Fusion in Stars (AQA GCSE Physics): Revision Note

Fusion in Stars

• 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

The Formation of New Elements

• 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

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

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

• Elements heavier than iron are produced in supernova explosions

Supernovae

• A supernova is a bright and powerful explosion that happens at the end of a massive star's life

  • It occurs when the star is bigger than the Sun

• The explosion releases a large amount of energy

• During a supernova, all of the elements which were produced by the fusion reactions are expelled along with neutrons

• The neutrons combine with the elements to form even heavier elements

• These elements are ejected into the universe by the supernova explosion and form new planets and stars

  • Since Earth contains many heavy elements, including ones heavier than iron, this is proof that it must have formed from the remains of one or more supernovae

A supernova

The explosion of a massive star, known as a supernova, is responsible for distributing the elements throughout the universe