In low-mass stars like our sun and up to 4 solar masses, when all helium is burned in the core into carbon and oxygen, burning of the shell helium and hydrogen signals the closely approaching death of the star, now a supergiant. After a cycle of contraction and reexpansion, the surface gravity of the supergiant will be so low that all of the outer layers will move out into space, creating a so-called planetary nebula around the now-naked, exposed very hot core. When all nuclear fuel is exhausted and fusion stops, the core will start again to collapse. The gravitational collapse of the core will stop at a density of approximately 1000 tons per cubic inch. Electrons, which are highly compressed at this density, resist all further compression and form what is called in physics degenerate electron matter. Degenerate matter, where electrons start to repel each other, is so resistant to further compression that nothing more can happen to the remaining naked core of the sun.
KeywordsBlack Hole Neutron Star White Dwarf Nuclear Density Supernova Explosion
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