If it's the long exposure that's puzzling, that's because you want a long enough exposure to allow the star trails to show up with the earth's rotation. Not enough time and the trails will be too short, too long and the sky can become too cluttered (imo). Many will include Polaris (North Star) in their star trail photos as all the other stars will appear to revolve around that star and give some star context.
As others have noted, dark skies are key as sky glow from light pollution from nearby towns may drown out your stars. A darker sky will allow fainter stars to show up against the sky when they might get lost otherwise.
This particular photo was taken with the 50 Distagon, wide open at f4, 400TX, for about an hour, maybe bit less. Push processed in Rodinal.