Fusion in Stars (AQA GCSE Physics)

• All the naturally occurring elements, apart from hydrogen, have been formed by nuclear fusion in stars
• Nuclear fusion occurs when two light nuclei collide at high speed and join to create a larger, heavier nucleus
• When the Universe was first formed, 13.8 billion years ago, the only element present was hydrogen
• If two hydrogen nuclei collide with enough energy they will fuse into a helium nucleus
  • For example, the nuclei of two different isotopes of hydrogen (protium and tritium) can join to form a helium nucleus by the process of nuclear fusion

An example of the fusion of protium and tritium to form helium

• The process of nuclear fusion releases energy
  • The energy is released in the form of heat and light

• During the majority of a star’s lifetime, hydrogen nuclei fuse together to form helium nuclei
• As the star runs out of hydrogen, other fusion reactions take place forming the nuclei of other elements
• For example, two helium nuclei (produced by the fusion of 2 hydrogen nuclei) could fuse together to form a beryllium nucleus

Two helium nuclei fusing together to form a beryllium nucleus

• The beryllium nucleus could then fuse with a helium nucleus to form a carbon nucleus

A beryllium nucleus fusing with a helium nucleus to form a carbon nucleus

• Elements lighter than iron are formed in fusion reactions like the ones above

Formation of Elements Heavier than Iron

• Elements heavier than iron are produced in supernova explosions
  • A supernova occurs at the end of a massive stars life
  • When the star explodes it releases very large amounts of energy and neutrons

• All of the elements which have been produced by the fusion reactions get thrown out and combine with the neutrons to form heavier elements