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 was one 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.
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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
stars, 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.
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. For example,
iron forms the major part of the Earth’s core. 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 oxides – 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 hydrogen and helium, which did not condense 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
1011 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
composition. 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.
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 – could 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
15 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 per cent 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.
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 nucleus 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 cannot 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 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. As explained in Inside Science
No 11, 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.
A tunnel through to heavier nuclei
THE PROTONS and neutrons inside nuclei are “glued” together by an active 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, therefore, 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.
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.
Further reading
The Origin of the Chemical Elements, by R. J. Taylor (Wykeham, 1975,
£8.00 – available from Taylor & Francis, London). The Elements: Their
Origin, Abundance and Distribution, by Tony Cox (Oxford University Press,
1989, £9.95) gives more details, and also discusses the elements on
Earth. The First Three Minutes, by Steven Weinberg (Fontana, 1983, £3.93)
remains the best account of the synthesis of elements in the big bang. The
Cambridge Encyclopedia of Astronomy (Cambridge University Press, 1984, approx.
£25) contains nicely illustrated articles on stars, including nuclear
reactions and supernovae.