Q. If there is no oxygen in space, then how do the sun and other stars burn with apparent flames?

R.C. Jr.

Goodyear, Ariz.

A. As hot as the sun may be making you in Arizona, it's not really burning at all. It's fusing.

Burning is a chemical reaction that involves the electrons in the outer shells of atoms in the fuel. Oxygen is part of what's necessary for the chemical reaction. But the sun and other stars create their energy by fusing the nuclei at the center of hydrogen atoms. The single-proton nuclei from each of four hydrogen atoms fuse into a single nucleus of helium that has two protons and two neutrons. But the helium nucleus winds up with a smaller mass than the sum of those four hydrogen nuclei that fused together.

The difference is energy. Lots of energy.

Einstein's famous equation E=mc2 translates to: energy equals mass times the speed of light squared. Well, the mass lost in fusing that tiny helium nucleus isn't very much at all. But the speed of light is 186,000 miles per second. Squared, that's 34,596,000,000. Multiply that times even the tiniest value for mass, and you've got lots of energy.

Fusion creates one million times more energy, per atom, than chemical burning. Who needs oxygen when you've got that kind of power available!

We can create the staggering power of the sun here on Earth, in the form of a hydrogen fusion bomb. But finding a way to use an on-going, self-sustaining nuclear fusion reaction for a controllable energy source has engineers stumped. Since temperatures reach five million degrees at the center of the sun, standard materials can't be used to build a container to hold such a reaction. Scientists have built machines called Tokamaks that use magnetic fields to do the job, but they haven't been able to hold the fuel long enough to reach ignition, the point at which the reaction becomes self-sustaining. That's what the sun does with fusion, and what nuclear power plants do with fission reactions, which split nuclei apart instead of fusing them together.