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M51 - the Whirlpool Galaxy is called a 'grand-design' spiral galaxy, because of it's prominent and well defined spiral arms. Estimated to be 23 million light-years away, it has a about mass of 160 billion Suns (or about 10% of the mass of the Milky Way) and an estimated diameter of about 76,000 light-years.
The Galaxy's pronounced spiral structure is thought to be due to the interaction with it's companion galaxy NGC 5195, which can be seen at tugging the end of of one of it's arms. The small galaxy has been gliding past the Whirlpool for hundreds of millions of years, with the interaction resulting in a region of new star formation.
One curious aspect about these well defined spiral galaxies, is that the structure of the spiral is thought to be rotating at a slower rate than the material itself. In the density wave theory the visible arms form as the result of gathering interstellar matter. As clouds of hydrogen approach an arm, the arm acts as a gravitational potential well, slowing the gas down & compressing it. We see this effect as dust lanes at the back of the spiral arms. As the cloud is compressed, star formation takes place, resulting in short-lived bright stars which mark the front of the spiral arms. These massive stars, in turn, have the effect of driving apart the clouds through stellar winds & supernova explosions. Hence the material in front of the arm once again resembles that which originally entered the back of the arm, and the cycle continues as the gas continues on to the next spiral arm.
For more info. see the Wikipedia entry.
