In this Instructable I'm going to show how a little statistics-based magic can help you get outstanding star photos, even with a regular compact digital camera. All you really need are a camera and a computer, although a remote control for the camera and a tripod will make your life much easier.
Step 1: Why stacking?
Firstly, the stars move around the sky, so on any exposure above about 15 seconds the stars will stretch out into curved streaks. This is a cool effect if it's what you're after, but if you want a picture of the sky as it appears to your eyes it's not helpful.
The second problem is that digital cameras pick up stray radio signals, cosmic rays, thermal vibrations and all sorts of other things which aren't starlight.
Thirdly, most cheap digital cameras will only take single exposures for a maximum of 15 or 30 seconds. An SLR with bulb mode will happily leave the shutter open for as long as you want, but this is a guide that will work with any camera. Fortunately, stacking images can get rid of all of these problems and increase the amount of detail visible in return for a little effort.
My examples will be taken from a set of photos I took of Cassiopeia and the surrounding sky. The image above shows a comparison of just Cassiopeia between one of my raw photos and the finished stack.