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Without any additions copy and past each star photo to a new layer, convert each of the layers above the base layer to lighten. This is a slow process if you have dozens to merge. If you know how to create an action, record the process and then you can use that action in future. If you don't understand actions, there are a few floating around on the web that you can download for free (here's one Star Trails Action) and place in your actions folder to do star trails.
Can you tell me why you would stack a number of images rather than just keep the shutter open for an hour?
Is this a kind of anti HDR type thing where the moving light trails are the only things really kept from frame to frame?
Quote: Can you tell me why you would stack a number of images rather than just keep the shutter open for an hour?
Is this a kind of anti HDR type thing where the moving light trails are the only things really kept from frame to frame?
A number of images are stacked together to reduce the noise on the final image. Also if you have the shutter open for an hour and you have excessive glare from objects that don't look that bright to look at but after an hour produce quite alot of reflective light. Dark frames are also taken by putting the lens cap back on and taking a few more images. When these are staked into the image they remove the hot pixels that are caused by the sensor getting hot whilst doing the long exposure.
I usually stack every image in its entirety and make any adjustments to the final image.
I hope i haven't rambled and everything makes sense.
Many thanks - You can almost learn something new here every day.
Much appreciated to all of you 
Quote: Can you tell me why you would stack a number of images rather than just keep the shutter open for an hour?
In case the battery runs out 59m into a 1h exposure.
Better to have 5 x 10min exposures then no exposure at all.
To merge the images, stack them as layers in Photoshop, then set blending mode to 'screen'
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