David Guza, age 10, of Decatur, Georgia, for his question:

What happens when a star stops burning?

Scientists tell us that a star can live only so long. Its life span may be a million years or as long as 20 billion years. It starts out as an enormous ball of hydrogen gas. Mighty forces in the universe turn it into a nuclear furnace. Its hydrogen starts burning and the newborn star bursts into a blaze. The hydrogen is its fuel and its nuclear furnace changes this fuel into helium gas. As it shines in the heavens, it uses more and more of its supply of hydrogen.

A star may keep blazing away for billions of years. But at last it uses up almost all of its hydrogen fuel. Then its nuclear furnace must slow down. The light grows dim and the fires go out. The star becomes a dead, dark ball of ashes. On and on it travels forever through space. But the cold burned out star has no warmth and no bright light to shine in the sky. Scientists tell us that our starry sun has enough hydrogen fuel to keep burning for at least another five billion years. It will keep shining for many ages to come.