Thanks @captainsailor :) I also live in a large city, but these photos were taken very far away from it (and on nights with as little moonlight as possible).

In terms of lens usage, your 50mm would be best (as it is f1.8: lower f stop = more light let into camera). However, the 50mm has a very narrow field of view, so your next best bet would be the 18-55, as its f3.5, and has a much larger field of view.

However, if you REALLY want to get into astrophotography, then I'd recommend picking a Rokinon 24mm f1.4. I use it for most of my photos, and the results speak for themselves :) I'm happy to answer any more questions you may have.