Stacking help in Elements 6 plz

I want to take pics of star trails, I understand the basic concept and need help figuring out a couple of things...

1)I want to do multiple 30 sec exposure pictures. What is the best way to keep the shutter going? How do you "tape it down"?

2)Once I have a bunch of exposures, how do I "stack them" in Elements 6? Is it essentially the same as "bracketing?

posting again for the day crew

OK< as I think about it here is another queastion.

If you stack them they all have to be exposed enough to get the picture you want, cuz just stacking under exposed pictures will not result in a more exposed picture, right?

so I need to figure out how long to expose then do a bunch of those exposures. Do you have to compensate as the night changes since the light will change too?