Deborah Newcombe, age 12, of Staten Island, New York, for her question:

How much energy does the sun have left?

The sun, of course, is a nuclear powerhouse somewhat like the con¬tinuous operation of a hydrogen bomb. Its fuel is hydrogen gas. Its operation is nuclear fusion, in which atoms of hydrogen are fused to form atoms of helium. And the nuclear operation is conducted on a stupendous scale at a fairly constant level. Astrophysicists estimate that the solar furnace is fed 564 million tons of hydrogen fuel each and every second. This produces an estimated 560 million tons of helium and the remaining four million tons are converted into various forms of solar energy.

At this rate, you would not expect the solar furnace to last very long. If the sun were made of coal, it would have burned to ashes in about 5,000 years. But its nuclear activity has been going at full blast for at least five billion years and scientists estimate that only about 50% of its hydrogen fuel has been consumed. As it grows older, the nuclear activity is expected to change. Many experts think that the sun will ration its remaining fuel and keep blazing away for another 10 to 12 billion years.