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Understanding Stacking
The primary point of stacking star trail captures is to extend the possible exposure time. Longer exposure times produce more apparent motion of the stars. It's as simple as that.
As I mentioned earlier, you'll need to use a fairly wide-open aperture to capture as much starlight as possible. With this wide-open aperture in mind, if you made a single long exposure to capture star trails, you'd have a limited amount of exposure time before considerably overexposing the image.
To reinforce this point, consider the four minute, f/4, and ISO 400 exposure. Leaving f/4 alone, if you dropped the ISO as far as it could go, hypothetically to ISO 100, you could extend the exposure time to 16 minutes at most. This is only a small fraction of the hours needed to create dramatic star trails.
As a digital darkroom technique, stacking is sometimes confused with high dynamic range (HDR) bracketing. Both techniques involve combining image sequences; however, their goals are quite different. HDR photography is all about extending dynamic range; whereas stacking images is actually about extending exposure time.
In other words, what you want from an exposure sequence created for stacking is a number of similarly exposed captures. These captures need to be spaced together with as little time gap between the exposures as possible (see Figure 3).
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Figure 3 The two-minute exposures occur regularly in sequence, so this strip of five exposures, shown in Adobe Bridge, takes place over 10 minutes.