The Whirlpool Galaxy, also known as Messier 51a, M51a, or NGC 5194,
is an interacting grand-design spiral galaxy with a Seyfert
2 active galactic nucleus in the constellation Canes Venatici.
It was the first galaxy to be classified as a spiral galaxy.
Recently it was estimated to be 23 ± 4 million light-years from
the Milky Way, but different methods yield distances between
15 and 35 million light-years. Messier 51 is one of the best known
galaxies in the sky. The galaxy and its companion, NGC 5195,
are easily observed by amateur astronomers, and the two galaxies
may even be seen with binoculars. The Whirlpool Galaxy is also
a popular target for professional astronomers, who study it to
further understand galaxy structure (particularly structure associated
with the spiral arms) and galaxy interactions.