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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.

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.

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.


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!

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.

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.