The Whirlpool Galaxy, one of the most famous deep sky objects, was discovered on October 13, 1773, by French astronomer and comet-hunter Charles Messier, who compiled an astronomical catalog of nebulae and star clusters eventually known as the 110 "Messier objects." His catalog, still in use, was intended to help observers distinguish between permanent and transient sky objects. Also known as M51a (in the Messier catalog), the Whirlpool Galaxy is a grand-design dwarf spiral galaxy, estimated to be 23-25 million light-years from our own Milky Way. It is interacting with a smaller companion galaxy, M51b (pictured). Easily observed even with binoculars, it is a popular target for study in understanding galaxy structure (particularly those with spiral arms) and interactions between galaxies.