Star trails are the curved streaks stars leave across a photo when a long enough exposure captures the Earth's rotation relative to the fixed stars overhead.
Star trails are the deliberate opposite of sharp-star astrophotography. Where a Milky Way shot fights to keep stars as points with a short enough shutter, a star-trail image lets the Earth's rotation drag them into arcs, turning a static sky into a record of time passing. The look depends on total exposure time, not one setting. Thirty minutes gives short, subtle arcs; two or three hours gives long sweeping curves that clearly circle the celestial pole.
Most photographers now build the effect from a stack of many 30-second frames rather than one continuous exposure, because that keeps noise down and means a plane or a passing headlight ruins one frame instead of the whole shot. Composition matters more than with most night work, since framing in the north or south celestial pole puts the centre of rotation on the picture and makes the circular motion obvious.
A photographer shoots star trails over a lighthouse by setting the camera to take 240 back-to-back 30-second frames at f/2.8, ISO 800, on a locked intervalometer, for a two-hour total. Pointed north, the finished stack shows stars wheeling in tight concentric circles around Polaris, which sits nearly still just above the lighthouse lamp. Pointed east instead, the same two hours of frames produce long straight diagonal streaks rising off the horizon, a different but equally strong effect.
Stacking software blends the frames by keeping the brightest pixel at each point, so the trails build up smoothly. One frame spoiled by a passing car is simply dropped from the stack, costing 30 seconds of arc length rather than the entire night's work. A dew heater on the lens earns its place here as much as the intervalometer, since two hours outdoors will fog an untreated front element and soften every frame after the first half hour.
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