The big bang resulted in the Universe and created the simplest elements.But heavier elements making up most of the Earth - and us - were created through the birth and death of generations of stars.
EVERYTHING that we see around us is made up of about 90 chemical elements. Their discovery and identification of the great achievements of chemistry in the 18th and 19th centuries. In the early part of this century, there came a better understanding of the atoms characteristic of each element. Negatively charged electrons appeared to "orbit" a positive nucleus, rather as the planets go round the Sun. The heavy atomic nucleus makes up nearly all the mass of an atom, although, relative to the size of the whole atom, it is very tiny. This nucleus is composed of positively charged protons together with some neutrons, which have no electric charge. The chemical nature of an element is controlled by the number of protons, ranging from one in hydrogen to 92 in uranium, and even more in elements that are not to be found naturally on Earth, but which chemists have succeeded in making in recent decades.
Where do the chemical elements come from? Chemical reactions rearrange atoms into different combinations, making and breaking chemical bonds. Only the outer electrons of an atom will take part in this process: the central nucleus is unaffected. Such reactions are not able, therefore, to turn one element into another element. That requires rearranging the particles making up the nucleus. This happens in radioactivity, when unstable nuclei split up,sometimes making lighter ones (see Inside Science, Number 17).
The nuclei of light atoms can also fuse together, making heavier atoms. This is a difficult process, because the two nuclei coming together need a large energy to get close enough to "stick" (see Box 1). Physicists use high-energy particle accelerators to study this type of nuclear reaction.
But atoms can also fuse at very high temperatures, when they move around quickly and randomly. Chemical reactions require some energy of this kind, and generally speed up at higher temperatures. But those reactions happen at temperatures of tens, hundreds, or at most a few thousand, degrees. For nuclei to react, very much hotter conditions are needed a minimum of 10 million degrees Celsius.
Temperatures such as these existed in the very early stages of the Universe, in the first few minutes after the big bang the fireball that started off the Universe some 15 billion years ago. Nuclear reactions also occur inside stats, and in fact nuclear reactions are the only possible source of the enormous amount of energy that keeps stars hot for billions of years. So the big bang at the beginning of the Universe and the interiors of stars provide the two environments where most elements have been made.
A tunnel through to heavier nuclei
|THE PROTONS and neutrons inside nuclei are "glued" together
by an attractive force called the strong interaction (see Inside
Science Number 17). An important feature of this force is that it operates
only over exceedingly short distances, around 10-13 centimetres,
about the size of nuclei themselves. There is another important force at
play: the electrostatic repulsion between positively charged protons. Inside
the nucleus, the strong interaction is sufficient to overcome this, and so
strong enough to ensure the stability of nuclei.
Suppose we try to force two nuclei together, to make a heavier element. This process happens quite easily, if we can get them close enough together for the strong interaction to operate. But the electrostatic repulsion acts over much longer distances. So before the nuclei can get very close, a very large repulsion operates.
This gives rise to an energy barrier, known as the Coulomb barrier and is shown in the diagram. In classical physics, two nuclei would have to have enough energy to surmount this barrier before the fusion reaction could take place. Quantum physics, however, introduces an important subtlety here. According to this theory, microscopic particles can pass through energy barriers which in classical physics are impenetrable. This process, known as tunnelling is very important in many processes of radioactive decay; it is also essential in the fusion reactions that make heavy elements.
|The amount of tunnelling depends on the energy
of the particles, and at ordinary temperatures it is negligible. High
temperatures, where atoms have large random velocities, are still required
for fusion to take place; but if it were not for tunnelling, these temperatures
would have to be much greater still, and the production of new elements would
be much harder.
The size of the Coulomb barrier increases with the charge on the approaching nuclei. To fuse heavier elements, there fore, requires higher temperatures. Neutrons, however, have no electric charge, and so they are able to approach nuclei without any repulsion. So, it is, therefore, much easier to make heavier nuclei by adding neutrons than it is by the normal fusion process.
Abundance of elements
Analysing the stars
THE elements on the Earth vary enormously in their abundance. Oxygen, silicon and iron are common; many elements, such as gold, are millions of times as rare. Most of the elements are important to us in various ways. Iron forms the major part of the Earth's core. For example, the surrounding layers of our planet, including the crust, are made of silicon and oxygen together with many other elements in smaller proportions. Nearly 30 elements are essential to life. These include carbon, oxygen, hydrogen and nitrogen, but also some quite rare ones such as selenium (see Inside Science No 20). In this century, industry has come to make use of nearly all the elements. The manufacture of a modern touch-dialling telephone, for example, will involve no less than 42 of them.
Many features of the Earth's chemical composition, however, are not at all typical of the Universe as a whole. We know that the two lightest elements, hydrogen and helium, make up more than 99 per cent of the visible Universe, with the others being present in very small proportions. On Earth, hydrogen, and especially helium, are much rarer because they are gases except at very low temperatures, and they largely escaped into space when the Earth formed. The common elements on Earth condensed into solids metallic iron, and silicon oxide-and so became concentrated in the dust particles that eventually collected together to make up our planet.
The abundance of elements in space is very important, not only because it influenced the ultimate composition of the Earth, but also because it can provide many clues to how the elements were originally formed. Clearly, a satisfactory theory about the origin of the elements should be able to account for the abundances that we observe.
How do we know these abundances? The 19th-century philosopher Auguste Comte thought that it would be impossible ever to know the composition of stars and other bodies in space. But even as he was writing, the evidence needed for this was becoming available. If you measure the spectrum of sunlight, by splitting the light into different colours, or wavelengths, with a prism or diffraction grating, you see many dark lines running across. Chemists see the same lines in spectra produced in the laboratory, when they put different elements in a flame. The lines are there because different elements absorb or emit light corresponding to characteristic, and extremely precise, wavelengths.
The wavelengths of the lines within the spectrum of the Sun reveal to us what elements are present in its outer layers, where atoms absorb some of the light coming from the interior. The strengths of these lines show us how much light is absorbed, and, therefore, the amount of each element that is present.
Astronomers have carried out this kind of analysis on the Sun and on many stars and galaxies, building up a picture of the abundance of elements in space. The information is supplemented by analysing meteorites, especially rare types called carbonaceous chondrites. These extraterrestrial rocks probably contain material, left over from the formation of the Solar System, that was not incorporated into the planets. The chemical composition of these meteorites matches that of the Sun very closely, except that they lack a few light elements such as, for example, hydrogen and helium, which did not con dense into solids.
The combination of data from the solar spectrum and the chemical analysis of meteorites allows us to know fairly precisely what the overall composition of the Solar System is. The abundances found are shown in the diagram on the facing page. Notice the scale and the enormous range of abundances: each scale mark in the abundance scale differs from the neighbouring one by a factor of 10. For every 1012 atoms of hydrogen there are about 10 of helium, fewer than 109 of the next commonest elements, carbon and oxygen, and fewer than 10 each of some rare elements such as uranium. The "zig-zag" curve shows that elements with even numbers of protons tend to be more common than those that have an odd number, and this is a reflection of the relative stability of nuclei: an even number of protons or neutrons gives extra stability.
The first few minutes
Elements one and two
ANALYSING other stars shows that the Solar System is fairly typical in its com position. There is, however, an important observation: very old stars, which started life as long as 10 billion years ago (the Sun is less than half this age), are made of hydrogen and helium, with relatively much less of the heavier elements. This suggests that the heavier elements were even rarer when these old stars were formed. In fact, theorists predict that only hydrogen and helium were made in the big bang. All the other elements must have been made since. Even today, they may be growing more abundant.
Jokers in the pack
|LITHIUM, beryllium and boron (elements with, respectively,
three, four and five protons) are comparatively rare. Their nuclei are not
very stable, and they are immediately consumed by nuclear reactions in stars.
A little lithium probably came from the big bang, but most of these light
elements are believed to have been made in a different way from the others
through collisions with cosmic rays. These rays are mostly nuclei
travelling through space at high speed. Their origin is still uncertain:
some may come from supernovae, or from other high-energy events in the Universe. Their energy is so large, however, that when they collide with other atoms in space, the nuclei can break into very much smaller fragments.
This process, known as spallation, is probably the origin of most lithium, beryllium and boron. Evidence for this comes from the atomic composition of the cosmic rays themselves: they do, indeed, contain these elements in very much higher relative proportions than does the Solar System, or even the Universe, as a whole.
According to the big bang theory, the Universe began as a "fireball" of extraordinarily dense and hot matter. In the early stages, it was so hot that not even atomic nuclei-let alone the molecules and solids familiar in everyday life would be stable. The chemical elements could not have been present 'in the beginning", but must have been made subsequently.
Some early speculations on the big bang theory suggested that all the known elements might have been produced very rapidly by putting light nuclei together in the early stages of the Universe. We now know that this was impossible, because the extremely rapid cooling and expansion of the Universe did not leave enough time. There was, however, some synthesis of elements in the first few minutes, and cosmological theory today can explain very nicely the apparent composition of the early Universe.
A few seconds from the beginning, the temperature was around 1010 °C. This is the maximum temperature at which atomic nuclei, other than simple protons, can exist. Protons were constantly changing into neutrons and vice versa, giving a ratio of about one neutron to every seven protons. Free neutrons are unstable, and under normal conditions last for only ii minutes on average, then decay to hydrogen atoms (protons and electrons). Before this decay process there was time for neutrons and protons to combine,forming deuterium, a heavy form of hydrogen. At the high temperatures that were then prevalent, the deuterium nuclei reacted rapidly with more protons, and the ultimate product was the stable nucleus of helium, containing two protons and two neutrons.
Under these conditions, the proportion of helium formed relative to hydrogen depends on how many neutrons are available at the temperature where nuclear reactions can begin. Physicists can calculate this quite precisely, and the theoretical value-about one helium atom to ten of hydrogen,or 23 to 25 per cent helium by mass-agrees very well with the proportions found in the Universe, especially in the older stars. A small amount of deuterium was also left unreacted; the predicted abundance of it also agrees well with what scientists have observed.
Any further nuclear fusion reactions, making heavier elements from helium, could not happen to an appreciable degree because the temperature was too low by the time helium was made. It seems, therefore, that 99 per cent of material in the Universe today owes its origin to the early stages. The agreement between theory and observation is impressive, and is one of the strongest pieces of evidence that ideas about the big bang are correct: no other theory of the origin of the Universe can explain the existence of hydrogen and helium in their observed proportions.
Cosmic cooking pots
The heavier elements
ALTHOUGH elements heavier than helium make up only 1 percent of the Universe, they are essential to us in many ways. The very existence of solid planets, such as the Earth, depends on elements such as iron, silicon and oxygen. We are made of highly complex molecules that contain carbon, nitrogen and many other elements.
A universe made of hydrogen and helium would be a very dull place in chemical terms. It is impossible to imagine how intelligent beings could arise to observe it or write about it. To make the heavier elements requires high temperatures sustained over a much longer period of time than was so after the big bang. But, such conditions do exist now-at the centre of stars. It is here that most of the remaining elements are made.
|The relative abundances of elements in the Solar System. Elements are shown with their atomic number (number of protons) and chemical symbol. The graph shows the numbers of atoms present for every 1012 hydrogen atoms. The vertical scale is a logarithmic one, each scale mark representing a whole factor of 10 different from the next one. The "zig-zag" curve shows that elements with an even number of protons are more abundant than those with an odd number|
A star begins when a large mass of gas contracts under its own gravity. Compression raises the temperature in the centre, to the point at which nuclei can start to fuse to form heavier nuclei. The output of energy from the nuclear fusion keeps stars hot, and prevents any further contraction, at least until the nuclear "fuel" has been used up. The first reaction to begin, at a temperature of about 10 million °C, is the fusion of hydrogen nuclei (protons) to form helium; this reaction occurs in a number of steps, in some of which half the protons are converted into neutrons. This is the so-called hydrogen burning phase of stars. It is not burning in the everyday sense of the word. Hydrogen burning does not produce new elements, but it is important because the energy produced keeps stars going for much of their lives.
The hydrogen burning phase leads to the build-up of a core of helium in the centre of the star. When hydrogen is exhausted, and the output of energy from the reaction declines, the centre of the star starts to contract again and becomes even hotter. As the core shrinks, the outer parts of the star expand. The star grows into a red giant.
What happens next depends on the star's mass. In the case of stars that have a relatively low mass, the core of helium simply becomes a compact object no larger than the Earth, known as a white dwarf, in which the helium nuclei are closely packed. The outer layers escape into space.
If a star is more massive than 0.4 Suns, the core becomes so hot (around 100 million °C) that the helium nuclei can react to form heavier nuclei. These fusion reactions require higher temperatures because the nuclei are more highly charged, and so need more energy to overcome their mutual electrostatic repulsion and fuse.
Two helium nuclei form beryllium (with four protons) but this is quite unstable and reacts quickly with further helium nuclei, to form first carbon and then oxygen. These two elements are the commonest in the Universe, after hydrogen and helium.
The relative amounts made depend on the temperature of the star, which in turn is controlled by its mass. But astronomers also know that some subtle features of nuclear physics are involved. In fact, it is something of an 'accident" that carbon does not react so quickly as to be effectively bypassed by this sequence. A world without carbon would be one without us!
As helium is consumed, a core of carbon and oxygen builds up. For a star with a mass between 0.4 and 8 times that of the Sun, this is the end of fusion reactions. The core becomes a white dwarf that is composed of carbon and oxygen.
In the most massive stars, the core gets so very hot that carbon and oxygen can in turn fuse together, forming elements as heavy as sulphur. Further reactions happen in stages, eventually producing iron (which has 26 protons) and a number of elements with similar masses, right at the centre. The reactions stop here, because iron has the most stable nucleus of all elements, and could not fuse under these conditions.
Around the iron core there are various layers in the star where the other reactions are still going on, so in cross-section the star tends to resemble an onion. As well as the reactions, that build hydrogen up to iron. other fusion processes are going on in these layers. These minor reactions can build up nuclei that are heavier than iron, in what astronomers call the s-process (meaning "slow"). The s-process occurs when some reactions produce neutrons, which are captured by other nuclei, so increasing their weight. Once a neutron has been captured, it may change into a proton. In this way, the s-process can increase the number of both protons and neutrons within a nucleus. It can produce elements up to bismuth (which has 83 protons).
|The "shell" structure of a heavy star, just before its explosion in a supernova. The main elements found in each shell are shown by their chemical symbols. The diagram indicates the relative mass of each part, but not its size: the inner shells are much denser, and occupy very much less space than indicated|
The death of a star
Elements of a supernova
THE life of a star reaches its final stage when the core of iron builds up in the centre. The iron nuclei cannot produce energy by fusion, but the gravitational force is remorseless: it continues to compress the core, raising the temperature to billions of degrees. Some of the elements formed in the core begin to disintegrate in this inferno, and the very centre of the star collapses suddenly into a dense mass of solid neutrons. The outer layers fall in, then "bounce back", spewing the contents of the star out into space in a supernova explosion.
The explosion itself creates more heavy elements, because it produces a flood of neutrons that are absorbed by existing nuclei. Unlike the s-process, where neutrons add on to nuclei one by one, there are now so many neutrons that several attach to a nucleus at once. This r-process (for "rapid") can make elements as heavy as uranium.
In a supernova explosion, the star becomes very much brighter, sometimes as brilliant as a billion Suns. Over the past 50 years, astronomers have found hundreds of supernovae in distant galaxies. These were so far away that they needed a telescope to be seen. When a supernova occurs in our Galaxy or a near neighbour galaxy, it is sometimes bright enough to be easily visible with the naked eye. We can find several supernovae in historical reports, including an observation by Chinese astronomers in AD 1054. The remains of this supernova now form the Crab Nebula, a cloud of hot gas still expanding outwards from the explosion. The spectrum of the expanding gas shows the presence of several elements made inside the star. The most recent supernova visible to the unaided eye was seen in 1987. The spectrum of its gases show many elements made in the explosion, including some that are radioactive and have gradually dwindled since 1987.
Some stars expel gas in more gentle ways, but supernovae provide the most important route for getting the elements out into space. Products from supernovae spread out, and eventually mix up with more gas. They then become incorporated into later generations of stars formed from the gas, eventually forming planets as well. Apart from direct observations on the remnants of old supernovae,the best evidence for the theory that the elements are produced in stars is that calculations confirm the observed abundances of elements. Such calculations are difficult and require the power of modern supercomputers. But the agreement is good. It appears from such calculations that almost all the material of the Solar System, apart from the hydrogen and helium remaining from the big bang, was produced by supernovae during the first few billion years of our Galaxy's existence.
© Reed Business Information 1999. Educational copying of Inside Science is permitted under licence of Copyright Licensing Agency Limited, 33 - 34 Alfred Place, London WC1 7DP
Back numbers of Inside Science are available from New Scientist, P.O. Box 666, London ElS 1DW. Inquiries 0181 5030589.
The Origin of the Chemical Elements, by R. J. Taylor
(Wykeham, 1975, £8.00- available from Taylor & Francis, London).
Tony Cox is a lecturer in inorganic chemistry at the University