Monday, March 30, 2009

A Tale of Two Tails

Most comets actually have two tails. The dust tail is usually broader and more diffuse than the ion tail, which is more linear. The ion tail is made up of ionized atoms—that is, atoms that have lost one or more electrons and that, therefore, are now electrically charged. Both the dust tail and the ion tail point away from the sun. But the dust tail is usually seen to have a curved shape that trails the direction of motion of the comet. Careful telescopic or binocular observations of nearby comets can reveal both of these tails.
What we cannot see optically is the vast hydrogen envelope that surrounds the coma and the tail. It is invisible to optical observations.
Common sense tells us that the tail would stream behind the fast-moving nucleus of the comet. This is not the case, however. The ion tail (far from the sun) or tails (the dust tail appears as the comet gets close to the sun) point away from the sun, regardless of the direction of the comet’s travel. Indeed, as the comet rounds the sun and begins to leave the sun’s proximity, the tail actually leads the nucleus and coma. This is because the tail is “blown” like a wind sock by the solar wind, an invisible stream of matter and radiation that continually escapes from the sun. It was by observing the behavior of comet tails that astronomers discovered the existence of the solar wind.

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