Image Credit of National Geographic. The Sun is the star at the center of the Solar System. It is a ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy mainly as light, ultraviolet, and infrared radiation.  The Sun's radius is about 695,000 kilometers or 109 times that of Earth. Its mass is about 330,000 times that of Earth, comprising about 99.86% of the total mass of the Solar System. Roughly three-quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron.  A star fuses hydrogen into helium first, then as it ages and runs out of hydrogen, the helium will fuse into heavier elements, until it reaches Iron at this point it will either have expanded out into a red gaint or it will implode into a super nova explosion. This will depend on the intial mass of the star. If the intial mass is passed a critical ...