Good old M51, one of the brightest galaxies visible in the northern hemisphere. At about 23 million light-years from us, discovered back in 1773, this nice looking pair of interacting galaxies is one of the most well known objects in the sky.
Not a particularly large galaxy, M51 would be about a third as large as the Milky Way, and maybe a fourth as massive.
This image, taken over two nights, on March 12th and 13th 2012, is the stack of about 6.8 hours of integration time (mode details by clicking on the image.)
In the full frame you can see some reflections that have afflicted my optical train, but that have since been solved.