Continuing in a series of coincidences in nature lumped under the term "anthropic principle". Quoting from Gribbin and Rees' COSMIC COINCIDENCES.
THE STELLAR PRESSURE COOKER
Making carbon, and heavier elements, inside stars solves only half the problem of how carbon-based life forms come to be here on Earth, puzzling over their origins. How do the heavy elements get our of the stars and spread across the Galaxy to become part of the clouds of material from which new stars and planets form? The simple answer is that the heavy elements are spread when a minority of stars explode as supernovae. But what makes a supernova blow its top? It turns out this spreading of the stuff of life across the cosmos also hinges on a close cosmic coincidence.
"Because of the failed resonance at Oxygen-16, life for a very massive star is both complicated and, ultimately, disastrious. All stars start their lives by 'burning' hydrogen nuclei, converting them into helium and releasing heat inthe process. When hydrogen is exhausted, helium in its turn can be burned to produce carbon-- at this stage in its life, a star like our Sun swells up to become a red giant. As long as helium is being converted into carbon, with a net release of energy for every carbon nucleus formed, the star can stay hot enough in the centre to support the weight of its outer layers. But eventually, after many millions of years, the helium is exhausted. What happens next depends upon the mass of the star. By far the majority of the stars run through some further nuclear reactions, in a last-ditch attempt to maintain their former glory, then collapse and cool down, huddling in upon themselves to form a ball of dead star material, a white-dwarf star that may have the mass of our Sun but occupies a volume no bigger than the Earth. (much deleted, believe it or not.)
THE SUPERNOVA CONNECTION
For the sake of argument, we will carry on describing what happens to a star with a mass of about twentyfive suns after silicon burning is complete it is left with a ball of iron, about as massive as our Sun, in its centre. Only the details are different for stars with different masses. The star now has no effective means of support, since there is no more energy being produced by nuclear burning in its core. The result is dramatic. The inner regions of the star are squeezed iinwards, and the pressure on the iron nuclei in the core become so great that electrons and protons are forced to merge into one another, forming neutrons. A ball of neutrons can indeed pack matter together more compactly than a ball of iron nuclei, andthe centre of the star begins to convert into a neutron star, still with as much mass as our Sun but now occupying only as much space as Mount Everest. It becomes, in effect, a single 'atomic' nucleus. Material from the inner part of the surrounding star has the floor pulled from underneath it, and plummets down onto the newly forming neutron star, reaching speeds as great as 15 percent of the speed of the light. When this fast-moving material hits the neutron star, from all sides at once, the shock actually squeezes the ball of neutron material like a golf ball being squeezed in an iron grip. But neutron stuff is very difficult to compress-- it is like trying to squeeze the nucleus of an atom--and quickly bounces back from this compression. Enormous pressures and temperatures are caused by this bounce, which turns the shock waves inside out and sends it speeding back through the giant star.
"Everything has happened in less than half a second. As the shock begins to move outwards through the star, which may have a diameter of 700 million kilometers, as big as the orbit of Jupiter, it encounters resistance and begins to slow down. It is, after all, trying to move bodily about twenty-four masses of solar material. Without help, it would fizzle out. But it is followed by a flood of neutrinos produced in the neutron core of the star when it was squeezed by the infalling matter. The matter in the slowing shock wave is so dense that it actually absorbs a significant number of neutrinos. The energy from the neutrinos gives the shock wave the boost it needs to finish the job of blowing apart the outer layers of the star.
"In all this energetic activity, quantities of elements heavier than iron have been formed, and many complex nuclear interactions have produced a variety of other elements from the basic products of nuclear burning. A supernova shines, for a few weeks, as brightly as a whole galaxy of normal stars put together, and the energy that makes it so bright comes from radioactivity, from unstable elements heavier than iron that were put together by the shock wave and are now breaking apart, releasing energy and forming more stable, tightly packed nuclei. From the site of this intergalactic beacon, much more than twenty times the mass of our Sun, in our specific example, is expelled, completely into space, and carries with it this heavy- element legacy from the dying star. The core, at last free from the bothersome pressure of the rest of the star, settles down as a spinning neutron star, perhaps to be detected by some civlization as a pulsar. The organic beings that study such pulsars, and the steel girders of which their radio telescopes are manufactured(not to mention the silicon in the chips of their computers) are equally the products of supernovae explosions in aeons gone by.
"The story is fascinating in its own right, but where is the anthropic coincidence? It lies in that burst of neutrinos, the crucial step in helping the shock wave to blow the star apart. Computer calculations in the 1980s had shown that the shock wave alone simply could not do the job, and that neutrinos must be involved. But some researchers were skeptical, because the properties of neutrinos had to be precisely "fine-tuned" to do the job. It all hinges upon the strength of the weak interaction, one of the four fundamental forces of nature. This is the force that determines how strongly neutrinos interact with baryons. If the weak interaction were a little too weak, then even the dense shock wave would be transparent to neutrinos, and they would flood out throughthe star without getting involved in pushing apart the outer layers of the star. If, on the other hand, the weak interaction were a little too strong, then the neutrinos would get involved in reacions in the core itself, and owuld never get out to the region where the shock wave was slowing and giving up the ghost. The weak intereaction has to be just right to allow neutrinos both to escape from the core and to interact with the shock wave.
"Some doubts about this scenario for the explosion mechanism were allayed by studies of the burst of neurinos from supernova 1987A. The energy of these neutrinos, and the implication, from the arrival of a handful in our detectors, of how many escaped from the core of the supernova, match the requirements of the models. Studies of the supernova match very well with the computer calculations, supporting the view that neutrinos are indeed the driving force in expelling large quantities of gas enriched with heavy elements into space--a phenomenon without which no planets like Earth or creatures like us would exist."
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