M 51 is the famous "Whirlpool Galaxy" in Canes Venatici (near the handle of the Big Dipper).
This is actually 2 interacting galaxies, the larger spiral (NGC 5194) and the smaller
(NGC 5195),which millions of years ago passed through the main disk of M 51, and now
is located just a bit behind the larger galaxy (37 million light years away from
earth). The gravitational interaction of these two galaxies has caused contraction
of gas and dust in the arms of the larger galaxy, which led to intense new star
formation, visible in the many blue knots along the arms of the spiral galaxy.

Image data:
Camera: SBIG STL-11000
Telescope: Hyperion 12.5"
Exposure: 6 hours 50 minutes total (Luminance = 10 minutes x 29; RGB = 5 minutes x 8 each)