PHYS 183 Lecture Notes - Lecture 20: Helium Flash, Subgiant, Triple-Alpha Process

• School: McGill University
• Department: Physics
• Course: PHYS 183
• Professor: Gilbert Holder

Document Summary

A sta(cid:396)"s fate depe(cid:374)ds (cid:272)(cid:396)iti(cid:272)ally o(cid:374) its (cid:373)ass. Low mass stars (m< 2sm): become white dwarfs. Intermediate mass stars ( 2< m< 8 sm): become red giants (other stuff), planetary nebulae, white dwarfs. Th(cid:396)oughout sta(cid:396)"s life, (cid:272)o(cid:374)sta(cid:374)t (cid:271)attle (cid:271)et(cid:449)ee(cid:374) i(cid:374)(cid:449)a(cid:396)d, (cid:272)(cid:396)ushi(cid:374)g pull of g(cid:396)a(cid:448)ity a(cid:374)d out(cid:449)a(cid:396)d pressure of gas and radiation. Star constantly readjusting to changing structure as fusion proceeds. Mai(cid:374) se(cid:395)ue(cid:374)(cid:272)e: lo(cid:374)gest pe(cid:396)iod of e(cid:395)uili(cid:271)(cid:396)iu(cid:373) i(cid:374) sta(cid:396)"s life. Other stages: equilibrium lost, adjustment to new equilibrium. Main sequence: core hydrogen burning, ie fusion: pp chain. Stellar core begins to shrink as gravity pulls inward, with less and less outward pressure available. But unfused hydrogen shell surrounding core contracts too, fusion can begin. Once shell burning stars, very efficient burning. Outer regions cool as they puff out. Core continues to contract inward due to gravity. At ~100 million k, helium starts to fuse. Heliu(cid:373) fusio(cid:374) p(cid:396)o(cid:272)ess (cid:272)alled (cid:862)t(cid:396)iple alpha p(cid:396)o(cid:272)ess(cid:863) Theoretical computer models show that he fusion commences explosively.