Third tail spotted on Hale-Bopp

On a clear evening, Comet Hale-Bopp appears to have two tails - but European astronomers have now discovered that it has a third, consisting of sodium atoms. The discovery could lead to new theories about the composition of comets and the effects of the sun on them.

The most visible tail of any comet consists of dust particles thrown off as the rock and ice forming the nucleus heats up during the approach to the sun. Another, subsidiary tail is also produced, consisting of charged ions of elements from the nucleus. This is thrown off and away by the "solar wind" of plasma emitted from the solar surface. The sodium tail, the first to be discovered, is emitted in a direction close to but slightly different from the second, "ion" tail. More unusually, the sodium is in the form of uncharged atoms, not ions.