first published on Shepherd on Climate on Friday 1st July 2011.
The first stage in the life of a newly formed star begins when its temperature has risen to about 10 million degrees. This is the hydrogen-burning stage of the star’s life cycle, when hydrogen nuclei fuse to form helium. In a middle-aged star like our own Sun about 600 billion kilograms of hydrogen are processed in this way each second.
When about 10 percent of the hydrogen in a star has been consumed, a further contraction takes place, and the central region of the star rises to over 100 million degrees. At the same time, the outer regions are pushed away from the central region by the activity within, and the inflated star becomes a Red Giant.
Now helium burning begins in the dense hot core, and helium nuclei fuse into beryllium, carbon and oxygen.
This phase continues until the helium in the core is exhausted and carbon and oxygen are present in approximately equal proportions. At this stage, the basic building blocks of life are present.
At the end of the helium-burning phase the inner regions of the core contract and the temperature rises. If the star has at least four times the mass of our sun the temperature can rise to a billion degrees and carbon burning and oxygen burning begin, resulting in the formation of heavier elements like sodium, magnesium, silicon and sulphur.
When the stellar core has been depleted of carbon and oxygen and is rich in silicon, the silicon-burning phase begins and silicon is converted to sulphur, argon, and other heavier elements. If contraction can raise the temperature of the interior to about 3 billion degrees, then the equilibrium phase of the star’s life cycle begins, and elements close to iron are formed. Iron is the most stable nucleus of all. If a star were to burn to its end, it would become a ball of iron.
Meanwhile in the outer regions of the star, nuclei may be subjected to intense streams of neutrons, which have been generated by the nuclear processes in the core. These neutrons are captured by the nuclei in collisions, and several additional neutrons may accumulate in a given nucleus.
At some stage of neutron accumulation, the nucleus becomes unstable and spits out an electron after a neutron collapses into a proton forming a heavier element. This process can extend beyond iron to uranium and beyond.
Stars do not burn smoothly from their inception to their death. As a star consumes atomic nuclei, it becomes depleted of fuel. At that stage, the outer regions of the star may collapse, like a falling roof, and plunge into the hot abyss below. The falling matter crashes onto the star’s dense core and bounces back; in this way, a star may shrug off its outer regions, scattering them through space.
The parent star may continue to burn, and may even explode again, but its precious ash has been scattered through space. Just how precious we will soon see. Now space no longer contains only tenuous clouds of primordial hydrogen and helium. There is an important contamination. The first great pollution has taken place with the potential for landscape, life, technology, and power now existing outside the stars.
Other stars may form from gases already enriched with many of the elements in the Periodic Table of Elements. These condensed, enriched clouds of gas ignite, burn in their nuclear way, and forge a richer soup. In time, it is their turn to die, in the explosive way typical of their kind. When the starquake comes, the splash and countersplash spread even more of the elements through the cosmos.
The newly liberated nuclei scattered through the cosmos have a complex composition that reflects the various modes by which they have been made. Stars of different mass burn in different ways: some never reach equilibrium; some never even reach the helium-burning stage; some burn rapidly, others slowly.
However, one common feature is that the mechanism of nucleosynthesis leads to very little lithium, beryllium, and boron, for these light elements are either bypassed in schemes of synthesis or are consumed almost as soon as they have been made.
During the nuclei's journey through the quiet of interstellar space it is bombarded with cosmic rays and passes through streams of rapidly moving particles. Collisions take place and chips are struck off heavier nuclei, a process called spalliation. These chips include nuclei of lithium, beryllium and boron.
Every element in the Periodic Table found on Earth was once forged in distant, long-extinguished stars and then scattered into space upon its death. After their birth, the elements had a gentler history, drifting through space, deflected by collisions and swept along by moving clouds of gas. Some of the atoms collect in clouds.
Four or five billion years ago…ten billion years after the universe began with a Big Bang…one particular cloud of hydrogen and helium…enriched by heavier elements...condensed as gravity drew the particles together. Eventually this cloud burst into nuclear activity, releasing energy as hydrogen fused into helium.
But some of the star stuff managed to keep clear of this nuclear fission reaction and encircled the incandescent star, coalescing together as grains, then rocks, then boulders and finally great spheres speeding around a sun…Our Sun. One of these spheres in due course becomes recognisable as the molten planet Earth.
After hydrogen and helium, the most abundant elements on Earth are carbon, nitrogen and oxygen. Beryllium with 4 protons and 5 neutrons, and boron, with 5 protons and 5 or 6 neutrons, only just managed to survive the tumult of the stars. Much of the lithium, beryllium and boron on Earth are fragments chipped from larger nuclei. It is as well for us that beryllium and boron did survive, because they are the doorway through which nucleosynthesis proceeds, carbon is formed and eventually minds evolved to appreciate what had been created over the billenia.
Although the formation of the Earth was a much gentler affair than the formation of the elements in the stars, it was still vigorous by current standards. The primitive Earth was molten throughout, as its insides still are today, and the heat drove off many volatile compounds.
Any elemental hydrogen trapped in bubbles returned to space...as did the chemically inert helium, being unable to anchor itself to other elements. Some hydrogen managed to bond and form non-volatile compounds but the only helium that remains on Earth comes from the radiaoactive decay of heavy elements such as radium and uranium.
Some elements were able to anchor themselves by forming compounds. In this category were silicon, aluminium, iron and nickel. Some elements formed compounds with sulphur but because of their volatility these were ejected from the boiling planet.
There is in fact something rather special about a nucleus of carbon which enables it to form quite rapidly. Resonance allows one pendulum couple of carbon to be strongly attracted to another pendulum couple of the same frequency. This, in turn, leads to the coupling between a nucleus and a proton of the right energy. Resonance also opens the door to the nucleosynthesis of other elements. The unlikely miracle of resonance is responsible for carbon being the third most abundant element in the universe. Without resonance there would be no life.
Keynes famously noted that in the long run we were all dead. He was talking of people. As far as planets are concerned he might have said that in the long run they are dead, flat space-time. But that's the bad news.
The good news is that this will not be for a while. Multiply 10 by itself a hundred times and you reach the halfway mark for our little corner of the cosmos. By then all our matter will have decayed into radiation. Meanwhile we have one or two more urgent concerns like what our planet has been up to recently...like in the past 11,000 years.
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