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FOR MORE than 30 years astronomers have believed that most of the elements
with a greater mass than helium come from nuclear reactions inside stars,
mainly through explosions of stars as supernovae. According to this picture,
every element except hydrogen and helium has been manufactured inside stars
and then scattered across the Universe in stellar explosions. The carbon
and oxygen that are essential to life as we know it, the steel sheet of
which our cars are made, the silicon in the chips inside our computers,
and the uranium and plutonium that power our nuclear reactors and bombs,
are all, literally, stardust.

But although the theoretical calculations hung together well, until
1987 there was no easy way to test this belief. No one had observed a supernova
exploding in our Galaxy since the invention of the telescope, and although
astronomers have seen many supernovae, these have been too far away to be
studied in detail. The explosion, nearly three years ago, of the supernova
SN1987A in the Large Magellanic Cloud (a close neighbour of our Galaxy)
changed the situation dramatically.

Not only is SN1987A near enough to be studied in detail, but astronomers
now have detectors that operate in all regions of the electromagnetic spectrum
from radio waves to gamma rays. Their observations – which are still continuing
– confirm that the energetic explosion of a supernova leads to the unstable,
radioactive isotope nickel-56. The decay of this isotope into cobalt-56
and then iron-56 accounts for almost all of the electromagnetic radiation
from the supernova in the first two years after the explosion.

This strongly confirms that established theories of how supernovae work
are accurate. SN1987A also provides important information about how different
layers of the star were mixed as a result of the explosion.

Astronomers have been studying the chemical composition of stars and
clouds of gas in space since the 19th century. They identify the different
elements present in a star by analysing its spectrum. Each element has a
set of characteristic lines. Geologists and geochemists have meanwhile investigated
the composition of accessible regions of the Earth and of meteorites, and,
more recently, samples of lunar rocks.

All these investigations show that the relative abundances of different
elements and different isotopes are very similar throughout the Solar System,
once we allow for the loss of volatile light elements from objects with
weak gravitational pull. But several important discoveries and developments
before the middle of the 20th century – natural radioactivity, artificial
nuclear transmutation, and the realisation that energy radiated by stars
comes from nuclear fusion – indicated that the chemical composition of the
Universe must be changing. So astronomers began to ask how the chemical
composition that we now see had come about.

At first, there did not seem to be much variety in the composition of
stars. We now know that this was because the first stars that were investigated
were all quite close to the Sun. The perceived similarity in composition,
coupled with the discovery at the end of the 1920s that the Universe is
expanding, led to the idea that nuclear fusion reactions during the ‘big
bang’, when the Universe was younger, hotter and more dense, had produced
all the elements we know today.

George Gamow, a Soviet-born physicist who spent mostof his life in the
US, developed the theory of the big bang inthe 1940s. But he was disappointed
to discover that the young,hot Universe would expand and cool so quickly
– according to the equations of general relativity – that nuclear reactions
would stop when the Universe contains only hydrogen and helium.

Fred Hoyle, then at the University of Cambridge, was at the same time
discussing the possibility that nuclear reactions inside stars could produce
all of the chemical elements from hydrogen. Astronomers realised that hydrogen
was being converted into helium (with the release of energy) in the Sun
and other stars during the main phase of their lives – so-called ‘main sequence’
stars. Once the hydrogen in the centre of a star was used up, could further
nuclear reactions occur to produce more massive elements?

Before such elements could be observable and become part of later generations
of stars (and planets), they would have to be expelled into space. The largest
type of stellar explosion, the explosion of a supernova, seemed the most
effective way of doing this. But at that time nobody knew how or why supernovae
exploded.

The theory of how stars produce elements requires that there are successive
generations of stars, each one more enriched with chemical elements than
its predecessors. So by the end of the 1940s, researchers anticipated that
the chemical composition of more distant stars would vary, with the oldest
stars closest in composition to the original composition – which, according
to Hoyle, is hydrogen.

With more powerful telescopes and detectors, this prediction was borne
out. Nuclear reactions in stars were important in determining the abundances
of elements, even if they were not the only factor. In 1957, Margaret and
Geoffrey Burbidge, Willy Fowler and Fred Hoyle collaborated to produce a
complete list of nuclear reactions that could occur in stars to provide
all but a few elements and isotopes from helium to transuranium elements.
Alastair Cameron came independently to much the same conclusions.

If all elements other than hydrogen were produced in stars, then the
oldest stars should contain very little helium, as well as little of the
heavier elements. But it gradually became clear that helium makes up about
25 per cent of any star. The rest is mostly hydrogen, and all the other
elements put together only make up about 1 or 2 per cent of the total. As
Hoyle and I pointed out in 1964, this would mean that the observed proportion
of hydrogen and helium closely matches Gamow’s calculations, taking into
account more recent developments in particle physics. So most of the observed
helium might indeed have been made out of hydrogen in the big bang, while
the other elements were produced from hydrogen and helium by nuclear reactions
in stars .

But what of the nuclear reactions that go on inside stars? Early on,
astronomers assumed that a succession of nuclear reactions could occur inside
a star and that eventually the star might explode, or blow matter away less
violently, returning products of the reactions to the gas clouds of interstellar
space. They thought that the reactions preceding the explosion were quasistatic
– in other words, the properties of the star would change slowly, so any
unstable (radioactive) nuclei would have time to decay before undergoing
another reaction. Later, astrophysicists realised that important nuclear
reactions could happen during a stellar explosion. Unstable nuclei might
react again before decaying, so these explosive nuclear reactions could
produce isotopes that cannot easily be made by quasistatic reactions. Including
explosive reactions in the calculations does seem to help explain the ratio
of isotopes of different elements on the Earth.

Astronomers think that there are two groups of reactions: nuclear fusion
reactions and neutron capture reactions.

Nuclear fusion reactions

1. Hydrogen burning: hydrogen->helium 2. Helium burning: helium->carbon
and oxygen 3. Carbon burning: carbon->neon, sodium, magnesium and others
4. Neon burning: neon->oxygen, sodium, magnesium and others 5. Oxygen burning:
oxygen->silicon, sulphur, phosphorus and others 6. Silicon burning: silicon
and others->manganese, chromium, iron, Cobalt and nickel (reactions need
successively higher temperatures going from 1-6) Neutron capture reactions
7. Slow neutron capture (s-process) 8. Rapid neutron capture (r-process)

As a star evolves, its central temperature rises, at least initially,
which can cause a succession of nuclear fusion reactions to occur, resulting
in elements of increasing mass. At each stage, when a particular kind of
nuclear fuel is exhausted, the central part of the star shrinks under its
own weight, and gets hotter as gravitational potential energy is released,
until the next fuel is ignited. If a star starts to cool down, fusion reactions
cease. Only the most massive stars produce all of the elements up to nickel
through quasistatic processes, although some stars of lower mass may produce
them explosively.

Fusion reactions cannot produce elements heavier than nickel because
that would require a net input, rather than a release, of energy. In this
sense, iron and nickel are the most stable nuclei. Neutron capture reactions
produce most of the more massive elements, as well as some less massive
isotopes. Light elements and isotopes are not produced by any of these processes:
deuterium, helium-3 and lithium-7 are produced cosmologically, in the big
bang; lithium-6, beryllium and boron are produced by the break-up of carbon,
nitrogen and oxygen isotopes in interstellar space. We still do not fully
understand how some rare isotopes with nuclei rich in protons are produced.
For a thorough explanation of where the elements come from we need to know
about four complicated astronomical processes: how galaxies and stars form,
how stars evolve and how they lose mass.

The way in which a galaxy forms provides the initial conditions for
star formation. Elliptical galaxies contain virtually no gas, and seem to
have had only one important period of star formation, long ago, whereas
stars are still forming in the spiral arms of a galaxy like our own Milky
Way. Star formation is not well understood, and we would like to know how
a star is formed. Observations of our Galaxy today may not be relevant to
what was going on when the Galaxy was young – the most important period
for the origin of the elements in our Solar System.

We know more about how stars evolve than how they form, at least as
far as the quasistatic stages of evolution are concerned. If we ignore the
loss of mass, we can calculate reasonably well how the chemical composition
of a star changes as it ages. But the crucial question is: when (if ever)
does a star lose mass, and what fraction of its mass is lost? In addition,
do explosive reactions modify the composition of the material that escapes?

The observations that will give us the answers involve the study of
material that has been processed by nuclear reactions and then carried to
the surface of the star or expelled into space. For example, theory tells
us that one of the reaction chains that converts hydrogen into helium, the
carbon-nitrogen (C-N) cycle, changes most of the carbon present into the
most abundant isotope of nitrogen, nitrogen-14. At the same time, it increases
the amount of carbon-13 relative to carbon-12. Element and isotope abundances
in line with the workings of the C-N cycle have been observed in the surfaces
of red giant stars – in which theory also says that material from the star’s
interior should have found its way to the surface.

Another observational test studies clouds of gas knownas planetary nebulae.
These are produced by relativelymild stellar explosions. Spectroscopy shows
they containthe ‘right’ element abundances, reflecting the kind ofnucleosynthesis
thought to go on inside stars.

The most important source of such material is probably the explosion
of a supernova. Supernovae are fairly rare events; none has been observed
in our Galaxy (which contains about a hundred billion stars) for almost
400 years. Although remnants of old supernovae have been identified and
studied, they cannot easily be used to test predictions about the origin
of the elements. This is because the expanding cloud of matter expelled
by a supernova sweeps up and mixes with interstellar matter. Hundreds or
thousands of years after the explosion, the newly processed material is
so inextricably mixed with older material that there is no way of extracting
unambiguous information about what was produced in the supernova itself.

This is why the explosion of SN1987A was so important. It gave us the
chance to observe the expelled matter before it had been diluted by interstellar
gas. As we could make observations in all parts of the electromagnetic spectrum,
we could also follow the decline in the total energy output of the supernova
during the months after the explosion.

SN1987A resulted from the explosion of a massive star. It is known as
a Type II supernova, although it is not entirely typical of its class. Shortly
before the explosion its interior structure might have looked like the structure
in Figure 2. Because a star is hottest in its centre, the various fusion
products form shells around the completely ‘burnt’ core. All of the nuclear
fusion reactions will have stopped, so the centre of the star is made of
iron group elements. In addition, thes-process has probably taken place
in regions where helium has burnt into carbon and oxygen, and possibly in
other parts of the star.

When the central regions of the star are burnt out, they can no longer
provide the star’s radiation by energy released in nuclear fusion, and contract,
releasing gravitational energy. Energy continues to flow outwards as the
core collapses, ultimately to form a neutron star. Matter that is still
falling inwards then bounces from the surface of the neutron star, sending
pressure waves outwards and expelling the material in the outer layers of
the star. During this implosion/explosion, the layers surrounding the core
are rapidly heated, leading to explosive fusion reactions, and probably
the r-process.

One important prediction of this model is that elements of the iron
group should be produced explosively. After hydrogen has been burnt to helium,
most of the nuclei produced by quasistatic reactions have equal numbers
of neutrons and protons (for example, oxygen-16 and carbon-12). If such
nuclei are processed explosively then, according to the theory, they are
converted into nuclei of nickel-56, each with 28 neutrons and 28 protons.
This nucleus is unstable, and turns into cobalt-56 with a half-life of about
six days; the cobalt-56 itself then decays into iron-56 with a half-life
of 77 days.

The decay of cobalt-56 to iron-56 should provide almost all of the energy
that the supernova can radiate during the first few hundred days after the
explosion. And this would explain the characteristic decay of its total
output of light.

Before the explosion, the light reaching us comes from the outermost
layers of a massive star. As material flows away from the star, after the
explosion, we should be able to see deeper into the star. We would see successive
layers of matter that have undergone quasistatic nuclear burning, and then
light from those regions within the star that have been processed by explosive
reactions.

The observations of SN1987A provide convincing evidence that nickel-56
was produced explosively. The total luminosity has decayed in step with
the half-life of cobalt-56. The characteristic lines of cobalt and iron
in the spectrum of the supernova indicate that a total mass of nickel-56
equivalent to 7 or 8 per cent of the mass of our Sun had been produced.

When cobalt decays it produces gamma-rays, which at first cannot escape
directly from the dense inner region of the supernova and are transformed
on their way out into electromagnetic radiation at lower energy (longer
wavelengths). Early in its life, most of the energy we receive from the
supernova is, therefore, in the form of ultraviolet radiation, light and
infrared. Eventually, the material surrounding the supernova becomes transparent,
and gamma-rays can escape. Astronomers have now observed X-rays and gamma-rays
from SN1987A, and when their energy is added to that observed at all other
wavelengths the total is still, more than two years after the explosion,
exactly in line with the prediction that the decay of cobalt-56 powers the
supernova.

But the supernova also came up with a few surprises. We observed both
iron and cobalt, and X-rays and gamma-rays, from SN1987A much earlier than
expected for a smooth, spherically symmetrical explosion. We could see deeper
into some regions of the star than we thought we would. One reason for this
would be deviation of the supernova from spherical symmetry. Some studies
of the shape of the supernova remnant, using a technique known as speckle
interferometry, have shown that it is not spherical. But this is not enough
to explain the observations. There must also have been some instability
in the ejected material that allowed fingers of cobalt to get closer to
the surface. Another surprise was that the radiation did not show an overall
doppler effect. Because the ejected material is moving outwards from the
star and towards us, we expected to see the gamma-rays blueshifted, at shorter
wavelengths than gamma-rays emitted when cobalt decays in the laboratory.
But some were redshifted and some blueshifted. The material of the remnant
must be patchily transparent, so that in some places we can see through
the shell of expanding material to the other side, which is moving away
from us.

The observations of cobalt decay are probably the most important and
exciting ones for the theory of the origin of the elements, as they confirm
that it is broadly correct. But they are not the only relevant ones. There
have been early observations of barium, strontium and scandium, which are
all produced by the s-process. These give insights into the way this process
must have occurred earlier in the evolution of the star. There is evidence
of enrichment of helium and nitrogen in the outermost layers of ejected
material, which tells us more about the mixing of C-N cycle material to
the surface. And astronomers have confirmed that most of the energy of the
explosion is emitted by the core as neutrinos. This has led to the imaginative
(and as yet unconfirmed) idea that neutrinos interact with neon, producing
an isotope which subsequently decays into fluorine.

Astronomers have learnt and are continuing to learn a great deal from
SN1987A. How much more will they learn if a supernova is observed in our
own Galaxy in the near future?

* * *

Cosmological element production

THE STANDARD model of the early Universe is a hot, dense, rapidly expanding
fireball – the big bang. If we wind back (in our imagination) the observed
expansion of the Universe today, we arrive at a ‘moment of creation’ when
density was infinite, some 15 billion years ago. Leaving aside exactly what
this infinity means, cosmologists can use information from particle physics
and general relativity to describe the evolution of the Universe from a
few seconds after this moment of creation.

At an ‘age’ of about 25 seconds, the temperature is about two billion
degrees(2 x 10**9 K) and the energy density roughly two tonnes per litre.
The fireball consists essentially of neutrinos and photons, with just a
trace of protons, neutrons and electron-positron pairs. The density of matter
is ‘only’ 10 grams per litre – 10 times the density of the air we breathe.
At this stage, protons cannot link up electromagnetically with electrons
to form hydrogen atoms, because atoms would be broken apart by energetic
radiation. They cannot even, for the same reason, combine with neutrons
to form deuterium nuclei.

But when the Universe is about a minute old, it has expanded and cooled
sufficiently for deuterium nuclei to hold together. This triggers a chain
of nuclear reactions that converts almost all of the deuterium into helium,
and produces very small quantities of a few other very light elements. With
no more free neutrons available, no more deuterium (and therefore no more
helium) is produced. A few hundred thousand years later, the Universe is
so cold (about at the temperature of the surface of our Sun) that naked
protons and helium nuclei link up with electrons to form atoms.

How much primordial material is converted into helium depends on how
rapidly the Universe expands in its early stages.

This in turn depends on the number of varieties of elementary particle
present, and the way they interact. Calculations of how much helium should
be produced have been refined since Gamow’s day for two main reasons. First,
we now know that there are three species of neutrino, not one. Secondly,
the half-life of the neutron was thought to be about 14 minutes when Gamow
did his calculations in the 1940s; it is now known to be about 10.2 minutes.

The standard model tells us that about 23 per cent of the matter in
the early Universe was processed into helium. It is very difficult to destroy
helium in large quantities inside stars, but some helium is produced there,
so theory predicts that at least 23 per cent, by mass, of any star should
still be helium today. This is exactly what we see in stars across our Galaxy,
and in other galaxies – a striking vindication of the standard modelof the
big bang.

* * *

The r- and s- processes at work

SOME nuclear reactions release neutrons. For example, when deuterium
and tritium fuse to produce helium-4, one neutron is released. Similar reactions
inside stars provide neutrons which may interact with other nuclei. Adding
a single neutron to a nucleus increases its mass by one unit, but does not
change its chemical properties – it is a different isotope of the same element.
In many cases, however, the newly formed isotope is unstable, and given
time it will eject an electron (beta-decay), converting one of its neutrons
into a proton and becoming a different element. Then, the whole process
can repeat, gradually building up heavier elements. This is the slow (s-)
process of neutron capture.

When large numbers of neutrons are available (for example, as a result
of explosive reactions occurring during the early stages of a supernova),
a nucleus may capture several neutrons before it has time to decay by emitting
an electron. The new nucleus formed by the sudden addition of several neutrons
will also decay to a stable isotope of some element, but in general the
isotopes produced by this rapid (r-) process of neutron capture are different
from the ones produced by the s-process.

In a diagram of the elements which plots neutron number against proton
number, stable isotopes lie on a zig-zag band known as the valley of stability.
Unstable isotopes produced by the r-process lie to the right, and their
decay products ‘rain down’ towards the valley, eventually producing stable
isotopes.

Roger Tayler is professor of astronomy at the University of Sussex,
and author of The Origin of the Chemical Elements, Wykeham, 1975