What is the source of energy in the Sun and other stars?

Nuclear fusion is the process in which two light atomic nuclei combine (fuse) to form a heavier nucleus, releasing a huge amount of energy. The energy comes from the fact that the mass of the new nucleus is slightly less than the total mass of the original nuclei. This "missing" mass (mass defect) is converted into energy according to Einstein's equation E = mc², where c is the speed of light.

In the core of the Sun, the temperature is about 15 million °C and the pressure is extremely high. Under these conditions, hydrogen nuclei (protons) move fast enough to overcome their electrical repulsion and fuse. Through a series of reactions called the proton-proton chain, four hydrogen nuclei are converted into one helium nucleus, releasing energy in the form of gamma rays and particles. Every second, the Sun converts about 600 million tonnes of hydrogen into helium, losing about 4 million tonnes of mass as energy. In heavier stars, a process called the CNO cycle also produces energy. This explanation was given by Hans Bethe in 1938-39, for which he received the Nobel Prize in Physics in 1967.

Because fusion needs very high temperatures (millions of degrees), it is also called a thermonuclear reaction. The hydrogen bomb is based on uncontrolled fusion.

Scientists are trying to achieve controlled fusion on Earth as a clean source of energy, because it uses abundant fuel (isotopes of hydrogen such as deuterium and tritium) and produces little long-lived radioactive waste. Devices called tokamaks use powerful magnetic fields to confine extremely hot plasma. The International Thermonuclear Experimental Reactor (ITER), being built in France, is the largest such project; India is one of its seven members. In December 2022, the US National Ignition Facility achieved a net energy gain from fusion for the first time, using lasers.

Why not the other options?

• ✗Nuclear fission, the splitting of heavy nuclei such as uranium-235, powers nuclear reactors and atom bombs.
• ✗Chemical combustion could not keep the Sun shining for billions of years.
• ✗Radioactive decay is a minor heat source inside the Earth, not the Sun's main source.