There are many kinds of objects in the sky besides stars: star
clusters, nebulae or clouds of interstellar dust and gas, and
galaxies. Because most of them are far away and visible only in
telescopes, astronomers use the term deep-sky object for them.
These objects, like stars, are catalogued. The most famous
catalog is the Messier Catalog. Charles Messier was a French
comet-hunter who became frustrated by objects in the sky that
looked like faint comets but were not, so he compiled a list of
110 of them. Messier's list is almost a tour guide to the
Universe, with many of the brightest and most spectacular deep-
sky objects on it. (Ironically, Messier sought fame by seeking
comets, but his list is his monument and his 21 comet
discoveries are largely forgotten. Moral: seek knowledge, and
you may find fame; seek fame, and you may be forgotten.) Objects
on the list are denoted by M plus a catalog number; for example,
the nearest spiral galaxy to us, The Andromeda Galaxy, is M31. A
far more extensive catalog, the