Stacking multiple exposure lengths.

[Buldric]

Hey guys,
I have another newb question for you guys refering to astrophotography.

I found, that when I look at my different exposures of M31, (30 seconds, 2 min, 5 min) that they show dust at different distances from the core. There seems to be "sweet spots" for each of them. A circle between where the core is overpoweringly bright, and the space beyond is too dark. Each exposure length therefore shows the dust at different distances from the core (as far as I can see).

Is it possible to stack the different exposure lengths in such a way that I can get a cumilative picture, that shows the full length of the arms(or what "arms" it has) on M31, or do I have to do a bunch of extremely long exposures, then use editing to try and bring out the arms from the brightness at the core?

What kind of effect might I gain from doing this, and if it is done, where would one use this technuiqe of stacking different exposure lengths.

Also if it is done, how would I go about it? would I stack ALL the exposures together, or do I stack all the 30s, the 2m's, and the 5m's, then stack those 3 on eachother afterwards?

All of this I could attempt going blindly through the night, however I thought "These forums are here, maybe I can get some tips, so that I'll at least have a stick when proding my way through the cave".

If you have any tips, or questions about what I'm talking about PLEASE feel free to post, so far there have been about 3 of us on these forums >.<
I have been trying to spread the word of these forums, but I think I have only reqruited one person who has yet to post.

Clear Skies,
-Eric