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The Deaths of Stars What happens to stars when the helium runs out? do they simply fade into oblivion? NO! stellar deaths produce some of the most spectacular phenomena in the universe and also play a vital role in enriching with heavy elements the material of which new stars are made Susan Cartwright Our Evolving Universe 1 Stellar lifetimes Our binary star data suggest that the lifetimes of stars with mass < 0.9 Msun are longer than 15 billion years (the age of the universe) But massive stars have lifetimes of only a few million years Susan Cartwright Our Evolving Universe 2 Stellar statistics A census of nearby stars: most stars are low mass red dwarfs a few percent are 1–2 x the Sun’s mass very massive stars are very rare (only 3 B and no O class blue stars in the 3800 stars within 75 light years of the Sun) Stars within 25 pc 12 00 10 00 80 0 60 0 40 0 Stellar deaths are rare M 20 0 K G F 0 I A II III IV B V WD (but crucial to our existence!) Susan Cartwright Our Evolving Universe 3 The deaths of Sun-like stars Giant stars are very unstable especially those which are fusing helium Outer layers of star easily lost mass loss seen in spectra of these stars rate increases towards end of helium fusion Susan Cartwright Our Evolving Universe 4 Mass loss in Sun-like stars Star of 1.7 solar masses Susan Cartwright Our Evolving Universe 5 Death of a star At end of helium fusion star has lost all its outer layers central, very hot, carbon core surrounded by expelled gas ultraviolet radiation from hot core causes gas to produce emission lines planetary nebula Susan Cartwright nothing to do with planet, just looks like one! Our Evolving Universe 6 Planetary nebulae Susan Cartwright Our Evolving Universe 7 The legacy of Sun-like stars Giant stars have convective outer layers elements formed in core are transported to surface thus ejected in planetary nebula nitrogen, barium, zirconium, etc. Planetary nebulae thus enrich the interstellar gas with many elements Stellar core is eventually revealed as small, hot white dwarf star Susan Cartwright Our Evolving Universe 8 Fusing heavy elements Massive stars have much hotter cores successfully fuse elements up to iron But this is a very temporary respite: for a star of 20 solar masses, hydrogen fusion lasts around 15 million years helium fusion lasts around one million years carbon fusion lasts 300 years oxygen fusion lasts for 7 months silicon fusion lasts for two days and produces an iron core not to scale! Susan Cartwright Our Evolving Universe H + He He C O Si Fe 9 The fate of massive stars Fusing iron requires (does not generate) energy iron core cannot support itself against gravity collapses to form neutron star neutron star is about 50% more massive than Sun, but is only 20 km across basically a gigantic atomic nucleus: protons and electrons have combined to form neutrons in extreme cases even this may not be stable, and a black hole is formed instead SN1994D: HST outer layers expelled in massive explosion: a supernova Susan Cartwright for a few weeks star is nearly as bright as a whole galaxy Our Evolving Universe 10 The legacy of massive stars Supernova remnants expanding gas cloud, formerly star’s outer layers Pulsars rapidly rotating neutron stars observed by their “lighthouse beam” of radio emission (sometimes also optical) emitted from magnetic poles of star Heavy elements formed in the core of the star as it implodes Susan Cartwright Our Evolving Universe 11 The Crab pulsar Spin axis and magnetic axis misaligned: see pulse In Crab see 2 pulses/cycle: angle must be ~90° Susan Cartwright Our Evolving Universe 12 Supernova remnants Cygnus Loop (HST): green=H, red=S+, blue=O++ Cas A in x-rays (Chandra) Vela Remnant of SN386, with central pulsar (Chandra) SN1998bu Susan Cartwright Our Evolving Universe 13 Building the elements Routes to building elements: Start with Add In Making carbon protons H-fusing stars nitrogen carbon helium nuclei (α-process) He-fusing stars oxygen-16, neon-20, etc. carbon-13, neon-22 helium nuclei He-fusing stars free neutrons silicon, sulphur silicon, sulphur pre-supernova stars iron iron, neon neutrons, slowly (s-process) He-fusing stars most stable isotopes up to Bi iron neutrons, rapidly (r-process) supernovae neutron-rich isotopes iron add photon, lose neutron? (p-pr) supernovae rare neutron-poor isotopes Susan Cartwright Our Evolving Universe 14 Adding neutrons— why does the speed matter? Slow addition of neutrons (s-process) cannot make isotopes that are “shielded” by unstable isotopes (e.g. 116Cd) Rapid addition of neutrons (r-process) cannot make those shielded by stable nuclei (e.g. 116Sn) Some isotopes (p-process) cannot be made by either method (e.g. 112Sn) 51 121 Sb 50 Sn 49 112 1.01% 115 d 112 In 48 Cd Susan Cartwright 113 110 14 m 111 114 115 116 117 118 119 0.67% 0.38% 14.6% 7.75% 24.3% 8.6% 113 114 115 116 117 4.3% 112 71.9 s 95.7% 14.1 s 113 114 115 44 m 116 12.5% 12.8% 24.1% 12.2% 28.7% 53.4 h 7.5% 62 63 64 65 66 67 68 Our Evolving Universe 122 57.3% 2.7 d 120 121 123 42.7% 122 32.4% 27.1 h 4.56% 117 118 2.4 h 69 50 m 70 71 72 15 Summary: the abundance of the chemical elements CNO The iron peak: made in the last stages of fusion Heavy elements: made by adding neutrons to iron not made in stars p-process isotopes Susan Cartwright Our Evolving Universe 16