09/10/2026
Our next feature is ‘F’ for ‘Fusion’ ✨
It’s over 2,000 years since the first suggestion that stars might be objects like the Sun, rather than being gods, or oil lamps in the sky, but that still left the question: what exactly are they? One mystery was how the Sun could keep burning for so long without burning out, as a normal fire would; it must have been fire, to produce the heat that it did…?
The mystery was left unsolved until the early 20th century, when the secrets of atoms began to be uncovered by:
✨ Albert Einstein, who showed that mass and energy are two sides of the same coin, with an enormous amount of energy contained in a small amount of matter.
✨ Arthur Eddington, who proposed that stars were powered by nuclear fusion, releasing huge amounts of energy in the process.
✨ Cecilia Payne-Gaposchkin, who concluded in her doctoral thesis that stars were made mainly of hydrogen and helium.
The immense pressure and temperature in the core of a star triggers nuclear fusion, forcing protons (hydrogen nuclei) to overcome their mutual electrical repulsion, and merge with neutrons to form helium nuclei. There are several different processes, depending on the mass of the star, but even the simplest process has several steps, and involves conversion of some protons into neutrons, the creation of some sub-atomic particles and the release of energy in the form of gamma rays.
As most of the available hydrogen becomes converted to helium, energy production slows down and the core starts to shrink under the pull of gravity; this shrinkage increases the pressure and temperature in the core until the next stage of nuclear fusion can begin, converting helium to carbon. Many stars don’t go beyond this stage, but the more massive the star, the more the pressure and temperature can build at each stage, with several more stages of fusion possible until silicon is fused to form iron.
Each stage is generally much shorter than the preceding one, with the final stage lasting only days. Iron cannot be further fused to form heavier elements, so a star reaching that stage will typically collapse under the pull of gravity when the energy supply stops, to produce a supernova explosion.
Fun fact! Every second the Sun loses over 4 million tonnes of mass through the fusion processes.
📷: NASA - National Aeronautics and Space Administration
