Energy production in the interior of stars like the

The energy produced by nuclear reactions in the interior of the Sun must equal the amount of energy radiated from the surface, since otherwise the Sun could not have been structurally stable over long periods of time. Evidence for the stability of the Sun comes from several sources. Stability over a period of nearly 3 x 109 years is implied by the relative stability of the temperature at the Earth's surface (oxidised sediments and fossil remains indicate that water in its fluid phase has been present throughout such periods). Stability over an even longer time is implicit in our understanding of the evolution of the Sun and other similar stars. As an indication of this stability, Fig. 2.2 shows the variations in the radius of the Sun believed to have taken place since its assumed formation from clouds of dust and gas.

The conversion of energy contained in the atomic constituents of main-sequence stars such as the Sun from heat of nuclear reactions (which transforms hydrogen into helium) to radiation escaping from the surface is largely understood. The advanced topic section 2.A at the end of this chapter gives further details of the stellar evolution and a specific model for energy transport within the Sun. The basis for regarding such radiation as a renewable source is that it may continue essentially unaltered for billions of years. Yet there is also a possibility of tiny variations in solar energy production that may have profound implications for life on the planets encircling it.

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