I propped the iPhone up on a small stand (the same one I used here). The app I used is called Night Cap Pro and it does a great job!
|Same photo as below, but I turned the brightness down a little bit to make the sky less pale gray and make the stars pop.|
|Same photo as above straight out of the iPhone, all star trails are done in-app with long exposure. This was about 10 minutes. That bright blob to the bottom left just coming over the roof is Venus, the rest is Orion, Gemini, and some planes.|
|I also used the star trail mode to make this image. The star trail mode actually does a better job than the noise reduction mode at cleaning up the sky for a nice regular star photo. This was about 20 seconds and you can just barely see trails.|