9781422283684

∆ Comet West, a striking sight in spring skies in March 1976. Here we see its two tails clearly—the blue one is the gas, or ion tail.

The hydrogen cloud is invisible in ordinary light, but shows up in pictures taken in ultraviolet light. Telling tails The most spectacular part of a comet is its tail , or rather tails,

because most comets have two tails. How do comets grow tails? Jets of gas and dust spurt out of the comet nucleus on the hottest side, which faces the Sun. As they spurt out, they come up against the solar wind . This is a stream of particles

flowing out from the Sun. The wind sweeps the gas and dust backwards. Soon two distinct tails develop, one made by the gas, one by the dust. The

particles in the solar wind hit the gas particles and make them electrically charged. They become what are called ions, and start

to glow. They form the gas, or ion tail. Sunlight pushes the dust particles into another tail, the dust tail .

∆ A false–color view of Halley’s comet in 1986, showing different levels of brightness.

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