PHYS 183 Lecture Notes - Lecture 21: Sn 1987A, Crab Nebula, Large Magellanic Cloud
21 Apr 2018
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In a solar type star, inert carbon core never ignites. Star insufficiently massive for core to contract and reach high enough temperatures: core shrinkage halted by electron degeneracy pressure. In more massive stars, carbon can be fused: forms o/ne/mg core. Slowly develops inion structure: layers of different elements. Averaged over universe, relative abundancies of the elements. Note a dip between he and c. Reactio(cid:374)s i(cid:374)volve alpha particles (cid:894)4he(cid:895): (cid:862)heliu(cid:373) capture reactio(cid:374)(cid:863) Continued evolution follows pattern: when core fusion ceases, hydrostatic equilibrium disrupted, core contracts; outer layers expand, core temperature increases, next heaviest element starts fusion, hydrostatic equilibrium restored. Burbidge, burbidge, fowler and hoyle 1958: worked out which nuclear reactions happen. Fusion of iron with anything requires energy, does not produce it. When iron forms, fusion reaches ultimate obstacle. Once iron forms, no more fusion can proceed. Note that only for very massive stars does this happen. Need sufficient gravity to get heavier elements to fuse.
