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Alicia Rutledge, age 12, of Atlanta, Georgia for her question:


Are planets dead stars?


In some ways, a dead star is like a planet, but the two are not the same thing at all. Astronomers tell us that a dead star shrinks to the size of a smallish planet and like a planet, it sheds no brilliant light or blazing heat. Both are round globes of solid materials. But a burned out star is at least a million times denser and heavier and besides, it was once a nuclear bonfire    but the planet was not.

Scientists know more or less how a sun and its planets are born and live through billions of years. From studying the nuclear fusion of the H bomb, they know how a star's fiery furnace burns    and finally burns out. By studying aging stars in the heavens, they know how the average star grows old and dies.

Our sun has a permanent family of nine planets. A few planets have been found orbiting other stars out in space and astronomers think that most other stars have solar systems of their own. In each case, the solar system is formed from an enormous cloud of cosmic gases. For reasons unknown, the huge hazy cloud begins to shrink and condense./ Cosmic forces set it spinning like a wheel, with most of the material packed together in the center. Smaller concentrations of material form the planets, whirling their separate orbits.

Perhaps 99 per cent of all the material in this budding solar system is concentrated in its central sun. Eventually, dynamic cosmic forces cause its enormous mass to ignite    and the embryo star becomes a blazing sun. Most of its material is hydrogen, the fuel which powers. the nuclear fusion of its furnace.

The average sun starts out with enough hydrogen fuel to run its furnace for many billions of years. As the fuel is consumed, it creates helium, which is the ashes of the nuclear furnace. Meantime, the blazing sun sheds life giving radiation on its planets and its enormous gravity keeps them revolving around their orbits.

Things may go on like this for ten billion years or more. But gradually the sun uses up its original quota of hydrogen fuel. Scientists predict what happens when about 15 per cent of the hydrogen is consumed. Its core of helium ash starts a second nuclear furnace, far hotter than the first. The star swells up and becomes a seething red giant, scorching the surfaces of the inner planets to ashes.

This sudden flare up soon subsides. Then the star shrinks and shrinks, burning with a blue glow as it changes its materials into metals. After a few hundred million years, its blazing blue fires subside but the weary old furnace continues to glow red for a hundred billion years or more.

Eventually, of course, the weary old star burns out altogether and becomes a ball of cold, dead ashes. Though smaller, it still has most of its original mass or material. Since gravity is related to mass, this means that the little dead star still holds it planet children in their orbits. Though as small as a planet, the dead densely packed sun still is perhaps a million times more massive than the average planet.

 

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