M 51: Whirlpool Galaxy

M 51 is an astronomical object part of the Messier catalog, it is made up of two different galaxies in the constellation Canes Venatici:
The bigger one is the famous Whirlpool Galaxy (also known as NGC 5194 or M51A) is a classic spyral galaxy discovered by Charles Messier;
The smaller one is known as NGC 5195 (or M51B), it is partially covered by an arm of the bigger galaxy (causing gravitational interactions between the two) ad was discovered a few years later.