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What happens when a comet nucleus races towards the sun?

Orbit
Comet tail pointing away from the sun

The following is the sequence of events as the comet falls towards the sun. The nucleus warms up as it comes closer to the sun. When the nucleus is at about the distance from the sun as planet Jupiter, the temperature is right for the frozen ices to evaporate directly to vapour and free some dust as well. These surround the comet nucleus. It is this dust cloud that reflects sun light and the comet becomes visible. This fuzzy ball resembles a head of hair and hence called coma, Greek for hair. If more dust surrounds the comet nucleus, the coma appears brighter. The size of the coma increases as the comet comes closer to the sun, and more and more dust is released. Around the time the comet crosses the path of the earth around the sun the water ice starts to evaporate and the comet starts to grow a tail - infact two tails! The dust is pushed away by the radiation pressure of sun light and forms a graceful curved tail, reflecting the sun light. This is the tail that has earned for it the name Puchal Tara in Hindi, Val nakshatram in Tamil and Thoka chukka in Telugu!
The second tail is the gas tail. The ultraviolet light and the stream of particles from the sun knock out one or more electrons from the gas molecules. This ionised gas glows through fluorescence. Usually the most of the fluorescence is due to the ionised carbon monoxide which glows blue. This gas tail is straight, as the ions are kept confined to the magnetic fields carried by the stream of particles that flow from the sun. Comet West that was visible in 1976 had a beautiful dust and gas tail. Comet Hyakutake seen in 1996 had a very prominent gas tail compared to its dust tail. Both the dust and gas tail can grow to enormous lengths, some hundreds of thousands of kilometers The gas tail always points away from the sun. The dust tail also, usually points away from the sun, but once in a while a comet sports a sunward tail!

Hyakutake
Comet Hyakutake 1996

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