evolução e o carbono ainda nas estrelas

Hubert Reeves em Origins, Arcade Publishing, New York, 1998.

"The progress of astrophysics has enabled us to bring the questions to the forefront of contemporary thinkers but has not enabled us to provide satisfactory answers. All we can say is that, contrary to the universe, which is in constant flux, these laws of physics do not change, either in space or in time. In the context of the Big Bang theory, these laws have presided over the making of complexity. What is more, the properties of these laws are even more astonishing. Their algebraic formulas and their numerical values appear particularly well adjusted.

In what way they are "well adjusted"?

Our mathematical simulations prove it: if they had been even slightly different, the universe would never have emerged from its initial chaos. No complex structure would have emerged, not even a molecule of sugar.

Why?

Just imagine for a moment that the nuclear force had been slightly stronger. All the protons would have quickly gathered into heavy nuclei. There would have been no hydrogen left to assure the longevity of the Sun and to form water masses here on earth. Instead, the nuclear force was just intense enough to produce some heavy nuclei (including carbon and oxygen) but not so intense as to completely eliminate the hydrogen - just the right dose. One could even go so far as to say that complexity, life, and consciousness were implicit from the very first instants of the universe, that they were inscribed, so to speak, in the very form of the laws - not as being 'necessary', but as being a possibility.

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"Our Sun has thus been 'fueled' by hydrogen for four and a half billion years. The more massive stars shine much more brightly and use up their hydrogen in a few million years, at which point they begin to contract again. Their temperature rises to more than 100 million degrees. Helium, the product of hydrogen fusion, in turn becomes a fuel, at which point a set of nuclear reactions allows for new combinations: three helium nuclei are going to come together as carbon nuclei, and four helium nuclei as oxygen nuclei.

But why didn't these reactions take place at the time of the original Big Bang?

The encounter and fusion of three helium nuclei is an extremely rare phenomenon. It takes a long time for it to occur. In the original Big Bang, the phase of nuclear activity lasted for only a few minutes, which is too short a time to manufacture a meaningful quantity of carbon. This time, in the more massive stars, the agglomerations are going to take place over millions of years.

Each of these more massive stars is therefore going to manufacture carbon and oxygen nuclei?

For the next several million years, the centers of the larger stars will indeed be stockpiled with heavy nuclei, including carbon and oxygen. These elements are going to play a fundamental role in the following phase of our history. Carbon in particular, with its special atomic configuration, lends itself easily to the manufacture of long molecular chains, which will play a key role in the appearance of life. Oxygen will become a component of water, another element that is indispensable to life."

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