The Sun is a self-gravitating sphere of gas, mostly hydrogen, that emits energy in the form of electromagnetic and corpuscular radiation; electromagnetic radiation is emitted at all wavelengths of the spectrum, from radio waves to gamma rays, with a maximum intensity in the visible band. Density and temperature increase from the surface to the center. The temperature varies from about 6000 K at the surface of the photosphere to 15-20 million K in the central regions; at these temperatures the gas is completely ionized (plasma). According to the so-called Standard Solar Model, the mechanism by which the Sun produces energy is thermonuclear fusion, which takes place in the central regions of the astro. The main thermonuclear reactions in the Sun are of the proton-proton type, which are more efficient at the relatively low temperatures found in the Sun than the carbon cycle reactions that power more massive stars. Such reactions produce a massive neutrino flux, which is the only source of direct information about the inner regions, since the radiation we observe, although produced in the center, is absorbed and re-emitted several times by the outer layers.

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