The sun is powered by a heat-producing process called nuclear fusion. In fusion, two lighter hydrogen nuclei combine to form a single, somewhat larger helium nucleus. (In fission reactions, the nuclei of certain heavy elements like uranium break apart and release energy.) The mass of the resulting helium nucleus is slightly less than the combined masses of the two lighter hydrogen nuclei, and the difference -- called the mass defect -- is released as energy. As Einstein stated in his famous equation E=mc², even a tiny amount of mass represents a large amount of energy. Fusion reactions -- also called thermonuclear reactions -- generate about 2.5 times more energy per atom of fuel than do fission reactions and have an even greater yield per kilogram of fuel.

The greatest challenge to artificially producing fusion reactions is getting two positively charged nuclei, which naturally repel each other, to combine. Scientists involved in energy and weapons research, then, must try to replicate the extreme conditions that exist on the sun, which means trapping a large quantity of fusion material in a contained space, raising its temperature to millions of degrees, and subjecting it to high pressure. Though huge amounts of energy are required to raise the fusion material to its ignition temperature, once fusion is underway, the released energy is powerful enough to produce further fusion.

At present, the only application of artificial fusion has been the hydrogen bomb. Nuclear fusion used for power generation could provide the earth with an almost unlimited source of power because the gaseous fuels needed -- hydrogen isotopes called deuterium and tritium -- come from common substances. Deuterium is made from water; tritium is produced from lithium, a metal widely found in minerals.

In one type of fusion reactor design, these gases are fed into a ring-shaped vacuum chamber and heated to the point at which they become a plasma -- a form of super-hot gas that is affected by magnetism. A powerful magnetic field then squeezes the plasma so that fusion occurs. Heat is transferred to a boiler to produce steam for electricity generation.

A second area of fusion research employs lasers: Tiny pellets of deuterium-rich material are dropped into a fusion chamber and compressed with powerful laser beams fired from around the chamber. The pressure of the beams confines and compresses the fusion material until ignition is reached.

Nuclear fusion represents a pollution-free alternative to current fission-based nuclear power technology. It is clean, meaning the process doesn't produce radioactive waste the way fission does. The only by-product of fusion is helium, a benign, inert gas. The realization of controlled commercial fusion power -- a technology that could replace fission-based nuclear power as well as fossil-fuel generation -- is gradually getting closer.