J is right, you just need as many pictures as possible to stack them. You can't stack a single image.
Another thing whith unguided astronomical images is, that you do not need to expose for a long time, in fact it is more problematic. The problem is, the light source is moving, and therefore the light reaches different pixels. Your object won't get brighter when you expose longer than the time which it needs to move significantly. So it is a lot better to do many short exposures, where the object hasn't moved further than about its own diameter and than stack them on the computer.