What causes the patterns made by star trails?

These patterns result from an optical phenomenon that's called the moiré effect. ... This extra second causes a gap between each of the shots required to make the image and the repetitive trails of the stars, causing the moiré patterns to appear.

What is responsible for the circular trails in the image above?

The circular trails result from the Earth's natural rotation – stars that never rise or set are said to be circumpolar. ... For instance, any star at my location will be circumpolar if it's less than about 39 degrees from Polaris. At the left side of the image (top to bottom), notice the blurred remnants of our Milky Way.