PHYSICS REVISIONTHE ORIGIN OF THE CHEMICAL ELEMENTS

STELLAR LIFE CYCLES

For most of the life of a star it is in a stable state in which the inward force of gravity is balanced by a force due to the increasing pressure towards the centre. The pressure force is caused by the rapid motion of the particles in the sun and by the pressure of the light hitting the particles (radiation pressure). If the pressure in the centre falls, the star will shrink causing the pressure to rise once more until a new equilibrium is established with the star smaller. If the pressure increases, the star will expand similarly.

STELLAR NUCLEAR FUSIONThe Big-Bang model suggests an initial composition for the universe of roughly 75% hydrogen and 25% helium with very small quantities of other light elements. Heavier elements come from their expulsion from a red supergiant’s supernova.

Most stars spend most of their lifetime as ‘Main-sequence stars’ that generate their

energy by the fusion of hydrogen to helium: 411H→

42He+2

01e (e is a positron, the

antiparticle of an electron, 0-1e, which, upon meeting, will annihilate each other and

release energy). This summarises the following: stage 1 – 2¿, stage 2 - 21H+11H→

32He+γ (the gamma ray becomes a light photon as it reaches the surface

of the star) and stage 3 - 32He+

32He→

42He+2

11H.

During nuclear fusion, some of the mass is converted to energy. To measure the mass of individual nuclei we use atomic mass units, u, where 1u=1.6605 X 10 -27kg. To calculate the energy released when this mass is converted to energy we use the formula E = mc2 (the speed of light is 3 X 108).

NUCLEOSYNTHESISNucleosynthesis is the word first used by Sir Fred Hoyle in 1946 to describe the process by which elements heavier than helium are produced in stars. Working fromexperimental results on the results of collision experiments between nuclei, and making assumptions about conditions in stars, including supernovae, Hoyle was able to show that heavy elements could be produced and able to explain why some elements are much more common than others, e.g. Carbon and Oxygen.

In post main-sequence stars such as – after the centre is exhausted of light elements to fuse causing pressure to drop making the nucleus shrink leading to density and temperature increasing – heavier elements can be fused. This is because heavier nuclei repel each other more strongly than light nuclei so require higher temperatures and pressures to fuse. A star will progressively produce heavier elements up to iron-56 (as heavier elements absorb the energy when they fuse and require fission instead) in shells around the nucleus as the lighter elements are used up in nucleosynthesis further out. This process is limited by the initial mass of the star, for example the sun will never be able to reach sufficient temperatures to fuse elements heavier than oxygen.

The amount of energy required to bind a nucleus together follows a definite pattern called a binding energy per nucleon curve. Generally, the binding energy per nucleon increases as lighter elements undergo nucleosynthesis via nuclear fusion - up to iron-56. Nuclei heavier than iron-56 have lower binding energies per nucleon and they tend to split up via nuclear fission.

STELLAR DEATHIn the final stages in the life cycle of solar-mass and giant stars nuclear fusion stops and the stars collapse. In solar-mass stars the pressure at the centre is not enough to hold up the outer layers. In doing so, its temperature rises (from converting GPE into heat) and so does its density. The remnant is a very dense white dwarf. The outer layers of the star are pushed off by radiation pressure forming a planetary nebula which ejects elements formed via nucleosynthesis into space.

The process of the core shrinking in giant stars is very sudden. The outer layers collapse onto the core, undergo very rapid fusion and the energy released blows the star apart in a supernova explosion.The elements produced, which are heavier than iron-56 up to uranium (and can only be produced from the energy released by the gravitational collapse and supernovae of massive stars), are ejected into space and form part of the nebulae from which new solar systems are formed. The core shrinks to become a neutron star – if the core is heavier than about 3.5 × the sun it becomes a black hole.