Francine DeFeo, age 11, of Staten Island; New York, for her question:

How long will the sun's energy last?

Scientists can only estimate the life span of our starry sun. But they have a large mass of evidence to help them, and more is expected from lunar rocks. It is thought that the sun and its planets were formed at about the same time. The earth's oldest rocks are weathered or crushed below massive layers of its restless crust. The more placid lunar landscape suggests that some of the oldest rocks may be five million million years old.

Dating the age of the solar system seems an odd way to estimate the future of the sun. But remember, it is a nuclear furnace, consuming its fuel at a certain rate. Originally it was made entirely from gaseous hydrogen fuel. Astronomers estimate that about half of its original fuel has been converted to helium and other elements. On this basis, if its age is five billion years., we can expect its energy to last another five billion years. However, astronomers think that its aging furnace may subside. Using less fuel, its energy might last another ten or 12 billion years.