I was out one night earlier in the month trying to image M31 with the Nikon P900. The results were pretty dismal. Below are my three best tries. The first one is from a starscape video so 25 sec ISO 100 and the others are ISO 6400 1/2 sec. In each case you can see the Andromeda Nebula as a smudge. The smudge is pretty blocky in the video frame. I looked at a number of the frames and they were similarly blocky so i doubt that stacking would make a difference. So the Nikon makes night sky photography easy and fun but not that rewarding – the sensor is just too small.