The Messier catalog is a list of astronomical objects compiled by the French astronomer Charles Messier in the 18th century. It includes 110 objects, such as star clusters, nebulae, and galaxies, which are primarily located in the northern hemisphere. The catalog was created to help astronomers distinguish between permanent celestial objects and transient ones, like comets. Each object in the Messier catalog is designated by the letter "M" followed by a number, such as M31 for the Andromeda Galaxy. The catalog remains a valuable resource for amateur and professional astronomers, serving as a guide to some of the most prominent deep-sky objects visible from Earth.