meant, NGC 5194 is sometimes called "M51A", and NGC 5195 is then "M51B".

M51 is the dominating member of a small group of galaxies, which also contains M63 and a number of fainter galaxies. As it is about 37 million light years distant and so conspicuous, it is actually a big and luminous galaxy. The value of M51's (and the whole group's) distance is still not very well known. Our value, of 37 Mly, is based on photometric methods and e.g. given by Kenneth Glyn Jones. Some authors give significantly lower values (less than 20 Mly), but a recent (2001) STScI Press Release gave 31 million light years.

*Much of the information regarding the Messier Objects and their origins has been graciously provided by www.seds.org/messier/