Caption: To those prepared to leave their brightly-lit suburbs and seek dark skies, Comet Halley was there to be enjoyed in the first months of 1986, especially in the southern hemisphere. 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 be seen. The blue one is primarily due to water vapor, dissociated by ultraviolet sunlight, fluorescing in the blue color of the cyanogen radical, while the faint yellow streak is sunlight reflected from dust particles liberated from the nucleus along with the volatile materials.
