If you're just getting in to astrophotography you might have heard about the stacking technique for startrails. You might even have followed a tutorial and created your first stacked image (if so, tweet me: @LongExposures). Most tutorials do a good job of explaining how, but what's not always obvious is why?
Here's a quick visual example of why stacking gets my vote.
The image above shows 80 seconds captured in a single continuous exposure at f/2. To allow me to shoot for this long, the ISO was set at a low rating of 80. There are some star trails visible but they're not widespread and neither are they long.
To capture more starlight I'd need to increase the camera's sensitivity to light (a higher ISO number); to make the trails longer I'd need to increase the exposure duration (a longer shutter speed).
If I tried to do both those things at once I'd very quickly have an overexposed file on my hands- and the brighter the sky, the less contrast there is to the star trails, rendering them less visible.
Time for a rethink. From our original ISO let's increase the rating to make it more sensitive; every scenario will be subtly different but ISO640 should do it here, that's 3-stops brighter. (80>160>320>640). We now need to compensate for the increase in sensitivity with a corresponding decrease elsewhere. Narrowing the aperture would work but would also reduce the amount of starlight recorded. We're only looking to reduce the ambient brightness, so for that let's drop shutter speed by 3-stops. 80>40>20>10. That's what you see below- note more stars, but shorter trails.
The trick, then, is to tweak ISO and aperture to gather the starlight and use shutter speed to control ambient illumination. Once you've found that balancing point, it's just a case of firing your camera continuously for an extended time. That's about 15 minutes worth of exposure shown below, probably the minimum you can get away with if your aim is a true star trail. It's worth remembering that as your focal length decreases trails will appear shorter for any given exposure duration thus wide angle shots will need more time.
One final benefit that may or may not be relevant depending on where you're shooting- if something passes through your frame during a stack, you can simply omit the files when compiling your final image. Much better to trim a little from your startrails than to lose the whole exposure if shooting a single file: