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Jacki Mannier, age 10, of Vander Wagen, N.M., for her question:

WHAT MAKES SOME COMETS SO BRIGHT?

Most comets cannot be seen by the eye alone. A telescope is necessary to bring them into sight for the scientists who study their movements through the solar system. Resembling fuzzy stars, comets move along definite paths and actually travel around the sun in oval shaped or elliptical courses.

Most famous of all comets is the one named for English astronomer Edmund Halley. It's Halley's Comet, and it last flashed through the skies in 1910. It circles the sun every 77 years and should be back on schedule in 1986, which isn't too far in the future.

Some faint comets return every seven years or so, while others come our way once every thousand years or even once every million years.

Remember Comet Kohoutek? It made a rather disappointing visit in 1973. It has an orbit that will bring it back for another look in 75,000 years.

The nucleus of a comet, scientists believe, resembles a dirty snowball that was formed in the icy, distant regions of the solar system. Making up the nucleus, most likely, are frozen gases and frozen water mixed with dust particles.

If a comet is bright, the sun makes it so. All of a comet's light actually comes from the sun, with part of the brightness being a reflection and part a release of energy which had been absorbed from the sun's rays.

As a comet moves toward the sun, the heat causes the outer layers of the icy nucleus to evaporate. The evaporation releases dust and gases which form a cloud like coma around the nucleus. The pressure of the sun's light may push particles and gas molecules away from the coma, forming one or more tails.

The tail of a comet may stream across space as far as 100 million miles. In most comets the nucleus measures less than 10 miles in diameter, but the cloud like,_.coma surrounding it, making up the comet's head, may be as large as 1 million miles.

Most comets develop the long, shining tail only when they come near the sun.

The pressure of the sun's light makes a comet's tail point away from the sun at all times. As the comet makes its approach toward the sun, the tail is pointing to the direction from which the comet came. As it passes the sun and moves away, the tail is not bringing up the rear but is actually pointing the way and leading.

 

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