Stellar processes, nucleosynthesis OUTLINE Reading this week: White 313-326 and 421-464 Today 1. Stellar processes 2. Nucleosynthesis Powerpoint credit: Using significant parts of a WHOI ppt 1
Question WHAT IS STEP 1 IN MAKING ELEMENTS? The Big Bang based on: 1) the observation of Edwin Hubble (1889-1953) that the galaxies were moving away from us HOW DO WE KNOW? 2) background cosmic microwave radiation can be heard linear relationship between distance and red-shift demonstrates uniform expansion, implying a point-source origin 2
Big Bang: Main steps 1) Universe is point at ~15 Ga 2) <10-32 seconds: quark soup + electrons mostly 3) ~1 second: free electrons, protons, neutrons, neutrinos and photons. 4) ~13.8 seconds, cooled enough for H and He nuclei 5) 380Ka: electrons added to H, He true atoms. Matter became organized into stars, galaxies and clusters Periodic Table: Z = atomic number or number of protons A = mass number or number of protons + neutrons Big Bang directly forms mostly H, some He that s only 2, how about the rest? 3
Nucleosynthesis: the process of creation of the elements Abundances of elements (and their isotopes) in meteorites and from observations on stars: Until stars formed, there was nothing except H and He Star formation due to gravity within molecular clouds At sufficient temperature and density (~10 7 K), nuclear fusion begins in star cores Young Magellan stars Supernova 1994D http://hubblesite.org Classification of stars The Hertzsprung-Russell diagram: luminosity vs surface temperature. Most stars, like the sun fall on the main sequence, but can evolve to red giants and supernovas (if they are at least 5 x as massive as our sun) or to white dwarfs, pulsars or even black holes. our sun 4
Most stars produce energy by 1 H burning fusion to 4 He 2 nd generation stars: 1 H burning by C-N-O cycle (catalysts) to make 4 He, Fusion processes make up to mass 40 Ca (He, C, O, Si burning) Eventually all H will be fused to He (sun has fused 10-20% H) Star < ~8 solar masses will undergo swelling to form a red giant, then gravitational collapse to a white dwarf: rapid nuclear rearrangement creating everything up to 56 Fe Nucleosynthesis (cont.) Star >8 solar masses implodes, then explodes into a supernova. Then neutron star or black hole, depending on mass Crab nebula 1054 supernova Collapse, supernova make elements > Fe, 2 pathways: s-process (slow): addition of neutrons to nuclei one at a time (only stable elements can be made), during collapse r-process (rapid): addition of neutron at a rapid rate so as to bridge areas of nuclear instability (only in supernovas and accounts for about half of elements beyond 56 Fe) 5
More massive star = heavier elements it can burn Burning processes account for elemental abundances up to Fe Note: little Li, stable Be, B produced H Burning PV = nrt Hi T keeps hi P: H keeps burning Once H = exhausted: ÞP decreases ÞH burning stops Hydrogen burning sequence: 1 1 H + 1 1 H 2 1 H + b+ + energy From C.T. Lee s notes 2 1 H + 1 1 H 3 2He + energy 3 2 He + 3 2 He 4 2 He + 1 1 H + 1 1 H +energy 6
He burning After H-burning gravitational collapse ÞNew release of gravitational energy, converted to thermal energy ÞEventually reaches temperatures sufficient for He burning ÞMainly Be, C generation, but can also make O, Ne, Mg Helium burning 4 2 He + 4 2 He 8 4Be + energy 4 2 He + 8 4 Be 12 6 C + energy (net reaction: 3 4 2 He 12 6 C + energy) From C.T. Lee s notes C burning After He-burning gravitational collapse ÞNew release of gravitational energy, converted to thermal energy ÞEventually reaches temperatures sufficient for C burning ÞMainly O generation, but can also make Ne, Si ÞEventually, C, N, O burning produces Fe or Ni at A ~ 60. Fusion reactions cannot go beyond A ~60 Carbon Burning 12 6 C + 4 2 He 16 8 O + energy Late-stage burning reactions Makes Ne, O and Si From C.T. Lee s notes 7
Energy and nuclear fusion The addition process stops at Fe because after that, energy is no longer gained by making heavier nuclei (> have to add energy). Fe = crossover where repulsion between nuclei outweighs overall attraction Fe Elements stability Energy consumption Energy production What is the most energy-productive step? 8
10/25/17 Chart of the nuclides number of protons Chart of the nuclides: greatest stability of proton/neutron ratio. At low atomic mass, greatest stability when neutrons = protons (N = Z), but as atomic mass increases, the stable neutron/proton ratio increases until N/Z = 1.5. number of neutrons Chart of the Nuclides number of protons Shows the nuclear, or radioactive, behaviour of nuclides Isobar: nuclides of equal mass number Isotope: nuclides of the same chemical element having different atomic masses number of neutrons 9
10/25/17 Nuclear processes (and the chart of nuclides) At higher masses need different processes: 1)Radioactive decay 2)s/p process during collapse and supernova number of protons So at low mass combine some isotopes in stellar fusion number of neutrons Radioactivity and the stability of nuclei Less energetically favorable nuclei are most unstable (shorter half life) and the harder it is to make in the first place. Decay occurs at characteristic rate for a given isotope. 5 main mechanisms of radioactive decay: b 10
Different decay schemes move you in different directions on the chart of nuclides Produces various nuclei, not necessarily stable ones Radioactivity and the chart of the nuclides From C.T. Lee s notes S versus R process From C.T. Lee s notes isobar S-process isotope R-process S Process leaves time between neutron additions for decay R Process just keeps adding neutrons with few decay steps, quick way to make nuclei with high neutron amount 11
Nuclear preferences So stellar processes make nuclei (fusion, neutrons), radioactivity removes unstable nuclei Nature has a preference for even numbers: lower energy when you get even numbers of opposite spin in protons (half of each), and same in case of neutrons A (mass) = Z(atomic number) + N(neutrons) A N Z # of nuclei even even even 166 odd odd 8 odd even odd 57 odd even 53 GG325 L28, F2013 Odd vs Even Nuclei Even mass nuclei are more stable and thus less likely to capture a neutron during r and s neutron addition processes. What would mass be if +1 N o? Correspondingly, their neutron capture cross section is smaller (a smaller target) Þeven nuclei are more abundant in nature 12
Solar abundance of the elements 1) Universe = 4% matter, 73% dark energy, 23% dark matter 2) General decrease in abundance with atomic number (H most abundant) 3) Big negative anomaly at Be, B, Li - moderate positive anomaly around Fe, sawtooth pattern from odd-even effect Solar system composition Abundances up to Fe from H, He, etc burning (fusion) in ancient stars that existed before the sun. Most abundant nuclei up to 56 Fe are multiples of 4 2He 13
Elements heavier than Fe ~all made by neutron addition to target nucleus by s and r process, combined with decay Neutrons come from late stage burning, and supernovae Nuclei with magic neutron numbers (82, 126, 184) are unusually stable, producing kicks or jogs in the neutron addition pathways. 14