Terri Overfield, age 13, of Wichita, Kansas, for her question:

How long is the world expected to last?

From time to time, some fiery eyed fanatic predicts that the world will end next Tuesday at 2 p.m. He adds that all but his very few chosen supporters will meet their doom. It never happens because this nonsense has nothing to do with the future of our sturdy old planet. Certainly no end is in sight for it. However, our world as we know it teems with life    and all its living things are supported by the radiant energy of the sun.

Life on earth is linked to the destiny of our stary sun. And a star's nuclear furnace burns only until its hydrogen fuel is consumed to ashes. Our sun is expected to shed its radiance for at least five billion and more likely 12 billion years into the far future. As it dies down, so will the life we know on the earth. After the last glimmer, the sun's cold, dark ball of ashes may continue on    and our lifeless world and the other planets may continue orbiting the dead star    indefinitely.