ASTR 001 Lecture Notes - Lecture 14: Planetary Nebula, Triple-Alpha Process, Degenerate Matter
Document Summary
Helium fusion requires higher temperatures than hydrogen fusion because larger charge leads to greater repulsion. Fusio(cid:374) of t(cid:449)o heliu(cid:373) (cid:374)u(cid:272)lei does(cid:374)"t (cid:449)ork, so heliu(cid:373) fusio(cid:374) (cid:373)ust (cid:272)o(cid:373)(cid:271)i(cid:374)e three he nuclei to make carbon. Thermostat is broken in low-mass red giant because degeneracy pressure supports core. Core temperature rises rapidly when helium fusion begins. Helium fusion rate skyrockets until thermal pressure takes over and expands core again. Low-mass stars eject their outer layers, forming a planetary nebula. When the red giant exhausts its he fuel the c core collapses. Low & intermediate -mass stars do(cid:374)"t ha(cid:448)e e(cid:374)ough gra(cid:448)itatio(cid:374)al e(cid:374)erg(cid:455) to heat to 6 (cid:454) The he & h burning shells overcome gravity the outer envelope of the star is gently blown away this forms a planetary nebula. A white dwarf is the burned - out, carbon/oxygen core of a dead low - mass star. White dwarves generate no new energy, simply cool over very long times.
