Read the following paragraph and answer the question. Give ONLY the answer, no other words or explanation.

For example:

Answer: <most likely answer, as short as possible>.

Paragraph: 
The Sun is Earth’s main source of energy. The Sun gives us both light and heat. The Sun changes hydrogen into helium through nuclear fusion. This releases huge amounts of energy. The energy travels to Earth mostly as visible light. The energy is carried through the empty space by radiation . We can use sunlight as an energy resource, called solar energy.


Question: What celestial body is the earth’s main source of energy?
sun
The Sun.