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Internal heat is the heat source from the interior of celestial objects, such as planets, brown dwarfs, and stars, caused by gravity, nuclear fission and decaying radioactive materials. The amount of internal heating depends on mass; the more massive the object, the more internal heat it has. The internal heating keeps celestial objects warm and active.
The internal heat is the heat left over from formation of celestial objects. The aging objects lose internal heat gradually except for stars.
The internal heating within terrestrial planets powers tectonic and volcanic activities. Earth has the most internal heating because it is the most massive of the terrestrial planets. Mercury and Mars have no significant internal heating because they are only 5 and 11% the mass of Earth respectively; they are "geologically dead".
The gas giants have much greater internal heating than terrestrial planets. Jupiter has the most internal heating with core temperature of 36,000 K. For the outer planets of our solar system, internal heating powers the weather and wind instead of sunlight that powers the weather for terrestrial planets. The internal heating within gas giant planets raise temperatures higher than effective temperatures, as in the case of Jupiter, this makes 40 K warmer than given effective temperature. The internal heating within giant planets that orbit very close to their stars make planets puffier, or planets that were expanded.
Brown dwarfs have greater internal heating than gas giants but not as great as stars. The internal heating within brown dwarfs is great enough to sustain thermonuclear reaction of deuterium to helium. Like gas giants, brown dwarfs can have weather and wind powered by internal heating.
The internal heating within stars are so great that they sustain thermonuclear reaction of hydrogen to helium and can make heavier elements. The Sun for example has a core temperature of 13,600,000 K. The bluer, more massive, hotter, and older the stars are, the more internal heating it has. During the end of its lifecycle, the internal heating of a star increases dramatically, caused by contracting core, eventually becoming hot enough to fuse helium then carbon or oxygen.
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