Saturday, December 12, 2009


Synonymous with life, it was born in the heart of stars

Although carbon has recently acquired a bad rap because of its association with greenhouse gases, it has also long been synonymous with biology. After all, “carbon-based life” is often taken to mean “life as we know it,” and “organic molecule” means “carbon-based molecule” even if no organism is involved.

But the sixth element of the periodic table—and the fourth most abundant in the universe—has not been around since the beginning of time. The big bang created only hydrogen, helium and traces of lithium. All other elements, including carbon, were forged later, mostly by nuclear fusion inside stars and supernovae explosions.

At the humongous temperatures and pressures in a star’s core, atomic nuclei collide and fuse together into heavier ones. In a young star, it is mostly hydrogen fusing into helium. The merger of two helium nuclei, each carrying two protons and two neutrons, forms a beryllium nucleus that carries four of each. That isotope of beryllium, however, is unstable and tends to decay very quickly. So there would seem to be no way to form carbon or heavier elements.

But later in a star’s life, the core’s temperature rises above 100 million kelvins. Only then is beryllium produced fast enough for there to be a significant amount around at any time—and some chance that other helium nuclei will bump into those beryllium nuclei and produce carbon. More reactions may then occur, producing many other elements of the periodic table, up to iron.

Once a star’s core runs out of nuclei to fuse, the outward pressure exerted by the nuclear fusion reaction subsides, and it collapses under its own weight. If a star is large enough, it will produce one of the universe’s most spectacular flares: a supernova explosion. Such cataclysms are good, because supernovae are what disperse carbon and the other elements (some of them forged in the explosions themselves) around the galaxy, where they will form new stars but also planets, life ... and greenhouse gases.

Source of Information : Scientific American September 2009

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