|The Cometary Tail|
There are two main types of cometary tail.
The Ion Tail (Type I)
The neutral gas species in cometary comae can be ionised by solar UV photons, as in
. The ions are susceptible to a magnetic force due to the solar magnetic field carried by the solar wind. Consequently, the ions are swept out of the coma into a long, distinctive ion tail. Because the most common ion, CO+, scatters blue light better than red, the ion tail often appears to the human eye as blue. Also, the magnetic force is very strong and produces ropes, knots and streamers that distinguish the ion tail from the dust tail. The solar wind sweeps past the comet at about 500 km/s, causing the ion tail to be swept almost exactly in the anti-solar direction.
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The Dust Tail (Type II)
The dust tail consists of dust particles that have been pushed out of the coma by radiation pressure from the sun. Compared to the ION tail, the dust tail is morphologically diffuse, and appears white or slightly pink (because the dust grains reflect sunlight slightly better at longer wavelengths than at shorter wavelengths). The dust particles in the tail are individually in orbit about the sun, each with slightly less attraction to the sun than the nucleus because of the effect of radiation pressure. This causes the dust tail to be curved as the comet swings around the sun.
Both the ion and the dust tails can attain great lengths, up to about an Astronomical Unit (10^11 m) in the most spectacular cases.
Last updated August 2009
Nucleus Comet Jewitt