Shooting star trail photos on film required a generous measure of luck (or experience) to judge whether the reciprocity failure inherent in a lengthy exposure would give a pleasing result. Digital capture makes the process apparently much simpler – you simply shoot a large number of almost consecutive exposures and stack them after the fact to mimic the effect of a long single exposure. The problem is how to treat the foreground. For the sky, “stacking” just means taking the maximum value at each pixel across the sequence of images. If its truly dark, applying the same algorithm to the whole frame will generally give only a silhouette for the foreground, since even at high ISO 20 or 30 seconds won’t be enough. So, instead, you might (a) start the sequence when there’s still a little light in the sky and stack for the maxima (in which case frame #1 will give the whole image, with the rest just contributing the trails), or (b) additively stack the frames to give the equivalent of a super-long exposure without reciprocity failure. In either case, there’s no such thing as a “real” image and it’s an aesthetic decision how bright the foreground is relative to the sky.
Enough theory, here’s my first attempt… looking north toward the Flatirons and Boulder from the Flatirons Vista trail south of town:
To make this image, I stacked 150 20s exposures (ISO 3200, f/5.6, about 135mm) starting about an hour after sunset. My 5D Mk3 didn’t seem to like shooting at 21s intervals (perhaps overhead with high ISO noise reduction?) so the intervals were 22s and as a result there are some gaps evident in the full resolution file. The images were then stacked using StarStax (very simple, free, and does what it says on the box).