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The huge amounts of energy generated by stars are due to nuclear fusion reactions occurring deep within their interiors. Using physical assumptions and observational data, an interior model of a star can be made which describes how energy is transported from it into space. The source of the sun's energy could not be chemical because if it was made out of coal, for instance, which produces heat and light from chemical energy, then it would have only lasted for about 300 years. As it does so the gases in its interior become compressed and when a gas is compressed its temperature increases . Kelvin and Helmholtz argued that gravitational contraction would cause the sun's gases to become hot enough to radiate heat energy into space.