ヘール・ボップ彗星
A hydrogen gas halo three times the size of the Sun was detected by Skylab around Comet Kohoutek in the 1970s. SOHO detected a hydrogen gas halo bigger than 1 AU in radius around Comet Hale–Bopp. Water emitted by the comet is broken up by sunlight, and the hydrogen in turn emits ultra-violet light. The halos have been measured to be ten billion meters across 10^10, many times bigger than the Sun. The hydrogen atom are very light so they can travel a long distance before they are themselves ionized by the Sun. When the hydrogen atoms are ionized they are especially swept away by the solar wind.