Published: 20 March, 2003

Nuclear fusion lets sun burn

How can the sun burn without oxygen in space? If there is oxygen in space why do astronauts need oxygen?

Excellent question. There are many ways in which something can burn. A log in your fireplace burns. Your skin, exposed too long to the sun, burns. An unanswered question in your mind burns and, as you observed, the sun burns. Only one of these types of burning is the chemical process of combustion, a reaction that generally needs oxygen. In the combustion of molecules made of carbon (most combustion is of this sort), some initial heat such as friction or another flame breaks the chemical bonds between carbon atoms. Oxygen in the air quickly fills the empty space where the old bonds were with new bonds. These new bonds are lower in energy and we observe the extra energy as the light and heat of a flame. In combustion old bonds are broken and new bonds are made, but the atoms remain the same.

The way that the sun burns is called nuclear fusion. In the sun, intense gravity forces four atoms of hydrogen to fuse into one atom of helium. One helium weighs a good bit less than four hydrogens, so what happened to the missing mass? According to Einstein's famous equation, E=mc², even a little mass (m) becomes a lot of energy (E). We observe this extra energy as the light and heat of our burning star, the sun. In fusion it is not the bonds between atoms that are changing but the atoms themselves.

Since the sun's burning occurs without oxygen and there is no oxygen in space, astronauts must carry it with them.