Or maybe the Sun continues to contract, generating fresh gravitational energy. No, that could work for millions of years, but not billions. This was once thought to be the Sun's source of energy, and the Earth and Sun were thought to be about 10 million years old. There must be some other source of the sun's energy. Also known as nuclear fission .
Radioactive materials keep the center of the earth hot.
They're also used to make nuclear weapons, and it's the energy source for nuclear power plants. But we know what elements are radioactive and which aren't. Hydrogen and helium are not. When conditions are right, certain light elements can combine together in an explosive reaction to create heavier elements. Hydrogen atoms can fuse together to create helium and release energy.