What you see of most galaxies is not all the galaxy,there is a halo of stars and matter that can reach almost a third of the size of the galaxy again, which is sometimes seen in very deep images,but only in the closer galaxies, a good example is M83.See my avitar for an example of a supernova seemingly outside a galaxy. astroron
Ken just seen your post, the supernova was 1987a was discoverd in Feb1987 in the LMC but not in the tarantula neb but close by