8.1.4Space Physics

Production of Elements — Oxford AQA International GCSE Combined Science - Physics Revision Notes & Topical Questions

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Part 1 of 4Stars as element factories

Source of energy

Source of energy Fusion reactions in the core of a star release the energy that makes it shine.

Source of elements

Source of elements The same reactions build heavier nuclei from lighter ones, so stars made all the naturally occurring elements.

The early Universe

The early Universe Before any stars existed there was no fusion, so the early Universe contained only hydrogen.
- Every heavier element had to be made later in stars.

1Main sequence

Main sequence Hydrogen nuclei fuse to form helium nuclei, releasing large amounts of energy.
- The huge supply of hydrogen lets the star shine steadily for millions of years.

2Hydrogen runs out

Hydrogen runs out When the hydrogen in the core is used up, fusion slows and the core contracts.
- The cooler outer layers swell and glow redder, so the star becomes a red supergiant.

3Heavier fusion

Heavier fusion Helium nuclei fuse to form carbon.
- Further fusion makes nuclei such as nitrogen and oxygen, and heavier elements up to iron.

4Core collapse

Core collapse When fusion can no longer continue, the core collapses suddenly.

5Supernova

Supernova The outer layers are blown away in a gigantic explosion called a supernova.

The sudden collapse of the core triggers an explosion that throws the outer layers of the star into space.

Temperatures inside the explosion are so high that nuclei can fuse to form elements heavier than iron.

All the elements heavier than iron are created in supernovae.

The explosion scatters these elements, along with those made earlier, out into space.

The material can later gather in new clouds of gas and dust, from which new stars and their planets can form.

Stage	Elements formed
Early Universe, before stars	Hydrogen only
Main sequence star	Helium, from the fusion of hydrogen
Red supergiant	Carbon, nitrogen, oxygen and heavier nuclei up to iron
Supernova	Elements heavier than iron, and dispersal of all the elements

Term	Meaning
Red supergiant	A very large, cool, red star formed when a massive star runs out of hydrogen in its core.
Supernova	The explosion of a massive star at the end of its life, which scatters elements into space.
Nuclear fusion	The joining of light nuclei to form a heavier nucleus, releasing energy.
Main sequence	The stable stage of a star's life in which hydrogen is fused to helium.
Naturally occurring element	An element found in nature, as opposed to one made artificially in a laboratory.

Saying that elements heavier than iron are made in the red supergiant stage. They form in the extreme conditions of a supernova.

Believing that the Big Bang made all the elements. The early Universe contained only hydrogen; fusion in stars made the rest.

Forgetting that the supernova also spreads the elements. Without the explosion the new elements would stay locked inside the star.

Describing the red supergiant as hotter than the main sequence star. Its surface expands and cools, which is why it looks redder.

Writing that stars run out of energy because the hydrogen has burned. It is used up in fusion reactions, not by burning.

Last updated: 20 October 2026