CAUSED BY EARTH'S CORE; THE LEAST USABLE SOURCE OF ENERGY

ASSOCIATED WITH ATMOSPHERIC OXYGEN, BIOMASS AND FOSSIL FUELS

FORMED FROM NUCLEAR FUSION OR RADIOACTIVE DECAY; USED TO GENERATE ELECTRICITY

GENERATED FROM INSIDE THE EARTH FROM RADIOACTIVE DECAY

RADIATED FROM THE SUN AS HEAT ENERGY

PROCESS OF AN UNSTABLE ATOM BY EMITTING PARTICLES OF IONIZED RADIATION

THE SIZE OF CERTAIN LARGE OBJECTS ATTRACTS OTHER SMALLER OBJECTS WITH A GRAVITATIONAL PULL