Several tutorials online if you google "star trails comets" with most all pointing to the same method used.
The easiest way (for me, at least, since I know it works) is to open Adobe Bridge, go to your folder and select your pics.
Click Tools menu, Photoshop, Load files into Photoshop Layers.
Once pics are loaded in PS as a individual layers change the Blending mode from Normal to Lighten on all of them.
Then beginning with the first image layer (on the very top of layer listing) incrementally change each until you reach the last image, which should be 100%. For example, if you have 20 images then you'll increment by 5%. ---- such as, first image layer at the top is 5%, second image layer 10%, third 15%, fourth 20%, etc., on down to the last image set at 100%. That's basically it.
There is a PS action available online to load all images and then change blending mode automatically to Lighten but its last step flattens the image, which then does not allow you to change the opacity on each layer. You could delete the Flatten step inside the action but I found when I did this the action started to produce errors in CS5, (not sure why, I didn't spend time investigating). The other issue the action includes every image in one single folder where you can't omit any pics if you wanted to, so then you'd have to first remove any unwanted pics in the folder and then run the action. Anyway, that download is available here if you want to play around with it--
One last thing, you can't include changing opacity in an Action, PS doesn't allow it. That step would need to be custom written in a Script or a Plug-in, which I have not done (obviously). If any of you wrote one one of these and willing to share then I'd be really grateful!!