11 Hour Star Trails . ... Of course, the length of the star trails will depend on the exposure time. While exposures lasting just five minutes produce a significant arc, in about 12 hours a given star would trace out half a circle . But in any long exposure, the background glow from light-polluted skies can build up to wash out the trails. ... To do it, he combined 128 consecutive five minute long digital exposures recorded in very dark night skies above Namibia . ...