2nd April 2011 Formation of Elements

Hydrogen, Helium, Lithium, Beryllium

The vast majority of hydrogen and helium are believed to have been formed in the Big Bang.

Protium, that is, ¹H is also formed  produced by spallation. Both helium-4 and protium  are formed by radioactive decay; protium by proton and neutron emission and helium as alpha particles. Helium is also formed in stars by the fusion of hydrogen.

Stars are clouds of hydrogen and helium which have collapsed under their own gravity.  At the core of the star, the atoms are under high temperature and pressure, and in these conditions can fuse together.

When our sun has run out, in 4-5 billion years, of hydrogen  to fuse into helium, it will collapse forming a white dwarf star, and then die.

The nuclei of these elements, along with some ⁷Li, and ⁷Be are believed to have been formed when the universe was between 100 and 300 seconds old. Because all this happened in a very short period of time, only the lightest of elements could form, that is, hydrogen, helium, lithium, beryllium and maybe boron.

The Elements up to & including Iron

The formation of elements other than isotopes of hydrogen and helium , and a small scattering of lithium, occurs in very massive stars.  When these stars run out of hydrogen at their core, they contract down which means their temperature increases and the helium can start to fuse together, gradually forming heavier and heavier atoms with successive collapses. This forms rings of different elements being compressed in to the core.

Image from Wikipaedia

The elements produced are those up to and including iron in the periodic table, that is, helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, neon, sodium, magnesium, aluminium, silicon, phosphorus, sulphur, chlorine, argon, potassium, calcium, tin, titanium, vanadium, chromium, manganese, iron.

Why does it stop at iron?

It's to do with something called binding energy.  This binding energy is calculated, using conservation of matter, from the mass deficit which is the amount of mass reduction when the atom forms compared with its component parts, namely (mass of individual protons and neutrons)- (mass of  atom).  The graph shows the average binding energy per nucleon (neutron or proton in the nucleus).

The Average Binding Energy per Nucleon

Because iron is at the peak of the graph, fusing iron atoms together doesn't release any additional energy, and furthermore it requires energy to occur. (Why can't two atoms of the left combine to jump past iron? Must be something like this: the gradients are such that (the binding energy per nucleon on left at x=a) > (binding energy per nucleon at x =2a) )

The Elements after Iron

Iron is the end of the process for energy producing fusion, since fusing iron atoms does not produce any energy for the star. Since the core is no longer producing outward thermal pressure, but gravity is still compressing things in, if the star has a mass of less than 5 times the mass of our sun, it collapses into a dense ball of neutrons.  This is called a neutron star and can be as little as 10km across. When the implosion is stopped by the neutrons, matter bounces off the iron core turning the implosion into an explosion and thus creating a type II supernova.  This explosion blasts the outer regions of the star into space.  It is thought that the heavier elements are formed in this explosion.

Note that if the mass of the core is greater than 5 times the mass of our sun, the neutrons can not stop the implosion and a black hole is formed.

Sources

1. Diamonds in the Sky - Universal Alchemy - a brief overview of the topic
2. Eureka: The Death of Stars - Balancing energy from fusion against gravity
3. Sxity Symbols: Element Creation - video explaning the formation of elements in the life of a star
4. Nuclear Binding Energy - explains the energy from fusion and why iron is significant
5. Supernovae - Types of supernovae, and what happens when they die
6. Type II supernova - the life of a star
7. Gravitational Energy
8. Nucleosynthesis