Doug Vaught, age 12, of Florence, South Carolina, for his question:

How much energy does the sun have left?_

Scientists estimate that the sun was ignited about five billion years ago. Its fuel is hydrogen gas, which is consumed at a fairly steady rate of 564 million tons per second. By the process of nuclear fusion, this amount of hydrogen becomes 560 million tons of helium    and the remaining four million tons is converted into energy. This can continue as long as the original supply of hydrogen fuel lasts.

It is estimated that about half of the sun's energy has been used during its first five billion years. There is enough left to last another five billion years, if the burning continues at the same rate. However, some scientists suspect that the nuclear furnace may slow down as its fuel dwindles, in which case the sun's energy would last longer than another five billion years.