Hand Bent Spoon said:
"So the answer is it is a depiction of the shephards being informed of the nativity. The angel looks so indistinct due to its great distance, and therefore lack of detail. A couple of careless strokes, and to the uneducated eye it looks like a flying saucer."
That explanation is not tenable I'm afraid.
Firstly, there were three shephards, not one as in the picture.
Secondly, the star of Bethlehem was a very bright astronomical event, not a dark object against the background of a lighter sky.
I dont think that renaissance artists were into "careless strokes". This is a religious painting and it all had to have a significance for the masses. It still doesn't make the object an alien craft, but it was almost certainly meant to look like it does.