Need help stacking images

[shaunly]

I'm a bit confuse about this technic. I don't understand how stacking 5, 10 or even more images gives you the same exposure as if you were to take one long image. I know in astrophotography this is something that must be done in order to get long exposure time that could take hours. Since I don't have all the gear for that yet, I can't really experiment with it. I have tried stacking 10 1sec. images using CS3 and PTGui, but the result basically look the same. The only thing I notice was that my noise level looks much better. I'm obviously missing something or doing something wrong. So if somebody could please explain to me, that would be great! Thanks in advance.