M 51 (NGC 5194), the Whirlpool Galaxy, is a face-on spiral galaxy in Canes Venatici. Charles Messier discovered it on October 13, 1773. If you look at M 51 through a medium aperture telescope, you will notice that it has a companion, NGC 5195. Messier's friend, Pierre Mechain, discovered NGC 5195 in 1781. In 1845 Lord Ross discovered the pinwheel-like structure in M 51 with his 72-inch telescope. In fact, M 51 was the first galaxy in which the spiral structure was detected.
Both galaxies are easy to locate. They lie on the border of Ursa Major and Canes Venatici. First look for the Big Dipper. Take the star Eta (η) Ursae Majoris at the end of the handle as a starting point. From there, go about two degrees west-southwest to a 4th magnitude star, 24 Canum Venaticorum. From there move 2 degrees in southwestern direction to find M 51 and it's companion, NGC 5195. Together, they form a triangle with the two stars Eta Ursae Majoris and 24 Canum Venaticorum.
Click here to download a printable finder chart (PDF)
M 51 is 8.2' by 6.9'. Its visual magnitude is 8.4 and the surface brightness is 12.6. NGC 5195 is 6.4' by 4.6' with a visual magnitude of 9.6 and a surface brightness of 13.1.
Under a really dark sky, it is possible to spot M 51 in 7 or 10x50 binoculars. You will see it as an 8th magnitude faint smudge of light. Under a light-polluted sky M 51 really suffers. With my binoculars, I have only spotted it once or twice, using my 15x80.
With an 8-inch telescope under light-polluted skies, both the cores of M 51 and NGC 5195 are visible, when the transparency is not too bad. The cores appear as two round patches of light, fuzzy on the outside, brighter towards the nucleus. Don't expect to see the spiral arms or "the bridge" between the two galaxies you see on all the images on the web or in astronomy books. These features are impossible to see from a suburban backyard with an 8-inch telescope. According to Phil Harrington (The Deepsky, An Introduction) you need a 12- to 14-inch telescope under suburban skies to see the pinwheel-like arms of M51. However, it is one of the brighter galaxies in the Messier list, that can be observed from a light-polluted observing site, so you should give it a try.