Susan Heymann, age 13, of LacGrange, Ky., for her question:

WHAT IS THE BIG BANG THEORY?

Cosmology, in astronomy, is the study of the structure and history of the universe. The big bang theory offers what many scientists say is the best explanation of the basic cosmological observations.

According to this big bang theory, the universe began as the result of an explosion about 13 billion years ago. Right after the explosion, the universe consisted chiefly of strong radiation. This radiation formed a rapidly expanding sphere called the primordial fireball.

After a few hundred years, most of the fireball changed into matter that consisted chiefly of hydrogen. The matter continued to move away from the point of the explosion. In time, the matter broke apart in huge clumps which became galaxies. Smaller clumps within the galaxies, says the big bang theory, became stars. Part of one clump became our solar system.