Alex McColgan

The leading theory was that the Sun's energy came from gravitational contraction.

Simply put, the idea proposed that as a star gradually radiates energy to space, it cools, and so collapses further under its own gravity, which in turn causes gravitational potential energy to be converted into heat in the star's core.

Today, we know that is a genuine process.

Not only is it involved in the formation of stars, it's also the reason that the gas giant Jupiter radiates more energy to space than it receives from the Sun.

In fact, the planet is shrinking by about 2cm every year under its own gravity.

And as the resultant internal heat works its way from deep in Jupiter's interior and out into space, it drives the intense storms that dance across the planet's surface.

In a 1920 paper, physicist Arthur Eddington wrote, If the contraction theory were proposed today as a novel hypothesis, I do not think it would stand the smallest chance of acceptance.

He argued that contraction would be hopelessly inadequate for powering a body that radiates as much energy as our sun.

Aston had shown that the mass of a helium nucleus was ever so slightly less than that of four hydrogen nuclei, so if a helium atom was just four hydrogens fused together, some mass was missing.

Eddington believed that this missing mass was converted into energy by Einstein's E equals mc squared.

And because c, the speed of light, is such a large number, the energy generated from even a tiny amount of mass is huge, meaning fusing hydrogen into helium would provide more than enough energy to power the sun.

Eddington's ideas were bold and unproven, but they soon inspired serious theoretical work, and in 1929, the first calculations of stellar nuclear fusion were published by Robert Descourt Atkinson and Fritz Hautemans.

It turned out that the Sun and all stars were giant fusion reactors, taking the most common element in the universe, hydrogen, and fusing it into other elements, starting with helium.

Now, replicating this process on Earth would be hugely advantageous, especially when compared to nuclear fission, the process that is used in nuclear power plants today.

Where fission relies on splitting large and rare unstable isotopes such as uranium-235 and plutonium-239, stellar nuclear fusion relies on hydrogen, an element that is widely available on Earth through the electrolysis of seawater.

And unlike the radioactive byproducts of fission,

that create a huge disposal problem for modern nuclear power stations, the byproduct of stellar fusion is helium, an extremely useful element.

Thanks to its exceptionally low boiling point, helium is used to cool the magnets in MRI machines for scientific research projects like CERN, as well as in the production of microchips.

It's an essential element that we actually need more of.

So how exactly do stars achieve this alchemy of fusing hydrogen into helium?