Final answer:
The Sun is the main source of heat energy through nuclear fusion, providing light and heat to Earth.
Explanation:
The Sun is referred to as the main source of heat energy because it undergoes nuclear fusion, converting hydrogen into helium and releasing vast amounts of energy in the process. This energy is primarily transmitted to Earth as visible light through radiation. The Sun's energy is essential for life on Earth and powers various processes such as photosynthesis and heating the planet and its atmosphere.
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