FIG. 1 is a perspective view of a hair straightener embodying my new design; FIG. 2 is a left side elevational view thereof; FIG. 3 is a right side elevational view thereof; FIG. 4 is a front elevational view thereof; the rear elevational view being a mirror image thereof; FIG. 5 is a top plan view thereof; and, FIG. 6 is a bottom plan view thereof.