AAO image reference UKS 19a. « Previous || Next »
Top left is NE. Image width is about 2 degrees
Image and text © 1986-2010, Australian Astronomical Observatory
Photograph from UK Schmidt plates by David Malin.
A colour photograph of a comet (or any astronomical object) does not always reveal every detail. The photograph above was made from the UK Schmidt plate taken in blue light that was used for our three-colour photograph of the comet. The turbulent structure was from the original glass negatives coping by a darkroom process known as unsharp masking.
Comet tails form two distinct types, a yellowish dust tail, made of tiny solid particles released as the icy, dusty cometary nucleus is evaporated by sunlight, and a blue ion tail, made from volatile compounds released along with the dust, then ionised by ultraviolet light. Both tails point away from the Sun, irrespective of the direction of the comet. They are driven by to solar wind, but often at different rates, so the dust tail is sometimes seen as curved.
The ion tail, seen above, is made of ionised gases evaporated from the nucleus of the comet as it approaches the Sun. These volatile materials can include water, carbon dioxide, carbon monoxide, ammonia, giving rise to the ionised species CO+, CN+ N2+, OH+, CO2+, and CH+, amongst many others. Some ions, mainly cyanogen (CN), glow or fluoresce blue under the influence of solar ultraviolet light, and that is what is captured in the photograph. The ion tail is affected by the sun's magnetic field and by the way it is liberated from the tiny nucleus of the comet. This image was made about a month after the comet has passed perihelion (closest to the Sun), so it was still very active.
Related images, other comets
AAT 46. Halley's Comet, December, 1985
AAT 117. Halley's Comet, April 9-10, 1986 (AAT image)
UKS 19. Comet Halley (colour version of above image)
UKS 33. Comet Hyukatake, March, 1996
UKS 34. Halley's Comet on April 9-10, 1986
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 object position and photographic exposure, search technical table by UKS reference number.