Source of the heat in the earth's core?

Question

What is the source of the heat in the earth's core? Is it because everything above it is so heavy, that it all compacts together so tightly and makes everything hot? I don't know, just a suggestion.

Answer 1

There are three main sources of heat in the deep earth: (1) heat from when the planet formed and accreted, which has not yet been lost; (2) frictional heating, caused by denser core material sinking to the center of the planet; and (3) heat from the decay of radioactive elements.

Answer 2

Almost all of it is from the continued radioactive decay of long-lived uranium isotopes and their daughter products.