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Interesting point. I think it depends on where the stars are. Stars near the North Pole (Polaris) will move slower and streak less than stars near the celestial equator. The stars in the picture appear to be moving parallel to the horizon, which means they are to the north. If they were to the east or west, they would be moving perpendicular to the horizon. Stars to the north will be moving more slowly.
There isn't enough information in the picture to easily determine the length of the exposure. You could measure the radius of the star streaks and then the length of each star streak to figure out the exact length of exposure, but the radius is so large (the streaks appear to be straight lines) that the error would be huge.
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