The Conditions for Fusion (Edexcel IGCSE Physics (Modular))

Fusion in stars

• Stars are huge balls of (mostly) hydrogen gas

• In the centre of a star, hydrogen nuclei undergo nuclear fusion to form helium nuclei

• An equation for a possible fusion reaction is:

• Where (deuterium) and (tritium) are both isotopes of hydrogen

  • These are formed through other fusion reactions in the star

• Fusion reactions release a huge amount of energy

• The heat from fusion provides a pressure that prevents the star from collapsing under its own gravity

The outwards and inwards forces within a star are in equilibrium. The centre red circle represents the star's core and the orange circle represents the star's outer layers

• In larger stars where the temperature gets hot enough, helium nuclei can fuse into heavier elements

The conditions for fusion

• Nuclear fusion requires

  • extremely high temperatures

  • extremely high pressures

• These conditions are required because of the electrostatic repulsion between protons

  • Since protons have a positive charge, they repel each other

  • Therefore, to overcome this repulsion, protons must have very high kinetic energies to allow them to get close enough to fuse

Hydrogen nuclei are positively charged protons which repel one another, making it difficult to achieve fusion under normal conditions

• For hydrogen nuclei to travel at such speeds, the gas has to be heated to millions of degrees Celsius

  • Such high temperatures are usually only achievable in the cores of stars

• In regular conditions, such as on Earth, the possibility of collisions between nuclei which result in fusion is very low

  • To increase the number of collisions (and hence fusion reactions) that occur between nuclei, high densities (and hence pressures) are also needed