Astronomy Picture of the Day

Explanation: The colours of Comet Hyakutake are caused by the action of sunlight on the dust and gas produced by the warming nucleus. The microscopic dust particles reflect sunlight while the sun's ultraviolet radiation excites and ionizes the gas molecules causing them to glow or fluoresce in a range of visible colours. This enhanced colour picture reveals subtle colour changes across the cometary coma and a faint multicoloured tail. It was made on the night of March 18-19 by combining separate green, red, and blue photographs, each about a 15 minute exposure. Some of the colour features in the tail may well represent real changes in its structure from one exposure to the next. The coloured star trails, created as the Anglo-Australian Observatory's UK Schmidt Telescope tracked the rapidly moving comet, indicate the order of the separate exposures. The cometary hues revealed here can not be seen directly due to the human eye's lack of colour vision at the low light levels involved.