What do things like “M42” and “NGC 205” mean?
These are catalog numbers referring to various objects (luminous gas clouds, star clusters, galaxies) in the sky. “M-objects” are on a list drawn up by Charles Messier, an 18th century French astronomer. He was trying to find comets, but he and his assistant Pierre Méchain kept finding various faint fuzzy things. At first annoyed by these repeated false alarms, he eventually realized that these were apparently different types of “deep-sky objects” and published a list giving the locations in the sky of said fuzzy things as a service to other astronomers. Messier published a couple of revisions over his lifetime; a few posthumous entries were added, bringing the total to 110. Messier of course did not know the actual natures of most of the objects he saw, and neither did any other astronomer of his day. NGC stands for “New General Catalog.” It is a much larger catalog of objects, first published under the editorship of John L. E. Dreyer in 1888, that covers both the Northern and Souther
