M73 is a tiny cluster of four (probably unrelated) stars in the sparse centre of Aquarius. Messier saw it as a 'cluster of three or four small stars, which resembles a nebula at first glance, containing little nebulosity'. He very probably included it in his list because it is very close to the globular cluster M72. Neither are easy to find, whether you look for the globular first or not. While some may doubt M73's claim to be a true Messier object, it is rewarding to find and rather pretty.