Terry Maker’s giant ball of cowgirl hats is a formidable piece of art. There are more than 500 straw hats in all, turned brim-up and assembled into a sphere 10-feet in diameter. It’s massive. The piece, in one form or another, has been rolling around the region — and the artist’s head — for about 15 years, since she debuted it at the Boulder Museum of Contemporary Art as an outdoor installation that was left to disintegrate in the elements. The current iteration is the star attraction of “Because the World Is Round,” a retrospective of Maker’s work currently at the Longmont Museum.