Comet Hale-Bopp. Optical image of comet Hale-Bopp over a natural rock arch. Hale-Bopp was one of the brightest comets of the 20th century. Its gas or 'ion' tail (blue) consists of ionized glowing gas blown away from the comet head by the solar wind. The dust tail (white) consists of grains of dust pushed away from the comet head by the radiation of sunlight. Comets have a nucleus of ice and dust. As they approach the Sun their surface evaporates, releasing a tail millions of kilometers long. Photographed on 30th March 1997 over Owachamo Natural Bridge, in the Natural Bridges National Monument, Utah, USA.