To me, an F seems like the best choice for shooting star trails, or long exposures in general. It's about as straightforward as it gets. No batteries to run out. Just put the camera on T and shoot (or use B and a locking cable release). They are also cheap for the amount of quality you get.
The problem with my suggestion is that I am not up to speed on the various Nikon lens designations, so I don't know if an AF-D lens has an aperture ring or not. If not, please ignore everything I've said...
AF & AF-D have aperture rings. The "D" in an afd stands for distance information that is relayed to the camera for flash calculations.
The newer ultrasonic type motor "G" lenses do not have aperture rings.
FM's are all manual, just like the old F's were. FM2's are the same and newer, to boot.
They have the advantage of being smaller, usually cheaper, and being newer than an F or F2 are usually (I say usually) in better shape.
n6006, has the T setting on it and has AF
Using the built in flash would demand some serious calculations.
1. Shoot the flash.
2. Go back home and calculate the distance to see in what year you should be back to open the shutter (in case any of you photons would make it back).
I like the idea
On a more serious note, from what I know of astrophotography (admittedly, not much), maybe it would be better to choose the lens first, then the camera.
While any lens which is sharp in the corners and has low distortion (for example, the Micro-Nikkor 55 2.8 AI - good also at infinity) should do, for best results, a lens which is good for night photography should be chosen.
This often includes, but is not limited to, "Noct" lenses. Apparently the trick is low coma and flare/reflections.
Also, many non-Noct lenses which do well are not made by Nikon. As it's a relatively small group of lenses, maybe you should do some more investigating, there are some astrophography sites, or repharse your original question.