AAO image reference AAT 117. « Previous || Next »
Top left is NE. Image width about 55 arc min
Image and text © 2001-2010, Australian Astronomical Observatory, Photograph from AAT plates by David Malin.
The warming action of sunlight on the tiny nucleus of the comet evaporates volatile materials from its surface which expand rapidly in the vacuum of space, producing the large coma. Solar radiation pressure sweeps back this tenuous cloud into the typical comet shape. Emerging from the coma, two distinct tails can often be seen. The blue one is primarily due to volatile molecules such as water, carbon dioxide and carbon monoxide dissociated by ultraviolet sunlight, fluorescing in the blue colour of the cyanogen radical, while the faint yellow streak is sunlight reflected from dust particles liberated from the nucleus along with the volatile materials. A more detailed view of the inner nucleus is seen on AAT 117a.
This picture was made after the comet had rounded the sun and was heading back into the cold of interstellar space. The comet's tails also point away from the sun, no matter what the direction of the comet. The multicoloured 'rain' is the trails of countless stars, photographed, like the comet, in red, green and blue light as the Anglo-Australian Telescope followed the comet's motion in front of the Milky Way. A deeper and wider field view was taken at about the same time by the UK Schmidt Telescope.
Related images, other comets
AAT 46. Halley's Comet, December, 1985
AAT 117a. Around the nucleus of Comet Halley on April 9-10, 1986
UKS 19. Halley's Comet on 12 March, 1986
UKS 33. Comet Hyukatake, March, 1996
UKS 34. Halley's Comet on 9-10 April, 1986 (UK Schmidt image)
MISC 20. Comet Halley hanging in the Milky Way in 1986.
B&W image Features in the dust tail of Comet Halley, 12 March, 1986
For details of photographic exposure, search technical table by UKS reference number.