These conditions of extremely high pressure and temperature can be found in the cores of stars. The pressure at the centre of the Sun, for example, is a staggering 100 billion times atmospheric pressure while the temperature is a whopping 15,000,000°C.

Look up to the sky and see... nuclear fusion in action © Getty Images

Under these conditions, the fusion of hydrogen into helium can easily be sustained. In a fusion reaction, the resultant nucleus (if it is lighter than iron) has a slightly smaller mass than the nuclei that combined to form it. That excess mass is released as energy, and it is that energy that powers the stars.

Fusion reactions are different to the ‘fission’ reactions which power nuclear power stations. There, heavy and unstable atoms are split apart to produce energy (and radioactive by-products too).

In contrast, fusion power could supply a clean, efficient and unlimited source of energy; it would require only water as a fuel (or lithium) and would produce only inert, non-toxic helium gas as a by-product.

One route for nuclear fusion is to use atoms of deuterium and tritium, both isotopes of hydrogen. They fuse under incredible heat and pressure, and the resulting products release energy as heat © Getty Images

The trouble is that initiating, containing, and sustaining nuclear fusion reactions offer significant engineering challenges. There are many research institutions working on the problem worldwide.

Recently, a laboratory in China achieved a fusion temperature of about 70,000,000°C for over 17 minutes – a fantastic achievement, but still some way off from being a commercial source of energy. Even so, scientists estimate that nuclear fusion energy will become commonplace by the second half of this century.