You can only see stars in a dark sky.

"While astronomy enthusiasts understand the importance of dark, pristine skies, the science of measuring light pollution is not well established. Now, researchers with the US National Park Service (NPS) have released data as part of a program to measure the impact of light pollution on the relatively unspoiled skies of western US National Parks. The NPS Night Sky program, the first of its kind, systematically detects and analyzes night-sky brightness in national parks to understand current dark-sky quality and identify light-pollution sources."

See also:

Polluted light vs. sight of the night

Post-twilight light shows and other photon phenomena