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, that are visible from Earth. The catalog was created to help astronomers distinguish between these permanent objects and transient comets. Each entry in the Messier Catalog is designated by the letter "M" followed by a number, like 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 interesting and accessible objects in the night sky.