M51

"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. It lies in the constellation Canes Venatici, and was the first galaxy to be classified as a spiral galaxy. Its pair with NGC 5195 (M51b) is among the most famous and relatively close interacting systems, and thus is a favorite subject of galaxy interaction models." (Wikipedia)