Nuclear fusion

Stars make light and heat.

They do this by joining tiny bits together. This is called fusion.

When tiny bits join, they make power. This power comes from a tiny bit of mass. That mass turns into energy.

It takes a lot of heat to make this happen. It also needs a lot of pressure. This is how the sun works.

Scientists use big machines to try this. They want to make clean power. This could help our world. It is a very big goal.

Stars are bright and hot because of nuclear fusion.

This is a way where tiny parts called nuclei join together. When they join, they make a larger nucleus. This process lets out a lot of power.

Fusion happens because of two forces. One force pulls things together. The other force pushes things apart. In small nuclei, the pulling force wins. This makes the nuclei stick together. This change turns a little bit of mass into energy.

Stars like our Sun do this every second. The Sun fuses 620 million metric tons of hydrogen each second. It makes 616 million metric tons of helium. The rest of the mass becomes energy.

Scientists want to use fusion for power on Earth. They use big machines called tokamaks. These machines use magnets to hold the hot parts. In 2022, scientists in the U.S. reached a big goal. They made more energy than they used to start the reaction. This is called break-even. It helps us learn how to make clean power.

Nuclear fusion is a way that tiny parts of an atom join together. These tiny parts are called nuclei. When two or more nuclei combine, they form one larger nucleus. This action releases a huge amount of energy.

This process is very important because it powers all the active stars in our sky. It is how the Sun creates the light and heat we feel every day. Scientists are working hard to use this same power to make electricity on Earth.

How does this work? It involves a battle between two different forces. One is the Coulomb force, which makes positively charged protons push away from each other. The other is the nuclear force, which acts like a strong glue to pull things together.

For small nuclei, the pulling force is stronger than the pushing force. When they get close enough, they stick together. This change in mass creates energy.

This happens because the new, larger nucleus has a different amount of binding energy than the smaller ones did.

People have been studying this for a long time. In 1915, a chemist named William Draper Harkins first proposed the idea. Later, in 1920, Arthur Eddington predicted that fusing hydrogen into helium powers the stars.

Scientists also learned about a trick called quantum tunneling. This allows a nucleus to pass through a barrier that it might not otherwise cross. In 1934, researchers Mark Oliphant, Paul Harteck, and Ernest Rutherford showed fusion in a lab. They discovered tritium and helium-3 during their experiments.

There are many important numbers to know about fusion. In the Sun, 620 million metric tons of hydrogen fuse every single second. This creates 616 million metric tons of helium each second.

On Earth, scientists use machines called tokamaks to try and control this. In 1993, the TFTR experiment in Princeton produced 1.6 gigajoules of fusion energy. A big milestone happened on December 5, 2022. In the United States, scientists achieved "break-even." This means they produced 3.15 megajoules of energy from only 2.05 megajoules used to start it.

Fusion is linked to many things we see in the world. It is the reason the stars shine and why planets stay warm.

Today, many private companies are spending billions of dollars to make fusion power a reality. Some use lasers, while others use magnets to hold the hot plasma. Large projects like ITER are being built to help us learn more. We are learning how to turn the power of the stars into a way to power our homes.

Nuclear fusion is a reaction where two or more atomic nuclei combine to form a single, larger nucleus. This process is fundamental to the universe because it powers all active stars. When nuclei fuse, the mass of the resulting product is different from the mass of the original parts. This difference in mass is released as a massive amount of energy. This energy comes from a change in the nuclear binding energy of the atoms.

Fusion is generally exothermic, meaning it releases energy, when creating nuclei lighter than nickel-62. This happens because of the positive gradient in the nuclear binding energy curve.

The mechanism of fusion relies on a battle between two opposing physical forces. The first is the Coulomb force, which causes positively charged protons to repel each other. The second is the nuclear force, which acts as a strong attraction to hold protons and neutrons together.

For light nuclei, the short-range nuclear force can overcome the infinite-range Coulomb repulsion. To make this happen, the particles must reach an extremely large triple product of temperature, density, and confinement time. Sometimes, nuclei use a process called quantum tunneling to cross the Coulomb barrier.

This allows a nucleus to pass through a barrier it might not otherwise be able to cross.

Different pathways exist for fusion depending on the size and type of the stars involved. In Sun-type stars, the proton–proton chain is the dominant process.

However, in higher-mass stars, the CNO cycle is more common.

The most fusible nuclei are among the lightest elements. These include isotopes like deuterium, tritium, and helium-3. In contrast, the process of nuclear fission is most energetic when using very heavy nuclei, such as the actinides.

This distinction defines how energy is harvested from different parts of the periodic table.

Our understanding of fusion grew through many decades of scientific discovery. American chemist William Draper Harkins first proposed the concept in 1915. In 1919, Francis William Aston invented the mass spectrometer. This tool helped prove that four hydrogen atoms are heavier than one helium atom. Consequently, Arthur Eddington predicted in 1920 that hydrogen fusion powers stars. In 1934, Mark Oliphant, Paul Harteck, and Ernest Rutherford performed the first intentional deuterium fusion experiment. This experiment led to the discovery of tritium and helium-3. By 1938, researchers observed deuterium-tritium (DT) fusion, which is considered the most favorable reaction for energy.

Fusion has been applied to both peaceful and military technologies. During the Manhattan Project, scientists like Enrico Fermi and Edward Teller studied fusion for thermonuclear weapons. The first artificial thermonuclear fusion occurred during the 1951 US Greenhouse test. This test produced 225 kilotons of energy, which was 15 times the yield of the Little Boy bomb. Later, the 1952 Ivy Mike test used a two-stage Teller–Ulam design to yield over 10 megatons. Modern devices often use solid lithium deuteride to provide tritium for the reaction. This creates a closed neutron cycle that makes the reaction highly energetic.

Today, scientists aim to use fusion for clean, controlled power on Earth. There are two main approaches: magnetic confinement and inertial confinement. Magnetic confinement uses toroidal designs, like the tokamak, to hold hot plasma.

One major international project is ITER, which is expected to begin full deuterium-tritium fusion in 2039. The other approach, inertial confinement, uses lasers to drive the reaction. A massive milestone occurred on December 5, 2022, in the United States. Scientists achieved "break-even," producing 3.15 megajoules of fusion energy from only 2.05 megajoules of input energy.

The scale of fusion is truly massive when looking at the stars. In the core of the Sun, 620 million metric tons of hydrogen fuse every second. This process produces 616 million metric tons of helium every second. In the fusion of two hydrogen nuclei into helium, about 0.645% of the mass is converted into energy. This energy is carried away as kinetic energy or electromagnetic radiation. On Earth, the pursuit of this power is growing rapidly. In 2025, a single Chinese state-owned fusion company received $2.1 billion in funding. Private companies also invested $2.6 billion in fusion startups in 2021 alone.