Shooting Star Trails
The 30s-at-a-time way...
My way of shooting star trails avoids the need for really long exposures, with their unwanted side effects. Instead, I use a sequence of 30s exposures, one shot right after the other, which I then combine in post.
Use a lens that is wide enough to capture a nice area of the night sky, as well as whatever foreground object you'd like to feature. Set your tripod up in an area where it is secure from extraneous light, or being inadvertently bumped. Frame your shot and focus at infinity then lock down that focus (e.g. gaffer tape).
Set your camera to Manual mode, your shutter speed to 30s, with your widest aperture - ideally f/2.8 or wider - available. Adjust your ISO to the lowest setting giving you a proper exposure of the night sky - typically between 1600 and 3200.
Make sure your camera is set to continuous shutter release and connect an external release that can be locked in the shutter released position. When you're ready to start your sequence, usually at or shortly after the end of astronomical twilight, hit the remote release and lock it. Now your camera will open the shutter, expose the sensor for 30s, then close the shutter. Since the remote is locked in "release shutter" mode, it will re-open the shutter right away again, expose for 30s, then close the shutter. And so on.
Letting this run for 2 hours, e.g., will yield around 240 thirty-second exposures of the night sky. Your are now free to use any continuous subset of these exposures, e.g. the last 200 frames (i.e. you'd ignore the first 20 minutes of exposures...) Keep in mind that, for shorter trails, you need less frames, especially if you're trying to composite a photo of a significant foreground element, with only a part of the sky visible - plan your shoot accordingly!
After the shoot, ingest your captures into your raw converter of choice. make sure their white balance is matched, and then create high resolution JPGs or TIFFs of all the exposures. Export them all to a separate folder. Once complete, open Photoshop and make sure your color space matches. Then highlight all your captures and bring the music into Photoshop as a "stack" (you should not need to align them, since you shot from a tripod. Once loaded, pick all the layers (i.e. highlight them all,) and choose 'Lighten" as your blend mode. This will reveal the 'trails' (i.e. the brightest parts of your captures.) If you don't want the trails to be 'fading,' you're done now.
If you prefer fading trails, check the number of layers you have (let's assume for this that you're using 200 frames,) and calculate the opacity gradient to apply. Since you want the layers to fade in from 1% to 100%, assign 1% to the first 2 layers, then set the next two to 2%, and so on, until you get to the 197th layer, which you assign an opacity value of 99%; same for layer 198. Finally, layers 199 and 200 will get 100% opacity. That's it. Now you have fading (in) star trails...