The clouds opened up just enough to show the Milky Way during this 20 second exposure. While the clouds show a noticeable amount of motion, 20 seconds is short enough for the stars to retain most of their roundness at this focal length.
The Sigma 18-35mm Art Lens is very sharp, even at its very wide f/1.8 aperture. It is a very good choice for your night sky photography. With this lens and a stable tripod, shots like this one are very easy to capture.