That makes it one of the most instructive oddities in the Messier catalog. Early telescopic observers could see a compact little group, but appearance alone could not establish whether its stars were physically associated. Modern measurements of stellar motion and distance let astronomers ask a question Messier could not settle from the eyepiece: are these stars actually traveling through space together?
The answer is no. M73 is a chance alignment.
That does not make it a mistake or a useless object. Quite the opposite. It preserves a basic problem of observational astronomy: the sky is a two-dimensional projection of a three-dimensional universe. Things that look grouped together may be separated by enormous distances.
M73 is therefore a catalog object whose most interesting property is that it is not the kind of object it once appeared to be.
Sources: modern astrometric studies of M73 and standard Messier catalog references.
