Ian states:
>A constellation is a collective entity
>defined by an arbitrary selection of ordered
>stars based upon a given point of view. A
>galaxy is also a collective entity defined by
>an ordering of stars, however, this ordering
>isn't arbitrary or based upon a given point
>of view, but may be caused by a black hole.
The salient difference is not "arbitrariness" but category of existence. A constellation exists entirely within someone's mind, whereas a galaxy is an objective, coherent configuration independent of any mental state. If you doubt this, try this thought experiment: imagine yourself 2 billion lightyears away orthogonally to the direction of the apparent position of the constellation. Where is the constellation now? Only in your memory. But you can still identify the Milky Way and other galaxies.
Dick