Discovered in 1773, M51 (& NGC5195) is one of the most well known galaxies in the sky and is a good example of a face-on spiral galaxy.
Its distance is approximately 23 million light years.

Just as M81 and M82, M51 and NGC 5195 (right part) are also gravitationally interacting galaxies.
Sometimes M51 is used to refer to both galaxies, in which case the individual galaxies are referred to as M51A (NGC5194) and M51B (NGC5195).

In 2005 as well as 2011 a supernova was observed in the Whirlpool Galaxy. These events helped to derive a more correct distance of 23 Mly as it was previously estimated to be around 37 Mly