The pattern was most likely caused by something between the camera and the sky, not by the star itself. From the answers, the strongest explanation is a window screen: its mesh can partially block the star’s light, and a point source like a star can reveal that pattern even when the screen is too out of focus to notice elsewhere in the frame. Shooting through window glass can also introduce reflections from indoor objects such as curtains, especially if the room is brighter than outside. For cleaner star trails, shoot outside if possible, and avoid shooting through glass or a screen. If you must shoot indoors, make the room very dark and place the lens close to the glass to reduce reflections.