Julia Kiraly, age 13, of New Brunswick, N.J., for her question:

IS IT TRUE THAT OUR SUN WILL BURN OUT?

Nowadays the news is full of grim predictions. Goodness knows, we don't want to worry about loosing our friendly neighborhood star. True, there is a limit to the life span of the sun. But the experts assure us this is not one of our present problems. Little or no changes are expected for another five billion years or so.

Our sun, of course, is a star and we now know that a star has a limited life span, a beginning and an end. Most experts agree that it is born when a stupendous cloud of dusty gases congeals to form a blazing central sun and perhaps a family of orbiting planets.

The newborn sun is made mostly of hydrogen gas, so dense and massive that gravity and other cosmic forces cause a fantastic change to occur. Deep within its heart, atoms of its hydrogen fuse to form larger atoms of helium. This is the process of nuclear fusion, during which a portion of the hydrogen matter is converted into various forms of radiant nuclear energy.