Hydrogen is converted to Helium in the sun.

Nuclear reactors use controlled chain reactions.

Energy is released spontaneously by radioactive elements.

1. Fusion
The source of nearly all the energy we have on earth is the fusion of hydrogen into helium which takes place inthe sun. Self sustaining fusion reactions can occur only under conditions of extreme temperature and pressure. Stellar interiors meet these specifications. The energy liberated in the fusion of light nucei into heavier ones is often called thermonuclear energy.

2. Fission
A nuclear reactor is a device in which a controlled chain reaction involving fission, releases energy. The energy liberated in a nuclear reactor is manifested as heat. It can be extracted by circulating a suitable coolant (liquid or gas) around the core of the reactor.

3. Radioactivity
The energy liberated during radioactive decay comes from within individual nuclei without external excitation.