Number Eight in a series of 'coincidences' that indicate how fine-tuned the universe is to be a haven of intelligent life.
Quoting Barry Parker's INVISIBLE MATTER AND THE FATE OF THE UNIVERSE.
"Of particular importance in relation to this, however, is a discovery made a few years ago by Brandon Carter of Cambridge University. He was examining the effects of changes in the fundamental constants of the universe on the stars (their makeup and evolution) and found that if these constants were changed, even slightly, many types of stars would not exist. For example, if they were adjusted in one direction all stars would be small--mostly red dwarfs. But we know that the elements (with the exception of a few of the light ones) are produced in massive stars. Therefore planets such as ours could not exist, On the other hand, if they were adjusted the other way, only giant stars would exist--none as small as our sun. And we know that stars like our sun are the best candidates for lifesupporting systems. This means that life would be impossible in a universe that had fundamental constants that were very much different from the ones our universe has. THis seems to imply that life is tuned to the fundamental constant that it now has. If they were much different there would be no life--and we would not be here. This idea sums up what is known as the Anthropic Principle."
A word of explanation, in case you are thinking, "But how do we KNOW giant stars cannot be teeming with life?" The giant stars lead relatively short lives, and do not stay in the main sequence for the billions of years needed for evolution, but millions only.
A second quote, from a different source. THE MATTER MYTH, by John Gribbin and Paul Davies, concerning Carter and suns.
"Astrophysicist Brandon Carter has shown how the structure of stars is very delicately dependant on the exact ratio of gravitational to electromagnetic forces. Our Sun is a middling sized, yellow star, and the conditions that make life on Earth possible are closely dependent on the Sun's basic nature. If these forces were very slightly different in their relative strengths, however, all stars would be either blue giants or white dwarfs, depending on which way the balance was tilted. Stars like our Sun, which seem to be ideal at providing conditions suitable for the emergence of life, would not exist.
"These apparent 'coincidences' and many more like them, have convinced some scientists that the structure of the Universe we perceive is remarkably sensitive to even the most minute changes in the fundamental parameters of nature. It is as though the elaborate structure of the cosmos was the result of highly delicate fine-tuning. In particular, the existence of life, and hence intelligent observers, is especially sensitive to the high-precision 'adjustment' of our physical circumstances."
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