Glue the second half with model glue on the first half.
Good luck! Update: If you use the "Weihnachtsstern_vase_mode.stl" model, printing is very quick. The hole for the cord must then be cut at the blunt tip of the star. It is good to print 2-3 lines of skirt / brim with a distance of 0 mm to the print object in order to obtain a surface for gluing.