The tiles in the borders around the star look square or rectangle to me, with the extra space to make the increasingly larger circles being provided by more grout space at the outer edges and less at the inner edges. So if using fabric instead of grout, either your squares/rectangles would have to distort at the outer edges, or your "background" pieces would have to adjust, depending on what method you are using (stained glass vs. just pieced shapes). Interesting project! Let us know if you draft it out, 'cause I've seen some sunflower centers and camelias I'd like to reproduce!