Magazine article UNESCO Courier

The Origin of the Elements

Magazine article UNESCO Courier

The Origin of the Elements

Article excerpt

As the Universe began to cool after the Big Bang in which it was born, clouds of gas were able to collapse under the pull of their own gravity and form stars. But those first stars did not contain the rich diversity of chemical elements that we find on Earth today. Only hydrogen and a little helium--about 20 percent of the total matter--were created in the Big Bang itself; all of the other elements, including the carbon in your body and the oxygen in the air that you breathe, have been manufactured inside stars and spread across space by stellar explosions--supernovae.

A first generation star, containing only hydrogen and some helium, would have formed as a ball of gas, contracting under the pull of gravity and getting hotter in the middle, as gravitational energy was converted into beat. Higher temperatures mean that the particles in the heart of the young star move faster and collide with one another more violently. At a few thousand degrees, electrons are stripped from their atoms, leaving a sea of protons (hydrogen nuclei) constantly colliding with one another. And at a temperature of about 10 million degrees K two protons that collide with one another stick together. Each of these protons carries a positive charge, but when they stick together the pair emit a positively charged counterpart of the electron, called a position, and one of the protons thereby becomes converted into a neutron. The combination of one proton and one neutron forms the nucleus of an atom of heavy hydrogen, deuterium. And when that deuterium nucleus collides with another proton, it can stick of form the nucleus of an atom of helium, a variety called helium-3 containing two protons and one neutron. Two of these helium-3 nuclei in turn combine to form one nucleus of helium-4, containing two protons and two neutrons, with the release of two protons--hydrogen nuclei--which go back into the stellar melting pot.

The net effect of this cycle of activity is to convert four protons into one helium-4 nucleus, and to release energy. This is nuclear fusion, the basis of the hydrogen bomb. A star like the Sun, indeed, is the cosmic equivalent of a hydrogen bomb, one which explodes continuously for ten thousand million years. The energy released keeps a star hot in the middle and holds it up against further collapse, for as long as the supply of hydrogen fuel lasts. Ultimately, however, the fuel must run out as all of the hydrogen in the middle is converted into helium. At that point, the star will begin to collapse again, getting hotter still in its heart, with pressure building up until conditions are so extreme that the helium nuclei themselves begin to combine, with three nuclei of helium-4 getting together to make one nucleus of carbon, which contains six protons and six neutrons. …

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