Nuclear Reactions: The Sun's energy comes from nuclear reactions in its core.

More specifically, the energy comes from the fusion (joining) of nuclei of hydrogen atoms into nuclei of helium atoms. The actual fusion process takes place in several steps, which may be summarized as follows:

Neutrinos are subatomic particles that travel close to the speed of light and only rarely interact with matter. The neutrinos produced during hydrogen fusion in the center of the Sun pass right through the overlying solar material and escape into space, carrying away a small fraction of the energy.

Where Does the Energy Come from in the Fusion of Hydrogen into Helium? During the fusion of hydrogen, approximately 0.7% of the mass of hydrogen is converted into energy. This means that the resulting helium has 0.7% less mass than the original hydrogen. Einstein's famous formula tells us how much energy this loss of mass generates:

Or, expressed more explicitly,

energy = (mass that disappears) x (speed of light) x (speed of light).

Since the speed of light (c) is very large (186,000 miles per second) and comes in squared, the amount of energy generated by the fusion of hydrogen is very large, even though the amount of mass that disappears is very small.

Nuclear vs. Chemical Energy: The energy that can be gotten from nuclear reactions is far greater than that of chemical reactions, such as the burning of coal or gasoline. For example, the fusion of one pound of hydrogen into helium yields as much energy as the burning of 10,000 tons of coal.