D'Arcy O'Connell, age 14, of Montreal, Quebec, Canada, for his question:

Why doesn't the sun burn up its matter?

It does, but its furnace is nothing at all like a fire in a grate. If our sun were burning coal, it would be ashes in about 5,000 years. Its matter is not solid fuel, but gaseous hydrogen. Its fiery furnace is run by nuclear fusion, the same atomic energy that explodes the hydrogen bomb. Scientists estimate that it has been burning for nearly five billion years and about half of its original fuel has been converted to ashes.

The sun burns by fusing small hydrogen atoms to create larger atoms of helium. Its gaseous hydrogen and its gaseous helium are both forms of matter. The fiery furnace converts its hydrogen fuel to helium ashes, releasing seething atomic energy. The sun is so enormous that it can afford to squander its fuel at a fantastic rate.

Every second through the ages, the sun consumes an estimated 564 million tons of its hydrogen fuel and the ashes become 560 million tons of helium. This leaves four million tons of matter unaccounted for, every second. A nuclear furnace proves that matter can be converted into energy. This is what happens to that four million tons of the sun's matter, every second.

Scientists suspect that the nuclear activity occurs in the sun's core, where perhaps temperatures are as high as 27 million degrees. Here the gaseous atoms are stripped of their electrons and crowded so close together that the material is ten times denser than steel. However, it is in a gaseous state. The enormous heat and pressure generate the furnace by fusing pairs of hydrogen atoms into single helium atoms. A helium atom is twice as heavy as hydrogen, which is the smallest of all atoms.

You would think that the two little hydrogens would fit neatly together to form the double weight helium atom. But this is a fusion of nuclei. And an atomic nucleus is a very complex, tight fisted package of matter and energy. There is a little more matter in two hydrogen nuclei than there is in one helium atom. During the fusion process, this surplus matter is converted into nuclear energy. Every second, enough atoms fuse to convert four million tons of the sun's matter into radiant energy. The sun pours out this energy and loses it forever. But the helium from the burning hydrogen is not lost.

The sun converts almost 15,000 million tons of its matter to energy every hour. Scientists tell us that it has enough fuel to burn at this rate for perhaps another 50 billion years. However, long before this its load of helium ash is expected to bank down the furnace. This may begin to happen about five billion years in the future. No doubt by that time the human race will have space traveled to another planet near a younger sun.