A star is a massive ball of plasma (very hot gas) held together by gravity. It radiates energy because of the nuclear reactions inside it.
It radiates heat and light, and every other part of the electromagnetic spectrum, such as radio waves, micro-waves, X-rays, gamma-rays and ultra-violet radiation. The proportions vary according to the mass and age of the star.
The energy of stars comes from nuclear fusion. This is a process that turns a light chemical element into another heavier element. Stars are mostly made of hydrogen and helium. They turn the hydrogen into helium by fusion. When a star is near the end of its life, it begins to change the helium into other heavier chemical elements, like carbon and oxygen. Fusion produces a lot of energy. The energy makes the star very hot. The energy produced by stars radiates away from them. The energy leaves as electromagnetic radiation.

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