Fusion is the process in which two small nuclei join together to make one larger nucleus.

N

N

+

N

+

N

+

+

Because the nuclei are positive, they repel each other. The only way to get them to connect with each other is to get them moving very quickly (get them very hot). In fact the temperature must be millions of degrees in order to make this happen! For this reason, the process is often called thermonuclear fusion.

These temperatures are found in the interiors of stars, where Hydrogen happens to be very abundant. Stars are hot because of thermonuclear fusion in their cores.

If you look at the Mass per Nucleon curve below, you will see that the nucleons in H have more mass than they do in He. Therefore, when two H combine to form one He nucleus, there is a net reduction in mass, and a great release of energy (e=mc² again!)

Fe has the lowest mass per nucleon, for this reason it is the largest nucleus that can be produced by fusion (without an input of energy), and the smallest nucleus that can be produced by fission.

Stars are fueled by the conversion of H to He, and of mass to energy.

Each second the sun converts 4,200,000 tons of mass into energy by converting H into He.

Humans are able to achieve these extreme temperatures during nuclear detonations. The hydrogen bomb (H-bomb), is basically a fission bomb (U-235 or Pu-239) in which neutron-rich isotopes of Hydrogen are present. The fission bomb gets the temperature up, then the hydrogen fuses.

Since the curve is steeper on the hydrogen side than on the uranium side, much more energy is released by a fusion bomb than by a fission bomb.

The United States has tested hydrogen bombs that are 2000 times more powerful than the one that leveled Hiroshima.