carbon

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carbon (C)

The fourth most common element in the universe (see elements, cosmic abundance) and the basis of all terrestrial life (see elements, biological abundance). It exists in a number of distinct forms, including graphite and diamond. Carbon is manufactured inside the core of evolved stars by a process that involves either a remarkable coincidence or a piece of cosmic tuning. In this process, two helium nuclei come together to make a nucleus of beryllium, which then has to capture a further helium nucleus to complete the synthesis of carbon. However, when the astrophysicist Fred Hoyle first looked closely at this reaction in the 1950s he realized there was a problem. According to what was then known, the capture of a helium nucleus by a beryllium nucleus was far too improbable to account for the observed cosmic abundance of carbon. He reasoned that the only way enough carbon could be made was if there existed a very specific match of nuclear energy levels, or resonance, between helium, beryllium, and carbon under precisely the conditions thought to prevail in the cores of stars at this stage in their evolution. Experiments promptly confirmed Hoyle's deduction—there was indeed a previously unsuspected resonance, very close to the energy value he gave. Crucially, for carbon-based life-forms, there is not a similar resonance at the same energy between carbon, helium, and oxygen. If there were, a large part of the carbon inside stars would quickly be changed into oxygen, and life as we know it would be impossible. These happy coincidences are cited by those who argue in favor of the anthropic principle, although Hoyle himself has put forward a more extreme interpretation:

If you wanted to produce carbon and oxygen in roughly equal quantities by stellar nucleosynthesis, these are the two basic levels you would have to fix, and your fixing would have to be just about where these levels are actually found to be. . . . A commonsense interpretation of the facts suggests that a superintellect has monkeyed with physics . . . and that there are no blind forces worth speaking about in nature.

Given the unique ease with which carbon forms long, complex chains and rings, it is difficult (though not impossible) to imagine life with a different chemical basis such as silicon

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Atomic number	6
Density (water=1)	2.25 (graphite), 3.52 (diamond)
M.Pt.	3.550C
B.Pt.	4,289C