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I have deliberately been vague about describing what a deep-sky object exactly is. You may define them simply as objects that are very far away. Other than that, there is nothing all of them have in common except that they be found somewhere in the Universe (not a very singular honor). The deep-sky objects on Messier's list can be classified into several different types as described below. After each description is a list of Messier objects of that type listed in my abridged version of the catalogue.