The Whirlpool galaxy, is also known as Messier 51a, M51a, or NGC 5194. It is a large galaxy with well-defined spiral arms, with a smaller companion galaxy. They have been through a collision, and are still interacting.
The Whirlpool was the first galaxy to be classified as a spiral galaxy. Different methods put its distance as between 15 to 35 million light-years. Recently it was estimated to be 23 ± 4 million light-years from the Milky Way. Messier 51 is one of the best known galaxies in the sky.
Its spiral arms are so impressive and well-defined that it is called a "grand design" spiral galaxy. It has an active galactic nucleus, no doubt fuelled by a massive black hole.
The galaxy and its companion, NGC 5195, are easily seen by amateur astronomers, in the constellation Canes Venatici. The two galaxies can be seen with binoculars. The Whirlpool galaxy is also a popular target for professional astronomers. They study its galaxy structure (particularly the spiral arms) and galaxy interactions.