The Whirlpool Galaxy in Canes Venatici. A classic spiral, it is believed to be approximately 35 million light-years distant. M51 is the dominating member of a small group of galaxies, which also contains M63 and a number of fainter galaxies. Being relatively close by and so conspicuous, it is also a big and luminous galaxy. The value of M51's (and the whole group's) distance is still not very well known. One 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.

Image taken April 6, 2009, with the ST-8 camera and F:3.5 focal reducer. Exposure is 30 minutes, with track and accumulate taking 60 30-second snapshots. I was amazed that it turned out so well, due to the fact that the moon was almost full at the time.
Color  image of M51, taken July, 2005, with 402 camera and f:3.5 focal reducer.  Exposure is 10 minutes with track and accumulate.