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UNIVERSITY
OF BRISTOL SCHOOL OF CHEMISTRY
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Stellar Alchemy
Of all the alchemists known, the greatest
are not human.
It is, in fact the stars which have mastered
elemental transmutations - capable of producing all the elements
in the universe from hydrogen.
It could be said that the stars have mastered
immortality, living as they do for millions of years - an
eternity for a human.
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Nuclear Fusion
When stars are born they are composed
of mostly hydrogen, (with a little helium).
Gravity compresses the core where,
at high temperature and pressure, nuclear fusion occurs.
Initially, two protons fuse, one of which decays to
give a deuteron (heavy hydrogen nuclei). Another proton
then fuses with this deuteron, and two of the products
fuse to give a helium-4 nucleus (ie. an alpha particle),
releasing two protons and energy.
When the hydrogen is used up, hydrogen
fusion stops, the radiative pressure from the core ceases
and the star contracts due to gravity. In stars having
less than 0.5 solar masses, this collapse has no further
effect and the star simply cools off.
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In stars with between 0.5 and 8 solar
masses, the gravitational contraction is strong enough
to heat the core to 100 million Kelvin. At this temperature,
the star is able to fuse two helium nuclei into beryllium,
and then this beryllium nuclei with another helium nucleus
to form carbon. This process is know as the triple alpha
process.
The gravitational contraction which
follows the end of the triple alpha process is not strong
enough to cause carbon fusion. The star sheds its outer
helium and hydrogen shells, exposing a bare carbon core
known as
a white dwarf, surrounded by a planetary nebula.
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Supernovae
A star which has more than 8 solar
masses is able to fuse carbon. The gravitational contraction
following the end of the triple alpha process heats
the core to 1 billion Kelvin. Temperature progressively
increases resulting in the formation of N, O, Na, Ne,
S and Si. At 6 billion Kelvin, the core can now fuse
silicon to iron.
The star develops an onion like structure,
with concentric shells of previously fused elements.
As iron is at the top of the binding
energy curve, the fusion of iron takes energy rather
than produces it. For the star this is catastrophic,
as it upsets the
balance between the contracting gravitational force
and the expansive radiative pressure!
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Gravity contracts the core further,
which does not resist as the radiative pressure is no
longer present. Gravity forces electrons into iron nuclei,
producing neutrons and neutrinos. Without the Coulombic
repulsions between nuclei present, gravity causes material
to rush to the core at up to quarter the speed of light.
It is at this point that a repulsive
force called neutron degeneracy pressure comes into
effect causing the core to rebound violently against
the inward rushing shells. This is the supernova.
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The force of the core and the huge
amounts of neutrinos on the dense outer shells is great
enough to cause fusion within them. It is here that
all the elements above and including iron are made by
the many different fusion pathways available, and the
subsequent decay pathways of radioactive elements, which
are also formed.
The following does not produce new
elements but is included for the sake of completeness.
The core now contracts inwards again.
With a mass of between 1.4 and 3 solar masses, the neutron
degeneracy pressure halts the contraction, and the core
becomes a neutron star. However if the core has a mass
greater than 3 solar masses nothing can stop the collapse.
The core shrinks to become a singularity - a point of
almost infinite density, the gravity of which not even
light can escape.
The result - a black hole.
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