Cool enough: I always thought it WAS a galaxy because that's what it looks like... a forming star has more foggy arms which in turn (you would think) would be visibly converging into the center mass. What you see at the end of EMPIRE... well, just looks like a regular ole galaxy with spiral arms full of stars.
VS 
As far as the trajectory of the Falcon, it's no different than the missing mug in the first shot of the Nostromo computers first coming to life (switch angle to funky helmet with a knob on it, back to computer console and BLAM! Coffee mug!) - not so much a blatant continuity error so much as it was just a shot that had to be done without any thought about "hey, where did this coffee mug come from?" or "why would the Falcon fly off to the left when there is a galaxy right in front of them"?
What most likely happened was that the Falcon model shot was done separately from the background plate, to the degree that when the Falcon model was shot they photographers probably didn't know what was going to be put into the blue screen behind it quite yet (or only had a concept drawing or written description).
Again, galaxy or forming star, I still consider that it would be more visually pleasing to the eye, make sense to the story elements (themes of separation, ie. showing the Falcon fly off to the left doesn't provoke as strong an emotional punch as seeing it fly directly away from us to possibly never be seen again) and fit logically to have the Falcon fly away from us as opposed to the left side of the screen (if this is done, another scene that can be used is when the Falcon is flying away from Cloud City right before it does an emmelman through the cloud to go back to get Luke. The angle of the Falcon would be right, as it would be from a lower angle below the window Luke and Leia are looking out of).
Regardless, I know I can only suggest.