What eventually happens to a white dwarf? It gets cooler and fainter (at the same radius) What happens to higher mass stars? • Gravity squeezes and heats the core enough to ignite Carbon • Then Oxygen, then Neon . . . as each fuel gets exhausted in the core, its burning moves to a shell • Concentric fusion shells form an “onion skin” structure • Formation of an Iron core is the last stage • Stars squiggle back and forth like crazy on HR diagram Why is iron formation the end of the line? • Creating elements heavier than Iron requires energy • With no more sources of energy, and Fe fusion taking energy from the gas, pressure support in the star’s core is lost . . . • The core quickly collapses under its own weight . . . • Protons and electrons are crushed together in the collapsing core, making neutrons • Eventually, the neutrons are so close together they “touch” neutron degeneracy pressure (which is very stiff) • The densities reach 100 billion kg/cm^3 (at those densities the whole Earth would fit in our football stadium!!) • The collapsing neutron core then bounces! An explosive shock wave propagates outward, expelling all outer layers (Supernova) • Crab nebula
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