Here's a 16 minute exposure taken too near a stray light source. Camera settings F3.5, ISO400, 963seconds, white balance cloudy. To the naked eye in the middle of the night he forest trees seemed to be completely black and the stars looked great.
Looked like a great place to take night phots. Wildcat Mountain State Park campground site number 8 as I recall. However about 150 yards away was a small building with an exterior safety light. The length of the exposure captured enough stray light on the trees the trees look almost like daytime. How much stray light is too much really is a trial and error process.
Long exposures its important to remember a math thing called inverse squared. Strong math skills great to have but I find math too dry a subject for an nature photography journal. Simply put brigthness of a light source is proprotionally reduced by the inverse square of the distance. The farther away the stray light the less night photos are affected. In camera terms double the distance and 1/4 the amount of light. Three times the distance 1/9 the brightness. Four times the distance 1/16 the amount of light.
Just looke around and remember it only takes very little stray light to ruin a shot.