The Whirlpool Galaxy (NGC 5194) is an unbarred spiral galaxy viewed from a distance of 37 million light years. It has a diameter of 50 thousand light years making it half the size and mass of our own galaxy. It was the first galaxy observed to have spiral structure.
The companion satellite galaxy (NGC 5195) is a barred spiral. It is travelling away from us roughly along the line of sight. It is now behind M51's spiral arm. We know this because the spiral arm's dust lane is above the companion galaxy. The gravitational tidal interaction between these two galaxies has made M51's spiral arms more prominent and it has also heavily distorted the barred spiral.
M51 is a Seyfert galaxy. This means that much of the energy emitted from the galaxy nucleus is due to a massive black hole at the center. Any material (stars or gas) that gets too close to this black hole get ripped apart and accelerated to very high speeds. This process produces the observed radiation. Of course we can only see the radiation from material that has not yet fallen within the black hole's event horizon. Almost all galaxies are thought to have a black hole at their centre but most are now relatively inactive because they have run out of near by material. The black hole in the Whirlpool galaxy is more fortunate because the gravitational disturbance from the companion galaxy is bringing material within it's reach again.
For more information, try: www.seds.org/messier/m/m051.html.